Search

Aims. Neutron stars (NSs) produced in the Milky Way are supposedly ten to the eighth - ten to the ninth, of which only $\sim 2 \times 10^{3}$ are observed. Constraining the phase space distribution of NSs may help to characterize the yet undetected population of stellar remnants. Methods. We perform Monte Carlo simulations of NS orbits, under different assumptions concerning the Galactic potential and the distribution of progenitors and birth velocities. We study the resulting phase space distributions, focusing on the statistical properties of the NS populations in the disk and in the solar neighbourhood. Results. It is shown that $\sim 80$ percent of NSs are in bound orbits. The fraction of NSs located in a disk of radius 20 kpc and width 0.4 kpc is $\lesssim 20$ percent. Therefore the majority of NSs populate the halo. Fits for the surface density of the disk, the distribution of heights on the Galactic plane and the velocity distribution of the disk, are given. We also provide sky maps of the projected number density in heliocentric Galactic coordinates (l, b). Our results are compared with previous ones reported in the literature. Conclusions. Obvious applications of our modelling are in the revisiting of accretion luminosities of old isolated NSs, the issue of the observability of the nearest NS and the NS optical depth for microlensing events. These will be the scope of further studies., Comment: 11 pages, 15 figures, accepted for publication in Astronomy and Astrophysics

We report here on the outburst onset and evolution of the new Soft Gamma Repeater SGR 0501+4516. We monitored the new SGR with XMM-Newton starting on 2008 August 23, one day after the source became burst-active, and continuing with 4 more observations, with the last one on 2008 September 30. Combining the data with the Swift-XRT and Suzaku data, we modelled the outburst decay over 160 days, and we found that the source flux decreased exponentially with a timescale of t_c=23.8 days. In the first XMM-Newton observation a large number of short X-ray bursts were observed, the rate of which decayed drastically in the following observations. We found large changes in the spectral and timing behavior of the source during the outburst, with softening emission as the flux decayed, and the non-thermal soft X-ray spectral component fading faster than the thermal one. Almost simultaneously to our XMM-Newton observations (on 2008 August 29 and September 2), we observed the source in the hard X-ray range with INTEGRAL, which clearly detected the source up to ~100keV in the first pointing, while giving only upper limits during the second pointing, discovering a variable hard X-ray component fading in less than 10 days after the bursting activation. We performed a phase-coherent X-ray timing analysis over about 160 days starting with the burst activation and found evidence of a strong second derivative period component (\ddot{P} = -1.6(4)x10^{-19} s/s^{-2}). Thanks to the phase-connection, we were able to study the the phase-resolved spectral evolution of SGR 0501+4516 in great detail. We also report on the ROSAT quiescent source data, taken back in 1992 when the source exhibits a flux ~80 times lower than that measured during the outburst, and a rather soft, thermal spectrum., 16 pages; accepted for publication in MNRAS

X-ray observations performed with ROSAT led to the discovery of 7 radio-silent isolated neutron stars (INSs) which are detected only through the relatively dim and purely thermal X-ray emission from the cooling star surface. A few of these INSs (a.k.a. X-ray Dim INSs, or XDINSs) have been also detected at optical wavelengths where they seem to feature thermal spectra. Optical studies of XDINSs thus play a crucial role in mapping the temperature distribution on the neutron star surface and in investigating the existence of an atmosphere around the neutron star. The aim of this work is to investigate the optical identification of the XDINS RX J0420.0-5022, tentatively proposed by Haberl et al. (2004) based on VLT observations. We re-analysed the original observations of Haberl et al. to assess the detection significance of the proposed counterpart and we performed deeper VLT observations aiming at a higher confidence detection. With a ~2 sigma detection significance and a re-computed flux of B=27.52+/-0.61, we can not rule out that the proposed counterpart was spurious and produced by the halo of a very bright nearby star. While we could not detect the proposed counterpart in our deeper VLT observations, we found evidence for a marginally significant (~3.9 sigma) detection of a similarly faint object (B= 27.5+/-0.3), ~ 0.5" north of it and coincident with the updated Chandra position of RX J0420.0-5022. Interestingly, the angular separation is consistent with the upper limit on the RX J0420.0-5022 proper motion (Motch et al. 2009), which suggests that we might have actually detected the Haberl et al. proposed counterpart. From the flux of the putative RX J0420.0-5022 counterpart we can rule out a > 7 optical excess with respect to the extrapolation of the XMM-Newton spectrum., 9 pages, 9 postscript figures, accepted for publication in Astronomy & Astrophysics

