Search

The quasi-persistent neutron star X-ray transient and eclipsing binary EXO 0748-676 recently started the transition to quiescence following an accretion outburst that lasted more than 24 years. We report on two Chandra and twelve Swift observations performed within five months after the end of the outburst. The Chandra spectrum is composed of a soft, thermal component that fits to a neutron star atmosphere model with kT^inf~0.12 keV, joined by a hard powerlaw tail that contributes ~20% of the total 0.5-10 keV unabsorbed flux. The combined Chandra/Swift data set reveals a relatively hot and luminous quiescent system with a temperature of kT^inf~0.11-0.13 keV and a bolometric thermal luminosity of ~8.1E33-1.6E34 (d/7.4 kpc)^2 erg/s. We discuss our results in the context of cooling neutron star models., Comment: Accepted for publication in MNRAS Letters, moderate revision according to referee report, added one plot to figure 2 and included new Swift observations, 5 pages, 2 figures

We present the results of the analysis of two Chandra observations of Circinus X-1 performed in 2007, for a total exposure time of ~50 ks. The source was observed with the High Resolution Camera during a long X-ray low-flux state of the source. Cir X-1 is an accreting neutron-star binary system that exhibits ultra-relativistic arcsec-scale radio jets and an extended arcmin-scale radio nebula. Furthermore, a recent paper has shown an X-ray excess on arcmin-scale prominent on the side of the receding radio jet. In our images we clearly detect X-ray structures both on the side of the receding and the approaching radio jet. The X-ray emission is consistent with being from synchrotron origin. Our detection is consistent with neutron-star binaries being as efficient as black-hole binaries in producing X-ray outflows, despite their shallower gravitational potential., Comment: Accepted for publication in MNRAS-Letters

We observed SAX J1808.4-3658 (1808), the first accreting millisecond pulsar, in deep quiescence with XMM-Newton and (near-simultaneously) Gemini-South. The X-ray spectrum of 1808 is similar to that observed in quiescence in 2001 and 2006, describable by an absorbed power-law with photon index 1.74+-0.11 and unabsorbed X-ray luminosity L_X=7.9+-0.7*10^{31} ergs/s, for N_H=1.3*10^{21} cm^{-2}. Fitting all the quiescent XMM-Newton X-ray spectra with a power-law, we constrain any thermally emitting neutron star with a hydrogen atmosphere to have a temperature less than 30 eV and L_{NS}(0.01-10 keV)<6.2*10^{30} ergs/s. A thermal plasma model also gives an acceptable fit to the continuum. Adding a neutron star component to the plasma model produces less stringent constraints on the neutron star; a temperature of 36^{+4}_{-8} eV and L_{NS}(0.01-10 keV)=1.3^{+0.6}_{-0.8}*10^{31} ergs/s. In the framework of the current theory of neutron star heating and cooling, the constraints on the thermal luminosity of 1808 and 1H 1905+000 require strongly enhanced cooling in the cores of these neutron stars. We compile data from the literature on the mass transfer rates and quiescent thermal flux of the largest possible sample of transient neutron star LMXBs. We identify a thermal component in the quiescent spectrum of the accreting millisecond pulsar IGR J00291+5934, which is consistent with the standard cooling model. The contrast between the cooling rates of IGR J00291+5934 and 1808 suggests that 1808 may have a significantly larger mass. This can be interpreted as arising from differences in the binary evolution history or initial neutron star mass in these otherwise similar systems., Comment: ApJ in press, 7 pages, 2 color figures

While iron emission lines are well studied in black hole systems, both in X-ray binaries and Active Galactic Nuclei, there has been less of a focus on these lines in neutron star low-mass X-ray binaries (LMXBs). However, recent observations with Suzaku and XMM-Newton have revealed broad asymmetric iron line profiles in 4 neutron star LMXBs, confirming an inner disk origin for these lines in neutron star systems. Here, we present a search for iron lines in 6 neutron star LMXBs. For each object we have simultaneous Chandra and RXTE observations at 2 separate epochs, allowing for both a high resolution spectrum, as well as broadband spectral coverage. Out of the six objects in the survey, we only find significant iron lines in two of the objects, GX 17+2 and GX 349+2. However, we cannot rule out that there are weak, broad lines present in the other sources. The equivalent width of the line in GX 17+2 is consistent between the 2 epochs, while in GX 349+2 the line equivalent width increases by a factor of ~3 between epochs as the source flux decreases by a factor of 1.3. This suggests that the disk is highly ionized, and the line is dominated by recombination emission. We find that there appears to be no specific locations in the long-term hardness-intensity diagrams where iron emission lines are formed, though more sources and further observations are required., Comment: 11 pages, accepted for publication in ApJ

We present a catalog of 9017 X-ray sources identified in Chandra observations of a 2 by 0.8 degree field around the Galactic center. We increase the number of known X-ray sources in the region by a factor of 2.5. The catalog incorporates all of the ACIS-I observations as of 2007 August, which total 2.25 Msec of exposure. At the distance to the Galactic center (8 kpc), we are sensitive to sources with luminosities >4e32 erg/s (0.5-8.0 keV; 90% confidence) over an area of one square degree, and up to an order of magnitude more sensitive in the deepest exposure (1.0 Msec) around Sgr A*. The positions of 60% of our sources are accurate to <1" (95% confidence), and 20% have positions accurate to <0.5". We search for variable sources, and find that 3% exhibit flux variations within an observation, 10% exhibit variations from observation-to-observation. We also find one source, CXOUGC J174622.7-285218, with a periodic 1745 s signal (1.4% chance probability), which is probably a magnetically-accreting cataclysmic variable. We compare the spatial distribution of X-ray sources to a model for the stellar distribution, and find 2.8 sigma evidence for excesses in the numbers of X-ray sources in the region of recent star formation encompassed by the Arches, Quintuplet, and Galactic center star clusters. These excess sources are also seen in the luminosity distribution of the X-ray sources, which is flatter near the Arches and Quintuplet than elsewhere in the field. These excess point sources, along with a similar longitudinal asymmetry in the distribution of diffuse iron emission that has been reported by other authors, probably have their origin in the young stars that are prominent at l~0.1 degree., Comment: 23 pages, 13 figures, 7 tables. Submitted to ApJ Supplements, referees' first comments incorporated. Data is available at http://www.srl.caltech.edu/gc_project/

In this paper we report on the spectral analysis of seven X-ray transients, which were found to be active during a monitoring campaign of the Galactic center carried out in 2006 and 2007 using the X-ray telescope aboard the Swift satellite. This campaign detected new outbursts of five known X-ray transients and discovered two new systems. Their 2-10 keV peak luminosities range from 1E34 to 6E36 erg/s. Two of the sources discussed in this paper are confirmed neutron star systems (AX J1745.6-2901 and GRS 1741-2853), while the five others have an unknown nature. We discuss the characteristics of the observed outbursts and the duty cycles of the various systems. Several of the detected transients seem to undergo enhanced X-ray activity with levels intermediate between quiescence and full outburst. We discuss the possibility that the subluminous appearance of the eclipsing X-ray burster AX J1745.6-2901 is due to line-of-sight effects. We detected two type-I X-ray bursts with a duration of 50-60 seconds from AX J1745.6-2901, which we discuss in view of the bursting behavior of low-luminosity X-ray transients. Assuming that we are dealing with accreting neutron stars and black holes, we estimate the time-average accretion rate, Mdot, of the transients, which is an important input parameter for binary evolution models that attempt to explain the nature of subluminous X-ray transients. Our estimates lie in the range of 3E-13 Msun/yr < Mdot < 1E-10 Msun/yr, if the systems are neutron star X-ray binaries and between 4E-14 Msun/yr < Mdot < 2E-11 Msun/yr for a scenario where the accreting object is a black hole. Some of the systems have such low estimated mass-accretion rates that they possibly pose a challenge for binary evolution models., Comment: Accepted for publication in A&A, some textual changes according to referee's report, 13 pages, 4 figures