There is an increasing theoretical and observational evidence that the external magnetic field of magnetars may contain a toroidal component, likely of the same order of the poloidal one. Such "twisted magnetospheres" are threaded by currents flowing along the closed field lines which can efficiently interact with soft thermal photons via resonant cyclotron scatterings (RCS). Actually, RCS spectral models proved quite successful in explaining the persistent ~1-10 keV emission from the magnetar candidates, the soft gamma-ray repeaters (SGRs) and the anomalous X-ray pulsars (AXPs). Moreover, it has been proposed that, in presence of highly relativistic electrons, the same process can give rise to the observed hard X-ray spectral tails extending up to ~200 keV. Spectral calculations have been restricted up to now to the case of a globally twisted dipolar magnetosphere, although there are indications that the twist may be confined only to a portion of the magnetosphere, and/or that the large scale field is more complex than a simple dipole. In this paper we investigate multipolar, force-free magnetospheres of ultra-magnetized neutron stars. We first discuss a general method to generate multipolar solutions of the Grad- Schluter-Shafranov equation, and analyze in detail dipolar, quadrupolar and octupolar fields. The spectra and lightcurves for these multipolar, globally twisted fields are then computed using a Monte Carlo code and compared with those of a purely dipolar configuration. Finally the phase-resolved spectra and energy-dependent lightcurves obtained with a simple model of a locally sheared field are confronted with the INTEGRAL observations of the AXPs 1RXS J1708-4009 and 4U 0142+61. Results support a picture in which the field in these two sources is not globally twisted., Comment: 12 pages, 17 figures, MNRAS accepted

We derive a strong upper bound on the amount of Primordial Black Holes (PBHs) that can still be present in the Universe. Gravitational capture of PBHs by the Milky Way stars during their formation and subsequent accretion would produce a dramatic depletion of disk stars and especially of white dwarfs, unless the average cosmic density and mass of PBHs are severely constrained. Our finding also helps to discriminate among the various production mechanisms of PBHs. Moreover, we show that a star becomes overluminous before its disappearance into a PBH for a time span independent of its mass, thereby providing a characteristic observational signature of the considered scenario. We stress that our result allows for the existence of stellar-mass black holes in a mass range that is forbidden by standard stellar evolution., Comment: 7 pages, no figures, submitted to PRL

We have carried out a search for radio emission at 820 MHz from six X-ray dim isolated neutron stars with the Robert C. Byrd Green Bank Radio Telescope. No transient or pulsed emission was found using fast folding, fast Fourier transform, and single-pulse searches. The corresponding flux limits are about 0.01 mJy for pulsed emission, depending on the integration time for the particular source and assuming a duty cycle of 2%, and 20 mJy for single dispersed pulses. These are the most sensitive limits to date on radio emission from X-ray dim isolated neutron stars. There is no evidence for isolated radio pulses, as seen in a class of neutron stars known as rotating radio transients. Our results imply that either the radio luminosities of these objects are lower than those of any known radio pulsars, or they could simply be long-period nearby radio pulsars with high magnetic fields beaming away from the Earth. To test the latter possibility, we would need around 40 similar sources to provide a 1 sigma probability of at least one of them beaming toward us. We also give a detailed description of our implementation of the Fast Folding Algorithm., Comment: 16 pages, 8 figures, 3 tables, accepted to ApJ