We have studied the rapid X-ray time variability in 99 pointed observations with the Rossi X-ray Timing Explorer (RXTE)'s Proportional Counter Array of the low-mass X-ray binary 1E~1724--3045 which includes, for the first time, observations of this source in its island and banana states, confirming the atoll nature of this source. We report the discovery of kilohertz quasi-periodic oscillations (kHz QPOs). Although we have 5 detections of the lower kHz QPO and one detection of the upper kHz QPO, in none of the observations we detect both QPOs simultaneously. By comparing the dependence of the rms amplitude with energy of kHz QPOs in different atoll sources, we conclude that this information cannot be use to unambiguously identify the kilohertz QPOs as was previously thought. We find that Terzan~2 in its different states shows timing behavior similar to that seen in other neutron-star low mass X-ray binaries (LMXBs). We studied the flux transitions observed between February 2004 and October 2005 and conclude that they are due to changes in the accretion rate., Comment: 16 pages, 13 figures, accepted for publication in ApJ

Millihertz quasi-periodic oscillations reported in three neutron-star low mass X-ray binaries have been suggested to be a mode of marginally stable nuclear burning on the neutron star surface. In this Letter, we show that close to the transition between the island and the banana state, 4U~1636--53 shows mHz QPOs whose frequency systematically decreases with time until the oscillations disappear and a Type I X-ray burst occurs. There is a strong correlation between the QPO frequency $\nu$ and the occurrence of X-ray bursts: when $\nu\gtrsim9$ mHz no bursts occur, while $\nu\lesssim9$ mHz does allow the occurrence of bursts. The mHz QPO frequency constitutes the first identified observable that can be used to predict the occurrence of X-ray bursts. If a systematic frequency drift occurs, then a burst happens within a few kilo-seconds after $\nu$ drops below 9 mHz. This observational result confirms that the mHz QPO phenomenon is intimately related with the processes that lead to a thermonuclear burst., Comment: Accepted for publication in ApJ Letters - Uses emulateapj (misspelled author name corrected)

The accreting millisecond pulsar SAX J1808.4-3658 may be a transition object between accreting X-ray binaries and millisecond radio pulsars. We have constrained the thermal radiation from its surface through XMM-Newton X-ray observations, providing strong evidence for neutrino cooling processes from the neutron star core. We have also undertaken simultaneous X-ray and optical (Gemini) observations, shedding light on whether the strong heating of the companion star in quiescence may be due to X-ray irradiation, or to a radio pulsar turning on when accretion stops., Comment: To appear in the proceedings of "Forty Years of Pulsars: Millisecond Pulsars, Magnetars and More" held in Montreal, Canada, August 12-17, 2007. 4 pages

We present the results from simultaneous radio (Very Large Array) and X-ray (Rossi-X-ray Timing Explorer) observations of the Z-type neutron star X-ray binary GX~17+2. The aim is to assess the coupling between X-ray and radio properties throughout its three rapidly variable X-ray states and during the time-resolved transitions. These observations allow us, for the first time, to investigate quantitatively the possible relations between the radio emission and the presence of the hard X-ray tails and the X-ray state of the source. The observations show: 1) a coupling between the radio jet emission and the X-ray state of the source, i.e. the position in the X-ray hardness-intensity diagram (HID); 2) a coupling between the presence of a hard X-ray tail and the position in the HID, qualitatively similar to that found for the radio emission; 3) an indication for a quantitative positive correlation between the radio flux density and the X-ray flux in the hard-tail power law component; 4) evidence for the formation of a radio jet associated with the Flaring Branch-to-Normal Branch X-ray state transition; 5) that the radio flux density of the newly-formed jet stabilizes when also the normal-branch oscillation (NBO) in the X-ray power spectrum stabilizes its characteristic frequency, suggesting a possible relation between X-ray variability associated to the NBO and the jet formation. We discuss our results in the context of jet models., Comment: Accepted by ApJ

We report on intermittent X-ray pulsations with a frequency of 442.36 Hz from the neutron-star X-ray binary SAX J1748.9--2021 in the globular cluster NGC 6440. The pulsations were seen during both 2001 and 2005 outbursts of the source, but only intermittently, appearing and disappearing on timescales of hundreds of seconds. We find a suggestive relation between the occurrence of type-I X-ray bursts and the appearance of the pulsations but the relation is not strict. This behavior is very similar to that of the intermittent accreting millisecond X-ray pulsar HETE J1900.1--2455. The reason for the intermittence of the pulsations remains unclear. However it is now evident that a strict division between pulsating and non-pulsating does not exist. By studying the Doppler shift of the pulsation frequency we determine an orbit with a period of 8.7 hrs and an projected semi major axis of 0.39 lightsec. The companion star might be a main--sequence or a slightly evolved star with a mass of $\sim$1 $M_\odot$. Therefore, SAX J1748.9--2021 has a longer period and may have a more massive companion star than all the other accreting millisecond X-ray pulsars except for Aql X-1., Comment: 5 pages, 4 figures, Accepted by ApJL - Uses emulateapj.cls - New figure added: "Pulse frequency as a function of orbital phase". Figure 4 has changed. Text has been rewritten as to fit the 4 pages limit

We report the discovery of an episode of coherent millisecond X-ray pulsation in the neutron star low-mass X-ray binary Aql X-1. The episode lasts for slightly more than 150 seconds, during which the pulse frequency is consistent with being constant. No X-ray burst or other evidence of thermonuclear burning activity is seen in correspondence with the pulsation, which can thus be identified as occurring in the persistent emission. The pulsation frequency is 550.27 Hz, very close (0.5 Hz higher) to the maximum reported frequency from burst oscillations in this source. Hence we identify this frequency with the neutron star spin frequency. The pulsed fraction is strongly energy dependent, ranging from <1% (3-5 keV) to >10% (16-30 keV). We discuss possible physical interpretations and their consequences for our understanding of the lack of pulsation in most neutron star low-mass X-ray binaries. If interpreted as accretion-powered pulsation, Aql X-1 might play a key role in understanding the differences between pulsating and non-pulsating sources., Comment: 5 pages, 3 figures, accepted by ApJ Letters after minor revisions. Slightly extended discussion. One author added. Uses emulateapj.cls

We observed the neutron star X-ray transient 2S 1803-245 in quiescence with the X-ray satellite XMM-Newton, but did not detect it. An analysis of the X-ray bursts observed during the 1998 outburst of 2S 1803-245 gives an upper-limit to the distance of <7.3 kpc, leading to an upper-limit on the quiescent 0.5-10 keV X-ray luminosity of <2.8x10^32 erg/s (3sigma). Since the expected orbital period of 2S 1803-245 is several hrs, this limit is not much higher than those observed for the quiescent black hole transients with similar orbital periods., Comment: 5 pages, 3 figures, accepted for publication in MNRAS

We present broadband (3-500 keV) INTEGRAL X-ray spectra and X-ray/optical light curves of the luminous black hole X-ray transient and relativistic jet source GRO J1655-40. Our analysis covers four Target of Opportunity observations of the outburst that started in February 2005. We find that the high energy emission of GRO J1655-40 can be modelled well with an unbroken power-law (with photon indices of 1.72+-0.03,2.21+-0.04 for the first and the second observations, respectively). These correspond to hard and thermal dominant states, respectively. In contrast to many other black hole spectra, high energy complexity in the form of a break or cut-off is not required for the hard state, contrary to previous expectations for this state. We show for the first time that Comptonization by non-thermal electrons is the dominant process for the high energy emission in the hard state. We discuss our results in terms of models for broad-band emission and accretion flows in stellar-mass black holes., Comment: 14 figures. Accepted for publication in ApJ. Revised text for typos and deleted last line of third paragraph in page 14 (last try)

We use four Chandra gratings spectra of the neutron star low-mass X-ray binary 4U 1820-30 to better understand the nature of certain X-ray absorption lines in X-ray binaries, including the NeII, NeIII, NeIX, OVII, and OVIII lines. The equivalent widths of the lines are generally consistent between the observations, as expected if these lines originate in the hot interstellar medium. No evidence was found that the lines were blueshifted, again supporting the interstellar medium origin, though this may be due to poor statistics. There is apparent variability in the OVIII Ly-alpha line equivalent width providing some evidence that at least some of the OVIII absorption arises within the system. However, the significance is marginal (2.4-sigma), and the lack of variation in the other lines casts some doubt on the reality of the variability. From calculating the equivalent hydrogen column densities for a range of Doppler parameters, we find they are consistent with the interstellar origin of the lines. Additionally we fit the spectra with photoionization models for locally absorbing material, and find that they can reproduce the spectrum well, but only when there is an extremely low filling factor. We conclude that both the ISM and local absorption remain possible for the origin of the lines, but that more sensitive observations are needed to search for low-level variability., Comment: 10 pages, 4 figures, 4 tables, accepted to ApJ

We discovered kHz QPOs in 80 archived RXTE observations from the peculiar low-mass X-ray binary (LMXB) Circinus X-1. In 11 cases these appear in pairs in the frequency range of ~230 Hz to ~500 Hz for the upper kHz QPO and ~56 Hz to ~225 Hz for the lower kHz QPO. Their correlation with each other, which is similar to that of frequencies of kHz QPO pairs in other LMXBs containing a neutron star, and their variation by a factor two confirm that the central object is a neutron star. These are the lowest frequencies of kHz QPO pairs discovered so far and extend the above correlation over a frequency range of factor four. In this new frequency range the frequency difference of the two kHz QPOs increases monotonically by more than ~170 Hz with increasing kHz QPO frequency, challenging theoretical models., Comment: 3 pages, 2 figures - one from astro-ph/0608089 (ApJ 653, 1435, 2006) in corrected form, to appear in the proceedings of the 11th Marcel Grossman meeting, improved wording, added reference

The Galactic bulge region is a rich host of variable high-energy point sources. Since 2005, February 17 we are monitoring the source activity in this region about every three days with INTEGRAL. Thanks to the large field of view, the imaging capabilities and the sensitivity at hard X-rays, we are able to present for the first time a detailed homogeneous (hard) X-ray view of a sample of 76 sources in the Galactic bulge region. We describe the successful monitoring program and show the first results for a period of about one and a half year. We focus on the short (hour), medium (month) and long-term (year) variability in the 20-60 keV and 60-150 keV bands. When available, we discuss the simultaneous observations in the 3-10 keV and 10-25 keV bands. Per visibility season we detect 32/33 sources in the 20-60 keV band and 8/9 sources in the 60-150 keV band. On average, we find per visibility season one active bright (>~100 mCrab, 20-60 keV) black-hole candidate X-ray transient and three active weaker (<~25 mCrab, 20-60 keV) neutron star X-ray transients. Most of the time a clear anti-correlation can be seen between the soft and hard X-ray emission in some of the X-ray bursters. Hard X-ray flares or outbursts in X-ray bursters, which have a duration of the order of weeks, are accompanied by soft X-ray drops. On the other hand, hard X-ray drops can be accompanied by soft X-ray flares/outbursts. We found a number of new sources, IGR J17354-3255, IGR 17453-2853, IGR J17454-2703, IGR J17456-2901b, IGR J17536-2339, and IGR J17541-2252. We report here on some of the high-energy properties of these sources. The high-energy light curves of all the sources in the field of view, and the high-energy images of the region, are made available through the WWW at http://isdc.unige.ch/Science/BULGE/., Comment: 27 pages, 42 figures, accepted for publication in A&A. Abstract abridged. Tables 3,4,6,7 appear at the end. Images have been compressed and are reduced in quality; original PostScript images can be retrieved from http://isdc.unige.ch/~kuulkers/bulge/

Thermal X-ray radiation from neutron star soft X-ray transients in quiescence provides the strongest constraints on the cooling rates of neutron stars, and thus on the interior composition and properties of matter in the cores of neutron stars. We analyze new (2006) and archival (2001) XMM-Newton observations of the accreting millisecond pulsar SAX J1808.4-3658 in quiescence, which provide the most stringent constraints to date. The X-ray spectrum of SAX J1808.4-3658 in the 2006 observation is consistent with a power-law of photon index 1.83\pm0.17, without requiring the presence of a blackbody-like component from a neutron star atmosphere. Our 2006 observation shows a slightly lower 0.5-10 keV X-ray luminosity, at a level of 68^{+15}_{-13}% that inferred from the 2001 observation. Simultaneous fitting of all available XMM data allows a constraint on the quiescent neutron star (0.01-10 keV) luminosity of L_{NS}<1.1*10^{31} erg/s. This limit excludes some current models of neutrino emission mediated by pion condensates, and provides further evidence for additional cooling processes, such as neutrino emission via direct Urca processes involving nucleons and/or hyperons, in the cores of massive neutron stars., Comment: 5 pages, 2 figures; slight revisions, accepted by ApJ

We report the discovery with the RXTE/PCA of twin kHz QPOs in Cir X-1. Eleven cases of simultaneous double QPOs occurred, with significances of up to 6.3 and 5.5 sigma and centroid frequencies ranging between approximately 56-225 and 230-500 Hz for the two QPO peaks, respectively, i.e., for the most part at frequencies well below those of other sources. The QPO properties clearly indicate that these double peaks are the kHz QPOs known from low magnetic field neutron stars, and not black-hole high-frequency QPOs, confirming that Cir X-1 is a neutron star. The kHz QPO peak separation varies over a wide range, \~175-340 Hz, and increases with QPO frequency. This is contrary to what is seen in other sources but agrees with predictions of the relativistic precession model and Alfven wave models; beat-frequency models require modification to accommodate this. In other observations single kHz QPOs can be seen down to frequencies as low as ~12 Hz, as well as a strong low-frequency (LF) QPO between 1 and 30 Hz. The relations between the frequencies of the kHz QPOs and the LF QPO are in good agreement with those found previously in Z sources, confirming that Cir X-1 may be a peculiar Z source. We suggest that the low frequencies of the kHz QPOs in Cir X-1 and to a lesser extent in (other) Z sources might be due to a relatively stronger radial inflow to the neutron star than in other kHz QPO sources., Comment: 24 pages, 12 figures, minor changes, accepted for publication in the Astrophysical Journal, for a version with high resolution images, see http://www.tat.physik.uni-tuebingen.de/~stratos/cirX1.pdf