After nearly a decade of quiescence, the soft gamma-ray repeater SGR 1627-41 reactivated on 2008 May 28 with a bursting episode followed by a slowly decaying enhancement of its persistent emission. To search for the still unknown spin period of this SGR taking advantage of its high flux state, we performed on 2008 September 27-28 a 120 ks long X-ray observation with the XMM-Newton satellite. Pulsations with P = 2.594578(6) s were detected at a higher than 6-sigma confidence level, with a double-peaked pulse profile. The pulsed fraction in the 2-12 keV range is 19% +/- 3% and 24% +/- 3% for the fundamental and the second harmonic, respectively. The observed 2-10 keV flux is 3.4E-13 erg/cm^2/s, still a factor of ~ 5 above the quiescent pre-burst-activation level, and the spectrum is well fitted by an absorbed power law plus blackbody model (photon index Gamma = 0.6, blackbody temperature kT = 0.5 keV, and absorption nH = 1.2E+23 cm^-2). We also detected a shell of diffuse soft X-ray emission which is likely associated with the young supernova remnant G337.0-0.1., Comment: Minor changes to match the final version (to appear in The Astrophysical Journal Letters). 5 pages in emulate-apj style, 1 table, 4 figures (1 color)

Over the last decade, X-ray observations unveiled the existence of several classes of isolated neutron stars (INSs) which are radio-quiet or exhibit radio emission with properties much at variance with those of ordinary radio pulsars. The identification of new sources is crucial in order to understand the relations among the different classes and to compare observational constraints with theoretical expectations. A recent analysis of the 2XMMp catalogue provided less than 30 new thermally emitting INS candidates. Among these, the source 2XMM J104608.7-594306 appears particularly interesting because of the softness of its X-ray spectrum and of the present upper limits in the optical, which imply a logarithmic X-ray-to-optical flux ratio greater than 3.1, corrected for absorption. We present the X-ray and optical properties of 2XMM J104608.7-594306 and discuss its nature in the light of two possible scenarios invoked to explain the X-ray thermal emission from INSs: the release of residual heat in a cooling neutron star, as in the seven radio-quiet ROSAT-discovered INSs, and accretion from the interstellar medium. We find that the present observational picture of 2XMM J104608.7-594306 is consistent with a distant cooling INS with properties in agreement with the most up-to-date expectations of population synthesis models: it is fainter, hotter and more absorbed than the seven ROSAT sources and possibly located in the Carina Nebula, a region likely to harbour unidentified cooling neutron stars. The accretion scenario, although not entirely ruled out by observations, would require a very slow (~10 km/s) INS accreting at the Bondi-Hoyle rate., Comment: 8 pages, 4 figures, accepted for publication in A&A (no changes relative to v1)

High-energy observations have unveiled peculiar classes of isolated neutron stars which, at variance with radio pulsars, are mostly radio silent and not powered by the star rotation. Among these objects are the magnetars, hyper-magnetized neutron stars characterized by transient X-ray/gamma-ray emission, and neutron stars with purely thermal, and in most cases stationary, X-ray emission (a.k.a., X-ray dim isolated neutron stars or XDINSs). While apparently dissimilar in their high-energy behavior and age, both magnetars and XDINSs have similar periods and unusually high magnetic fields. This suggests a tantalizing scenario where the former evolve into the latter.Discovering so far uninvestigated similarities between the multi-wavelength properties of these two classes would be a further step forward to establish an evolutionary scenario. A most promising channels is the near infrared (NIR) one, where magnetars are characterized by a distinctive spectral flattening with respect to the extrapolation of the soft X-ray spectrum.We observed the two XDINSs RX J0420.0-5022 and RX J1856.5-3754 with the Multi-Conjugate Adaptive Optics Demonstrator (MAD) at the Very Large Telescope (VLT) as part of the instrument guaranteed time observations program, to search for their NIR counterparts. Both RX J1856.5-3754 and RX J0420.0-5022 were not detected down to K_s ~20 and Ks ~21.5, respectively. In order to constrain the relation between XDINSs and magnetars it would be of importance to perform deeper NIR observations. A good candidate is 1RXS J214303.7+065419 which is the XDINS with the highest inferred magnetic field.