We report our analysis of a Chandra X-ray observation of the rich globular cluster Terzan 5, in which we detect 50 sources to a limiting 1.0-6 keV X-ray luminosity of 3*10^{31} ergs/s within the half-mass radius of the cluster. Thirty-three of these have L_X>10^{32} ergs/s, the largest number yet seen in any globular cluster. In addition to the quiescent low-mass X-ray binary (LMXB, identified by Wijnands et al.), another 12 relatively soft sources may be quiescent LMXBs. We compare the X-ray colors of the harder sources in Terzan 5 to the Galactic Center sources studied by Muno and collaborators, and find the Galactic Center sources to have harder X-ray colors, indicating a possible difference in the populations. We cannot clearly identify a metallicity dependence in the production of low-luminosity X-ray binaries in Galactic globular clusters, but a metallicity dependence of the form suggested by Jordan et al. for extragalactic LMXBs is consistent with our data., Comment: 15 pages, 10 figures (3 color). Resubmitted to ApJ after incorporating referee comments. v2: Added references to introduction

The Galactic Bulge region is a rich host of variable high-energy point sources. These sources include bright and relatively faint X-ray transients, X-ray bursters, persistent neutron star and black-hole candidate binaries, X-ray pulsars, etc.. We have a program to monitor the Galactic Bulge region regularly and frequently with the gamma-ray observatory INTEGRAL. As a service to the scientific community the high-energy light curves of all the active sources as well as images of the region are made available through the WWW. We show the first results of this exciting new program., Comment: 5 pages, 2 figures, to be published in: "The Transient Milky Way: a perspective for MIRAX", eds. F. D'Amico, J. Braga & R. Rothschild, AIP Conf. Proc

We present the first results of coordinated multi-wavelength observations of the Galactic black hole GX 339-4 in a canonical low-hard state, obtained during its 2004 outburst. XMM-Newton observed the source for 2 revolutions, or approximately 280 ksec; RXTE monitored the source throughout this long stare. The resulting data offer the best view yet obtained of the inner accretion flow geometry in the low-hard state, which is thought to be analogous to the geometry in low-luminosity active galactic nuclei. The XMM-Newton spectra clearly reveal the presence of a cool accretion disk component, and a relativistic Fe K emission line. The results of fits made to both components strongly suggest that a standard thin disk remains at or near to the innermost stable circular orbit, at least in bright phases of the low-hard state. These findings indicate that potential links between the inner disk radius and the onset of a steady compact jet, and the paradigm of a radially-recessed disk in the low-hard state, do not hold universally. The results of our observations can best be explained if a standard thin accretion disk fuels a corona which is closely related to, or consistent with, the base of a compact jet. In a brief examination of archival data, we show that Cygnus X-1 supports this picture of the low/hard state. We discuss our results within the context of disk-jet connections and prevailing models for accretion onto black holes., Comment: 13 pages, 10 figures (6 in color), ApJ, in press

We report on new X-ray observations of the large-scale jets recently discovered in X-rays from the black hole candidate 4U 1755-33. Our observations in 2004 show that the jets found in 2001 are still present in X-rays. However, sensitive radio observations in 2004 failed to detect the jets. We suggest that synchrotron radiation is a viable emission mechanism for the jets and that thermal bremsstrahlung and inverse-Compton emission are unlikely on energetic grounds. In the synchrotron interpretation, the production of X-rays requires acceleration of electrons up to ~ 60 TeV, the jet power is ~ 4x10^35 erg s^-1, and the radio non-detection requires a spectral index alpha > -0.65 (S_\nu \propto \nu^\alpha) which is similar to the indexes found in lobes surrounding some other compact objects. We find an upper limit on the flux of 4U 1755-33 in quiescence of 5x10^-16 erg cm^-2 s^-1 in the 0.3-8 keV band., Comment: accepted by ApJ, 11 pages, 2 color figures

We present a 19 ks Chandra ACIS-S observation of the globular cluster Terzan 1. Fourteen sources are detected within 1.4 arcmin of the cluster center with 2 of these sources predicted to be not associated with the cluster (background AGN or foreground objects). The neutron star X-ray transient, X1732-304, has previously been observed in outburst within this globular cluster with the outburst seen to last for at least 12 years. Here we find 4 sources that are consistent with the ROSAT position for this transient, but none of the sources are fully consistent with the position of a radio source detected with the VLA that is likely associated with the transient. The most likely candidate for the quiescent counterpart of the transient has a relatively soft spectrum and an unabsorbed 0.5-10 keV luminosity of 2.6E32 ergs/s, quite typical of other quiescent neutron stars. Assuming standard core cooling, from the quiescent flux of this source we predict long (>400 yr) quiescent episodes to allow the neutron star to cool. Alternatively, enhanced core cooling processes are needed to cool down the core. However, if we do not detect the quiescent counterpart of the transient this gives an unabsorbed 0.5-10 keV luminosity upper limit of 8E31 ergs/s. We also discuss other X-ray sources within the globular cluster. From the estimated stellar encouter rate of this cluster we find that the number of sources we detect is significantly higher than expected by the relationship of Pooley et al. (2003)., Comment: 10 pages, 4 figures, 2 tables. Accepted for publication in MNRAS

We present the first results of our X-ray monitoring campaign on a 1.7 square degree region centered on Sgr A* using the satellites XMM-Newton and Chandra. The purpose of this campaign is to monitor the behavior (below 10 keV) of X-ray sources (both persistent and transient) which are too faint to be detected by monitoring instruments aboard other satellites currently in orbit (e.g., Rossi X-ray Timing Explorer; INTEGRAL). Our first monitoring observations (using the HRC-I aboard Chandra) were obtained on June 5, 2005. Most of the sources detected could be identified with foreground sources, such as X-ray active stars. In addition we detected two persistent X-ray binaries (1E 1743.1-2843; 1A 1742-294), two faint X-ray transients (GRS 1741.9-2853; XMM J174457-2850.3), as well as a possible new transient source at a luminosity of a few times 1E34 erg/s. We report on the X-ray results on these systems and on the non-detection of the transients in follow-up radio data using the Very Large Array. We discuss how our monitoring campaign can help to improve our understanding of the different types of X-ray transients (i.e., the very faint ones)., Comment: Accepted for publication in A&A, 10 December 2005

We observed the bright phase of the 2003 outburst of the Galactic black hole candidate H 1743-322 in X-rays simultaneously with Chandra and RXTE on four occasions. The Chandra/HETGS spectra reveal narrow, variable (He-like) Fe XXV and (H-like) Fe XXVI resonance absorption lines. In the first observation, the Fe XXVI line has a FWHM of 1800 +/- 400 km/s and a blue-shift of 700 +/- 200 km/s, suggesting that the highly ionized medium is an outflow. Moreover, the Fe XXV line is observed to vary significantly on a timescale of a few hundred seconds in the first observation, which corresponds to the Keplerian orbital period at approximately 1 E+4 gravitational radii. Our models for the absorption geometry suggest that a combination of geometric effects and changing ionizing flux are required to account for the large changes in line flux observed between observations, and that the absorption likely occurs at a radius less than 1 E+4 radii for a 10 Msun black hole. Viable models for the absorption geometry include cyclic absorption due to an accretion disk structure, absorption in a clumpy outflowing disk wind, or possibly a combination of these two. If the wind in H 1743-322 has unity filling factor, the highest implied mass outflow rate is 20 percent of the Eddington mass accretion rate. This wind may be a hot precursor to the Seyfert-like, outflowing "warm absorber" geometries recently found in the Galactic black holes GX 339-4 and XTE J1650-500. We discuss these findings in the context of ionized Fe absorption lines found in the spectra of other Galactic sources, and connections to warm absorbers, winds, and jets in other accreting systems., Comment: 18 pages, 7 figures, 5 in color, subm. to ApJ. Uses emulateapj.sty and apjfonts.sty

We have analyzed spectra of the Galactic black hole GX 339-4 obtained through simultaneous 76 ksec XMM-Newton/EPIC-pn and 10 ksec RXTE observations during a bright phase of its 2002-2003 outburst. An extremely skewed, relativistic Fe K-alpha emission line and ionized disk reflection spectrum are revealed in these spectra. Self-consistent models for the Fe K-alpha emission line profile and disk reflection spectrum rule-out an inner disk radius compatible with a Schwarzschild black hole at more than the 8 sigma level of confidence. The best-fit inner disk radius of 2-3 r_g suggests that GX 339-4 harbors a black hole with a ~ 0.8-0.9 (where r_g = GM/c^2 and a=cJ/GM^2, and assuming that reflection in the plunging region is relatively small). This confirms indications for black hole spin based on a Chandra spectrum obtained later in the outburst. The emission line and reflection spectrum also rule-out a standard power-law disk emissivity in GX 339-4; a broken power-law form with enhanced emissivity inside ~6 r_{g} gives improved fits at more than the 8 sigma level of confidence. The extreme red wing of the line and steep emissivity require a centrally--concentrated source of hard X-rays which can strongly illuminate the inner disk. Hard X-ray emission from the base of a jet - enhanced by gravitational light bending effects - could create the concentrated hard X-ray emission; this process may be related to magnetic connections between the black hole and the inner disk. We discuss these results within the context of recent results from analyses of XTE J1650-500 and MCG-6-30-15, and models for the inner accretion flow environment around black holes., Comment: 5 pages, 2 color figures, uses emulateapj.sty and apjfonts.sty, ApJ Letters, accepted

We present the analysis of a 35 ksec long Chandra observation of the neutron star soft X-ray transient (SXT) SAX J1810.8-2609. We detect three sources in the field of view. The position of one of them is consistent with the location of the ROSAT error circle of SAX J1810.8-2609. The accurate Chandra position of that source coincides with the position of the proposed optical counterpart, strengthening the identification as the counterpart. We detected the neutron star SXT system in quiescence at an unabsorbed luminosity of ~1x10^32 erg s^-1 (assuming a distance of 4.9 kpc). This luminosity is at the low-end of quiescent luminosities found in other neutron star SXTs. This renders support to the existence of a group of faint soft X-ray transients of which the accreting millisecond X-ray pulsar SAX J1808.4-3658 is the most prominent member. The quiescent spectrum of SAX J1810.8-2609 is well-fit with an absorbed power law with photon index of 3.3+-0.5. With a value of 3.3x10^21 cm^-2 the Galactic absorption is consistent with the value derived in outburst. Since the spectra of quiescent neutron star SXTs are often fit with an absorbed blackbody or neutron star atmosphere plus power-law model we also fitted the spectrum using those fit functions. Both models provide a good fit to the data. If cooling of the neutron star core and/or crust is responsible for the soft part of the spectrum the time averaged mass accretion rate must have been very low (~5.7x10^-13 Msun yr^-1; assuming standard core cooling only) or the neutron star must be massive. We also discuss the possibility that the thermal spectral component in neutron stars in quiescence is produced by residual accretion., Comment: 5 pages, 1 figure, accepted for publication by MNRAS

We studied the aperiodic X-ray timing behaviour of the accreting millisecond pulsar SAX J1808.4-3658. The source was recently found to be the first accreting millisecond pulsar that shows the kilohertz quasi-periodic oscillations (kilohertz QPOs) that are found in many other X-ray binaries with accreting neutron stars. The high frequency of these signals reflects the short dynamical time scales in the region near the compact object where they originate. We find that in addition to the kilohertz QPOs SAX J1808.4-3658 shows several low frequency timing features, based on which the source can be classified as a so-called atoll source. The frequencies of the variability components of the atoll sources follow a universal scheme of correlations. The correlations in SAX J1808.4-3658 are similar but show a shift in upper kilohertz QPO frequency. This discrepancy is perhaps related to a stronger or differently configured magnetic field., Comment: 4 pages, 3 figures. To appear in the proceedings of the "The Restless High-Energy Universe" (Amsterdam, The Netherlands), 2003, eds. E.P.J. van den Heuvel, J.J.M. in 't Zand, and R.A.M.J. Wijers

We observed the Galactic black hole GX 339-4 with the Chandra High Energy Transmission Grating Spectrometer (HETGS) for 75 ksec during the decline of its 2002-2003 outburst. The sensitivity of this observation provides an unprecedented glimpse of a Galactic black hole at about a tenth of the luminosity of the outburst peak. The continuum spectrum is well described by a model consisting of multicolor disk blackbody (kT = 0.6 keV) and power-law (Gamma = 2.5) components. X-ray reflection models yield improved fits. A strong, relativistic Fe K-alpha emission line is revealed, indicating that the inner disk extends to the innermost stable circular orbit. The breadth of the line is sufficient to suggest that GX 339-4 may harbor a black hole with significant angular momentum. Absorption lines from H-like and He-like O, and He-like Ne and Mg are detected, as well as lines which are likely due to Ne II and Ne III. The measured line properties make it difficult to associate the absorption with the coronal phase of the interstellar medium. A scenario wherein the absorption lines are due to an intrinsic AGN-like warm-absorber geometry -- perhaps produced by a disk wind in an extended disk-dominated state -- may be more viable. We compare our results to Chandra observations of the Galactic black hole candidate XTE J1650-500, and discuss our findings in terms of prominent models for Galactic black hole accretion flows and connections to supermassive black holes., Comment: 20 pages, 11 postscript figure files (many in color), uses emulateapj.sty and apjfonts.sty, slightly expanded, accepted for publication in ApJ