We report on a multi-band high-energy observing campaign aimed at studying the long term spectral variability of the Anomalous X-ray Pulsar (AXP) 1RXS J170849.0-400910, one of the magnetar candidates. We observed 1RXS J170849.0-400910 in Fall 2006 and Spring 2007 simultaneously with Swift/XRT, in the 0.1-10 keV energy range, and with INTEGRAL/IBIS, in the 20-200 keV energy range. Furthermore, we also reanalyzed, using the latest calibration and software, all the publicly available INTEGRAL data since 2002, and the soft X-ray data starting from 1999 taken using BeppoSAX, Chandra, XMM, and Swift/XRT, in order to study the soft and hard X-ray spectral variability of 1RXS J170849.0-400910. We find a long-term variability of the hard X-ray flux, extending the hardness-intensity correlation proposed for this source over 2 orders of magnitude in energy., 5 pages, 2 figures, accepted for publication in Astronomy & Astrophysics main journal

We report on two new XMM-Newton observations of the Anomalous X-ray Pulsar (AXP) 4U 0142+614 performed in March and July 2004, collecting the most accurate spectrum for this source to date. Furthermore, we analyse two short archival observations performed in February 2002 and January 2003 (the latter already reported by Gohler et al. 2005) in order to study the long term behaviour of this AXP. 4U 0142+614 appears to be relatively steady in flux between 2002 and 2004, and the phase-averaged spectrum does not show any significant variability between the four epochs. We derive the deepest upper limits to date on the presence of lines in the 4U 0142+614 spectrum as a function of energy: equivalent width in the 1-3 keV energy range < 4 eV and < 8 eV for narrow and broad lines, respectively. A remarkable energy dependence in both the pulse profile and the pulsed fraction is detected, and consequently pulse-phase spectroscopy shows spectral variability as a function of phase. By making use of XMM-Newton and INTEGRAL data, we successfully model the 1-250 keV spectrum of 4U 0142+614 with three models presented in Rea et al. (2007a), namely the canonical absorbed blackbody plus two power-laws, a resonant cyclotron scattering model plus one power-law and two log-parabolic functions., 9 pages, 5 figures; accepted for publication on MNRAS (typos corrected and few references added)

We report on X-ray, optical, infrared and radio observations of the X-ray dim isolated neutron star (XDINS) 1RXS J214303.7+065419 (also known as RBS 1774). The X-ray observation was performed with the High Resolution Camera on board of the Chandra X-ray Observatory, allowing us to derive the most accurate position for this source (alpha = 21h43m3.38s, delta= +6deg54'17".53; 90% uncertainty of 0."6). Furthermore, we confirmed with a higher spatial accuracy the point-like nature of this X-ray source. Optical and infrared observations were taken in B, V, r', i', J, H and Ks filters using the Keck, VLT, Blanco and Magellan telescopes, while radio observations were obtained from the ATNF Parkes single dish at 2.9GHz and 708MHz. No plausible optical and/or infrared counterpart for RBS 1774 was detected within the refined sub-arsecond Chandra X-ray error circle. Present upper limits to the optical and infrared magnitudes are r'>25.7 and J>22.6 (5 sigma confidence level). Radio observations did not show evidence for radio pulsations down to a luminosity at 1.4 GHz of L < 0.02 mJy kpc^2, the deepest limit up to date for any XDINS, and lower than what expected for the majority of radio pulsars. We can hence conclude that, if RBS 1774 is active as radio pulsar, its non detection is more probably due to a geometrical bias rather than to a luminosity bias. Furthermore, no convincing evidence for RRAT-like radio bursts have been found. Our results on RBS 1774 are discussed and compared with the known properties of other thermally emitting neutron stars and of the radio pulsar population., 8 pages, 9 figures, accepted for publication on MNRAS