We report on the detection of X-ray emission from the unusual Galactic radio source G357.7-0.1 (the "Tornado"). Observations made with the Chandra X-ray Observatory demonstrate the presence of up to three sources of X-ray emission from the Tornado: a relatively bright region of dimensions 2'x1' coincident with and interior to the brightest radio emission at the "head" of the Tornado, plus two fainter extended regions possibly associated with the Tornado's "tail". No X-ray point sources associated with the Tornado are seen down to a 3-sigma luminosity (0.5-10 keV) of 1e33 ergs/s, for a distance to the system of 12 kpc. The spectrum of the brightest region of X-rays is consistent with a heavily absorbed (N_H ~ 1e23 cm^-2) thermal plasma of temperature kT ~ 0.6 keV; an absorbed power law can also fit the data, but implies an extremely steep photon index. From these data we tentatively conclude that the Tornado is a supernova remnant (SNR), although we are unable to rule out the possibility that the Tornado is powered either by outflows from an X-ray binary or by the relativistic wind of an unseen pulsar. Within the SNR interpretation, the head of the Tornado is a limb-brightened radio shell containing centrally-filled thermal X-rays and which is interacting with a molecular cloud. We therefore propose that the Tornado is a "mixed morphology" supernova remnant. The unusual tail component of the Tornado remains unexplained in this interpretation, but might result from expansion of the SNR into an elongated progenitor wind bubble., Comment: 4 pages of text, plus one embedded EPS figure. Minor changes following referee's report. ApJ Letters, in press

We have discovered extended X-ray emission from the microquasar Cyg X-3 in archival Chandra X-ray Observatory observations. A 5" wide structure lies approximately 16" to the NE from the core point source and may be extended in that direction. This angular scale corresponds to a physical extent of roughly 0.8 lyr, at a distance of 2.5 lyr from Cyg X-3 (assuming a 10 kpc distance). The flux varied by a factor of 2.5 during the four months separating two of the observations, indicating significant substructure. The peak 2-10 keV luminosity was about 5e34 ergs/s. There may also be weaker, extended emission of similar scale oppositely directed from the core, suggesting a bipolar outflow. This structure is not part of the dust scattering halo, nor is it caused by the Chandra point spread function. In this Letter we describe the observations and discuss possible origins of the extension., Comment: Submitted to ApJ Letters. 5 pages, 2 figures (1 color). Uses emulateapj

We present our analysis of Chandra observations obtained when the soft X-ray transient RX J170930.2-263927 (XTE J1709-267) returned to quiescence after an outburst. Using the type I burst peak luminosity found by Cocchi et al. (1998) and the value of N_H we derived from our spectral fits, the distance to RX J170930.2-263927 can be constrained to 2.5-10 kpc. RX J170930.2-263927 is probably associated with the low-metalicity Globular Cluster NGC 6293, which has a tidal radius of 14.2 arcminutes, since the projected distance to the centre of the cluster is approximately 25 parsec (9-10 arcminutes). If the association is correct, RX J170930.2-263927 would be at ~8.5 kpc. We determined that L_outburst / L_quiescence is approximately 10^5 for this outburst. If the quiescent luminosity is caused by cooling of the neutron star core then enhanced core cooling processes were at work implying a neutron star mass of \~1.7-1.8 Msun. Combining our Chandra observations with archival ROSAT observations we show that the source most likely exhibits periods of sustained low-level accretion. Variable, low-level activity could provide an alternative explanation for some of the sources in the recently proposed category of faint soft X-ray transients. We found excess emission at ~0.6 keV. If such an excess is a unique feature of ultracompact systems, as was recently proposed, RX J170930.2-263927 must have a short orbital period as well. From the constraints on the distance and the non-detection of the optical counterpart with m_V<20.5, we conclude that this system must have a low-mass companion., Comment: 11 pages, 5 figures, accepted for publication by MNRAS

We report on X-ray observations of the the large-scale jets recently discovered in the radio and detected in X-rays from the black hole candidate X-ray transient and microquasar XTE J1550-564. On 11 March 2002, X-ray emission was detected 23 arcsec to the West of the black hole candidate and was extended along the jet axis with a full width at half maximum of 1.2 arcsec and a full width at 10% of maximum intensity of 5 arcsec. The morphology of the X-ray emission matched well to that of the radio emission at the same epoch. The jet moved by 0.52 +/- 0.13 arcsec between 11 March and 19 June 2002. The apparent speed during that interval was 5.2 +/- 1.3 mas/day. This is significantly less than the average apparent speed of 18.1 +/- 0.4 mas/day from 1998 to 2002, assuming that the jet was ejected in September 1998, and indicates that the jet has decelerated. The X-ray spectrum is adequately described by a powerlaw with a photon index near 1.8 subject to interstellar absorption. The unabsorbed X-ray flux was 3.4 x 10^-13 erg cm^-2 s^-1 in the 0.3-8 keV band in March 2002, and decreased to 2.9 x 10^-13 erg cm^-2 s^-1 in June. We also detect X-rays from the eastern jet in March 2002 and show that it has decelerated and dimmed since the previous detections in 2000., Comment: accepted for publication in ApJ, 11 pages, several figures in color

We have discovered at x-ray and radio wavelengths large-scale moving jets from the microquasar XTE J1550-654. Plasma ejected from near the black hole traveled at relativistic velocities for at least four years. We present direct evidence for gradual deceleration in a relativistic jet. The broadband spectrum of the jets is consistent with synchrotron emission from high energy (up to 10 TeV) particles accelerated in shock waves formed within the relativistic ejecta or by the interaction of the jets with the interstellar medium. XTE J1550-654 offers a unique opportunity to study the dynamical evolution of relativistic jets on time scales inaccessible for active galactic nuclei jets, with implications for our understanding of relativistic jets from Galactic x-ray binaries and active galactic nuclei., Comment: Published in Science, 2002, volume 298, p. 196-199. PDF file at at http://www.sciencemag.org/cgi/reprint/298/5591/196

We have analyzed a Chandra HETGS spectrum of the Galactic black hole Cygnus X-1, obtained at a source flux which is approximately twice that commonly observed in its persistent low-intensity, spectrally-hard state. We find a myriad of absorption lines in the spectrum, including Ly-alpha lines and helium-like resonance lines from Ne, Na, Mg, and Si. We calculate a flux-weighted mean red-shift of ~100 km/s and a flux-weighted mean velocity width of 800 km/s (FWHM) for lines from these elements. We also detect a number of transitions from Fe XVIII-XXIV and Ni XIX-XX in absorption; however, the identification of these lines is less certain and a greater range of shifts and breadth is measured. Our observation occurred at a binary phase of phi = 0.76; the lines observed are consistent with absorption in an ionized region of the supergiant O9.7 Iab companion wind. The spectrum is extremely complicated in that a range of temperatures and densities are implied. Prior Chandra HETGS spectra of Cygnus X-1 were obtained in a similar transition state (at phi = 0.93) and in the low/hard state (at phi= 0.84). Considered together, these spectra provide evidence for a companion wind that is focused as it flows onto the black hole primary in this system., Comment: 10 pages, 7 color figures, uses emulateapj.sty and apjfonts.sty. Submitted to ApJ

We present an analysis of the first high-resolution spectra measured from an accretion-driven millisecond X-ray pulsar in outburst. We observed XTE J1751-305 with XMM-Newton on 2002 April 7 for approximately 35 ksec. Using a simple absorbed blackbody plus power-law model, we measure an unabsorbed flux of (6.6 +/- 0.1) * 10^(-10) erg/cm^2/s (0.5--10.0 keV). A hard power-law component (Gamma = 1.44 +/- 0.01) contributes 83% of the unabsorbed flux in the 0.5-10.0 keV band, but a blackbody component (kT = 1.05 +/- 0.01 keV) is required. We find no clear evidence for narrow or broad emission or absorption lines in the time-averaged spectra, and the sensitivity of this observation has allowed us to set constraining upper-limits on the strength of important features. The lack of line features is at odds with spectra measured from some other X-ray binaries which share some similarities with XTE J1751-305. We discuss the implications of these findings on the accretion flow geometry in XTE J1751-305., Comment: 5 pages, 3 figures (2 color). ApJ Letters, accepted. Uses emulateapj.sty

Broad Fe K-alpha emission lines have recently been reported in a number of Galactic black holes. Such lines are useful accretion flow diagnostics because they may be produced at the inner accretion disk and shaped by relativistic effects, but in general they have only been observed at luminosities of L_X ~ 10^(37-38) erg/s in soft X-rays. The Galactic microquasar V4641 Sgr -- widely known for its 12.2 Crab (1.5-12 keV) outburst in 1999 September -- displayed low-level activity in 1999 March. BeppoSAX observed the source in this state and Fe K-alpha line emission was found (in 't Zand et al. 2000). In re-analyzing these data, we find strong evidence that the Fe K-alpha line profile is broadened. For the most likely values of the source distance and black hole mass measured by Orosz et al. (2001), our fits to the total spectrum indicate that the source was observed at a luminosity of L_X = 1.9 (+1.0, -0.8) * 10^(36) erg/s (2-10 keV), or L_X/L_Edd. = 1.8 (+0.9, -0.8) * 10^(-3). Advection-dominated accretion flow (ADAF) models predict a radially-recessed disk in this regime. In contrast, fits to the observed Fe K-alpha emission line profile with a relativistic line model constrain the inner disk to be consistent with the marginally stable circular orbit of a Schwarzschild black hole., Comment: 5 pages, 3 color figures. ApJ Letters, in press

We investigated the temporal behavior of the new black hole transient XTE J1650-500 with the Rossi X-ray Timing Explorer (RXTE) as the source made a transition to the low/hard state during the decay of the 2001 outburst. We find QPOs in the 4 - 9 Hz range, enhanced time lags and reduced coherence during the state transition. We also observe a shift in the peak frequency of the noise component with energy during the transition. The evolution of the power spectrum as well as the lag and coherence behavior during the state transition are similar to the state transitions observed for other black hole sources, especially Cyg X-1. The temporal properties during the transition to the low state put constraints on the accretion geometry of XTE J1650-500 and may have implications for all black hole binary systems. We suggest a possible geometry and evolution of a jet+corona+disk system based on enhanced lags and peak frequency shift during the transition., Comment: 22 pages, 7 figures, submitted to ApJ

We report on a detailed analysis of data obtained over nearly four years with the Rossi X-ray Timing Explorer of the Z source GX 5-1. From a spectral analysis using a hardness-intensity diagram it was found that the source traced out the typical Z-shaped pattern. The study of the power spectral properties showed that when the source moved on the Horizontal Branch towards the Normal Branch the fractional rms amplitudes and timescales of all variability decreased, while their FWHMs increased. The frequency separation of the two kHz QPO peaks decreased from 344+-12 Hz to 232+-13 Hz. At low frequencies, besides the horizontal branch oscillation (HBO) and its second harmonic, two additional broad Lorentzian components were needed to obtain acceptable fits. These broad Lorentzians have Q-values of ~1-2 and have frequencies 0.5 and 1.5 times the HBO frequency. When interpreted as being related to the HBO, they seem to favor disc models for the HBO over the magnetic beat-frequency model. We compare the timing properties and colour-colour diagrams (CDs) of GX 5-1 with those of the back hole candidate XTE J1550-564 and the atoll source 4U 1608-52. The CDs are strikingly similar when a colour scheme is used that is commonly employed in back hole studies. However, this may be a degeneracy as the CDs turn out to be more complicated when colours common in neutron star studies are employed. Apart from some remarkable similarities between the CD of XTE J1550-564 and that of 4U 1608-52, several differences can be seen between these CDs and that of GX 5-1. Conclusions on spectral states or properties based solely on the use of CDs using the ``black hole scheme'' should be regarded with caution., Comment: 19 pages, 15 figures, Accepted for publication in MNRAS, 3 figures reduced in size, abstract abbreviated

We observed the Galactic black hole candidate XTE J1650-500 early in its Fall, 2001 outburst with the XMM-Newton European Photon Imaging pn Camera (EPIC-pn). The observed spectrum is consistent with the source having been in the "very high" state. We find a broad, skewed Fe K-alpha emission line which suggests that the primary in this system may be a Kerr black hole, and which indicates a steep emissivity profile that is hard to explain in terms of a standard accretion disk model. These results are quantitatively and qualitatively similar to those from an XMM-Newton observation of the Seyfert galaxy MCG--6-30-15. The steep emissivity in MCG--6-30-15 may be explained by the extraction and dissipation of rotational energy from a black hole with nearly- maximal angular momentum or material in the plunging region via magnetic connections to the inner accretion disk. If this process is at work in both sources, an exotic but fundamental general relativistic prediction may be confirmed across a factor of 10^6 in black hole mass. We discuss these results in terms of the accretion flow geometry in stellar-mass black holes, and the variety of enigmatic phenomena often observed in the very high state., Comment: Published in ApJL (Miller et al., 2002, ApJ, 570L, 69)

We observed the Galactic black hole Cygnus X-1 with the Chandra High Energy Transmission Grating Spectrometer for 30 kiloseconds on 4 January, 2001. The source was in an intermediate state, with a flux that was approximately twice that commonly observed in its persistent low/hard state. Our best-fit model for the X-ray spectrum includes narrow Gaussian emission line (E = 6.415 +/- 0.007 keV, FWHM = 80 (+28, -19) eV, W = 16 (+3, -2) eV) and broad line (E = 5.82 (+0.06, -0.07) keV, FWHM = 1.9 (+0.5, -0.3) keV, W = 140 (+70, -40) eV) components, and a smeared edge at 7.3 +/- 0.2 keV (tau ~ 1.0). The broad line profile is not as strongly skewed as those observed in some Seyfert galaxies. We interpret these features in terms of an accretion disk with irradiation of the inner disk producing a broad Fe K-alpha emission line and edge, and irradiation of the outer disk producing a narrow Fe K-alpha emission line. The broad line is likely shaped predominantly by Doppler shifts and gravitational effects, and to a lesser degree by Compton scattering due to reflection. We discuss the underlying continuum X-ray spectrum and these line features in the context of diagnosing the accretion flow geometry in Cygnus X-1 and other Galactic black holes., Comment: Accepted for publication in ApJ

After 3 years of quiescence, the globular cluster NGC 6440 exhibited a bright transient X-ray source turning on in August 2001, as noted with the RXTE All-Sky Monitor. We carried out a short target of opportunity observation with the Chandra X-ray Observatory and are able to associate the transient with the brightest of 24 X-ray sources detected during quiescence in July 2000 with Chandra. Furthermore, we securely identify the optical counterpart and determine that the 1998 X-ray outburst in NGC 6440 was from the same object. This is the first time that an optical counterpart to a transient in a globular cluster is securely identified. Since the transient is a type I X-ray burster, it is established that the compact accretor is a neutron star. Thus, this transient provides an ideal case to study the quiescent emission in the optical and X-ray of a transiently accreting neutron star while knowing the distance and reddening accurately. One model that fits the quiescent spectrum is an absorbed power law plus neutron star hydrogen atmosphere model. We find an intrinsic neutron star radius of 17_{-12}^{+31} km and an unabsorbed bolometric luminosity for the neutron star atmosphere of (2.1+/-0.8)E33 erg/s which is consistent with predictions for a cooling neutron star., Comment: Accepted for publication in ApJ Letters