We present an empirical spectral modelling of the high energy emission of the anomalous X-ray pulsar 4U 0142+614, based on simultaneous Swift and INTEGRAL observations from X to gamma-ray energies. We adopted models contained in the XSPEC analysis package, as well as models based on recent theoretical studies, and restricted ourselves to those combinations of up to three components which produce a good fit while requiring the lowest number of free parameters. Only three models were found to fit satisfactorily the 0.5-250keV spectrum of 4U 0142+614: i) a ~0.4keV blackbody and two power-laws, ii) a resonant cyclotron scattering model plus a power-law and iii) two log-parabolic functions. We found that only the latter two models do not over-predict the infrared/optical emission observed simultaneously from this AXP, and only the log-parabolic functions can naturally account for the upper limits set by COMPTEL in the gamma-ray range. A possible interpretation of the two log-parabolae in terms of inverse Compton scattering of soft X-ray photons by very energetic particles is discussed., 5 pages, 2 figures; accepted for publication in ApJ Letter

Recent progress on anomalous X-ray pulsars.- XMM-Newton observations of soft gamma-ray repeaters.- MMIV: de SGR 1806-20 Anno Mirabili.- Long term spectral variability in the soft gamma-ray repeater SGR 1900+14.- The radio nebula produced by the 27 December 2004 giant flare from SGR 1806-20.- The continuum and line spectra of SGR 1806-20 bursts.- Unveiling soft gamma-ray repeaters with INTEGRAL.- Our distorted view of magnetars: application of the resonant cyclotron scattering model.- Spitzer space telescope observations of SGR and AXP environments.- Anomalous X-ray pulsars: persistent states with fallback disks.- The anatomy of a magnetar: XMM monitoring of the transient anomalous X-ray pulsar XTE J1810-197.- PSR J1119-6127 and the X-ray emission from high magnetic field radio pulsars.- Chandra smells a RRAT.- QED can explain the non-thermal emission from SGRs and AXPs: variability.- Resonant Compton upscattering in anomalous X-ray pulsars.- Newborn magnetars as sources of gravitational radiation: constraints from high energy observations of magnetar candidates.- Astrophysical input for gravitational wave searches.- Dim isolated neutron stars, cooling and energy dissipation.- Accretion by isolated neutron stars.- Non-LTE modeling of supernova-fallback disks.- Chandra observations of neutron stars: an overview.- XMM-Newton observations of the isolated neutron star 1RXS J214303.7+065419/RBS1774.- Persistent and transient blank field sources.- The Magnificent Seven in the dusty prairie.- The magnificent seven: magnetic fields and surface temperature distributions.- Isolated neutron stars: magnetic fields, distances, and spectra.- Studies of neutron stars at optical/IR wavelengths.- Radio emission from AXP and XDINS.- Measuring proper motions of isolated neutron stars with Chandra.- The trigonometric parallax of the neutron star Geminga.- The puzzling X-ray source in RCW103.- Evidence for a binary companion to the central compact object 1E 1207.4-5209.- Towards self-consistent models of isolated neutron stars.- Neutron star surface emission: Beyond the dipole model.- Molecular systems in a strong magnetic field.- Thin magnetic hydrogen atmospheres and the neutron star RX J1856.5-3754.- Ultraviolet emission from young and middle-aged pulsars.- Studying millisecond pulsars in X-rays.- X-ray observations of PSR B0355+54 and its pulsar wind nebula.- New phase-coherent measurements of pulsar braking indices.- Thermal X-ray emission from hot polar cap in drifting subpulse pulsars.- The example of effective plasma acceleration in a magnetosphere.- Impact of neutron star oscillations on the accelerating electric field in the polar cap of pulsar.- Heat blanketing envelopes and thermal radiation of strongly magnetized neutron stars.- Equation of state of neutron star cores and spin down of isolated pulsars.- Equation of state constraints from neutron stars.- Neutron star masses: dwarfs, giants and neighbors.- A microscopic equation of state for protoneutron stars.- Modelling the dynamics of superfluid neutron stars.- Cooling of neutron stars with strong toroidal magnetic fields.- Internal heating and thermal emission from old neutron stars.- Thermal emission areas of heated neutron star polar caps.- Cooling of magnetars with internal layer heating.- What do exotic equations of state have to offer?.- Precession as a probe of the neutron star interior.- Spin-one color superconductivity in compact stars?-an analysis within NJL-type models.- Exotic bulk viscosity and its influence on neutron star r-modes.- Nucleon superfluidity versus thermal states of isolated and transiently accreting neutron stars.- The drift model of "magnetars".- Importance of Compton scattering for radiation spectra of isolated neutron stars.- Search for fast optical activity of SGR 1806-20 at the SAO RAS 6-m telescope.- Instabilities in rotating relativistic stars driven by viscosity.- 10 years of RXTE monitoring of anomalous X-ray pulsar 4U 0142+61: long-term variability.- Exotic ion H 3 ++ in strong magnetic fields.- The ion H 3 + in a strong magnetic field.- X-ray intensity-hardness correlation and deep IR observations of the anomalous X-ray pulsar 1RXS J170849-400910.- RX J1856.5-3754 as a possible strange star candidate.- Chandra monitoring of the candidate anomalous X-ray pulsar AX J1845.0-0258.- Spin-down of young pulsars with a fallback disk.- Search for radio pulsations in four anomalous X-ray pulsars and discovery of two new pulsars.- On the iron interpretation of the 6.4 keV emission line from SGR 1900+14.- Mountains on neutron stars.- Optical spectroscopy of the radio pulsar PSR B0656+14.- On the peculiarities in the rotational frequency evolution of isolated neutron stars.- Gravitational waves from r-modes.- Giant pulses of pulsar radio emission.- On the role of the current loss in radio pulsar evolution.- Force-free magnetosphere of an aligned rotator with differential rotation of open magnetic field lines.- Oscillations in the neutron star crust.- Two decades of pulsar timing of Vela.- Slow glitches in the pulsar B1822-09.- Short time scale pulse stability of the Crab pulsar in the optical band.- Using XMM-Newton to measure the spectrum of the Vela pulsar and its phase variation.- A toy model for global magnetar oscillation.- Structure of pair winds from compact objects with application to emission from bare strange stars.- The complex X-ray spectrum of the isolated neutron star RBS1223.- High frequency oscillations during magnetar flares.- Magnetar corona.- Intrinsic spectra of the AXPs.- The first multi-wavelength campaign of AXP 4U 0142+61 from radio to hard X-rays.