We present a detailed study of the correlations between the burst properties and the inferred mass accretion rate for the X-ray transient MXB 1659-298. The bursts which exhibited oscillations were observed when the source was at relatively high mass accretion rate, similar to what has been seen for other sources. However, due to the limited number of observations at lower mass accretion rates, no bursts were observed at such accretion rates and it is still possible that when MXB 1659-298 accretes at such low mass accretion rates, bursts can occur which might still exhibit burst oscillations. No clear correlations were found between the different burst properties and accretion rate, in contrast to what has been found for KS 1731-260 and 4U 1728-34, but similar to what has been reported for Aql X-1. However, this lack of correlation for MXB 1659-298 and Aql X-1 might be due to the limited range of mass accretion rate observed for those sources compared to KS 1731-260 and 4U 1728-34., Comment: Accepted for publication in ApJ, 18 October 2001

We report on an XMM-Newton Reflection Grating Spectrometer observation of the black hole candidate and Galactic microquasar GRS 1758-258. The source entered a peculiar "off/soft" state in late February, 2001, in which the spectrum softened while the X-ray flux -- and the inferred mass accretion rate -- steadily decreased. We find no clear evidence for emission or absorption lines in the dispersed spectra, indicating that most of the observed soft flux is likely from an accretion disk and not from a cool plasma. The accretion disk strongly dominates the spectrum in this lower-luminosity state, and is only mildly recessed from the marginally stable orbit. These findings may be difficult to explain in terms of advection-dominated accretion flow, or "ADAF" models. We discuss these results within the context of ADAF models, simultaneous two-flow models, and observed correlations between hard X-ray flux and jet production., Comment: 14 pages, 4 figures (3 in color), ApJ accepted Oct. 1 2001

We present an analysis of the high frequency timing properties of the April-May 2000 outburst of the black hole candidate and Galactic microquasar XTE J1550-564, measured with the Rossi X-ray Timing Explorer. The rapid X-ray variability we measure is consistent with the source being in either the ``very high'' or ``intermediate'' black hole state. Strong (5-8% rms) quasi-periodic oscillations (QPOs) are found between 249-276 Hz; this represents the first detection of the same high frequency QPO in subsequent outbursts of a transient black hole candidate. We also present evidence for lower-frequency QPOs at approximately 188 Hz (3.5 sigma, single trial), also reported previously and likely present simultaneously with the higher-frequency QPOs. We discuss these findings within the context of the 1998 outburst of XTE J1550-564, and comment on implications for models of QPOs, accretion flows, and black hole spin., Comment: 14 pages, 4 figures; minor changes from initial version; accepted for publication in the Astrophysical Journal

On two occasions, we obtained nearly simultaneous ~ 4 kilosecond snapshot observations of the Galactic black hole and microquasar XTE J1550-564 with Chandra and RXTE near the peak of its May, 2000 outburst. The low-energy sensitivity of Chandra and the resolution of the High Energy Transmission Grating Spectrometer (HETGS), coupled with the broad energy range and large collecting area of RXTE, have allowed us to place constraints on the outburst accretion flow geometry of this source in the ``intermediate'' X-ray state. The 0.65-25.0 keV continuum spectra are well-described by relatively hot (kT ~ 0.8 keV) accretion disk and hard (Gamma ~ 2.3) coronal power-law components. Broad, relatively strong Fe K-alpha emission line (EW ~170 eV) and smeared absorption edge components consistent with Fe XXV are strongly required in joint spectral fits. The resolution of the Chandra/HETGS reveals that the broad Fe K-alpha emission lines seen clearly in the individual RXTE spectra are not due to an intrinsically narrow line., Comment: 11 pages, 4 figures (2 in color). MNRAS, accepted. Significant updates from the initial version

We report evidence for an Fe K-alpha fluorescence line feature and disk reflection in the Very High, High, and Low State X-ray spectra of the galactic microquasar XTE J1748-288 during its June~1998 outburst. Spectral analyses are made on data gathered throughout the outburst by the Rossi X-ray Timing Explorer Proportional Counter Array. Gaussian line, relativistic disk emission line, and ionized disk reflection models are fit to the data. In the Very High State the line profile appears strongly redshifted, consistent with disk emission from the innermost stable orbits around a maximally rotating Kerr black hole. In the High State the line profile is less redshifted and increasingly prominent. The Low State line profile is very strong (~0.5 keV equivalent width) and centered at 6.7 +/- 0.10 keV; disk line emission model fits indicate that the inner edge of the disk fluctuates between ~20 and ~100 gravitational radii in this state. The disk reflection fraction is traced through the outburst; reflection from an ionized disk is preferred in the VHS and HS, and reflection from a relatively neutral disk is preferred in the LS. We discuss the implications of our findings for the binary system dynamics and accretion flow geometry in XTE J1748-288., Comment: 32 pages, 8 figures, 5 tables. ApJ accepted 8/16/00

Pairs of quasi-periodic oscillations (QPOs) at kilohertz frequencies are a common phenomenon in several neutron-star low-mass X-ray binaries. The frequency separation of the QPO peaks in the pair appears to be constant in many sources and directly related to the neutron star spin frequency. However, in Sco X-1 and possibly in 4U 1608-52, the frequency separation of the QPOs decreases with increasing inferred mass accretion rate. We show that the currently available Rossi X-ray Timing Explorer data are consistent with the hypothesis that the frequency separations in all sources vary by amounts similar to the variation in Sco X-1. We discuss the implications for models of the kilohertz QPOs., Comment: 8 pages, 3 b&w figures and 1 color figure; to appear in the Astrophysical Journal Letters

We analysed all X-ray data on the low-mass X-ray binary Cygnus X-2 obtained with Ginga. The analysis of the spectral and fast timing behaviour of these 4 years of data provides new insights in the behaviour of this Z source. We confirm the previously observed recurrent patterns of behaviour in the X-ray colour-colour and hardness-intensity diagrams (HIDs) consisting of shifts and shape changes in the Z track. The source behaviour is correlated with overall intensity. When the overall intensity increases, the mean velocity of the motion along the normal branch (NB) of the Z track increase, as well as the width of the NB in the HID. Contrary to previous results we find that, during different observations, when the source is at the same position in the NB of the Z track the rapid X-ray variability differs significantly. During the Kuulkers et al. (1996) ``medium'' level, a normal branch quasi-periodic oscillation (QPO) is detected, which is not seen during the ``high'' overall intensity level. During the high overall intensity level episodes the very-low frequency noise on the lower NB is very strong, whereas during the medium overall intensity level episodes this noise component at the same position in the Z track is much weaker. The explanation of the different overall intensity levels with a precessing accretion disk is difficult to reconcile with our data. Furthermore, the frequency of the horizontal branch QPO decreases when Cyg X-2 enters the upper NB, giving a model dependent upper limit on the magnetic field strength at the magnetic equator of ~8.5x10^9 G. We also report 5 bursts, with durations of ~5 s, whose occurrence are uncorrelated with location in the Z track, overall intensity level or orbital phase. The burst properties indicate that they are not regular type I bursts., Comment: 15 pages including 11 figures. Tables 4 and 5 only available in postscript format. Accepted for publications in A&A Main Journal