Retraction Note to: Astrophys. Space. Sci. (2007) 308: 309–316 The authors have retracted this article (McGowan et al. 2007) which was unintentionally submitted in full length as part of a conference proceedings. However, the authors had already published the article in another journal (McGowan et al. 2006). Therefore this article is redundant. All authors agree with this retraction.

We report XMM-Newton observations of the isolated neutron star RBS1774 and confirm its membership as an XDINS. The X-ray spectrum is best fit with an absorbed blackbody with temperature kT=101 eV and absorption edge at 0.7 keV. No power law component is required. An absorption feature in the RGS data at 0.4 keV is not evident in the EPIC data, but it is not possible to resolve this inconsistency. The star is not seen in the UV OM data to mAB∼21. There is a sinusoidal variation in the X-ray flux at a period of 9.437 s with an amplitude of 4%. The age as determined from cooling and magnetic field decay arguments is 105–106 yr for a neutron star mass of 1.35–1.5 M⊙.

In the last years, optical studies of Isolated Neutron Stars (INSs) have expanded from the more classical rotation-powered ones to other categories, like the Anomalous X-ray Pulsars (AXPs) and the Soft Gamma-ray Repeaters (SGRs), which make up the class of the magnetars, the radio-quiet INSs with X-ray thermal emission and, more recently, the enigmatic Compact Central Objects (CCOs) in supernova remnants. Apart from 10 rotation-powered pulsars, so far optical/IR counterparts have been found for 5 magnetars and for 4 INSs. In this work we present some of the latest observational results obtained from optical/IR observations of different types of INSs., 8 pages, 6 Postscript figures

A large body.of observational evidence has gathered over the last three decades for the occurrence of both high speed flows and strong gravitational fields in astrophysical objects. Galactic X-ray sources and Active Galactic Nuclei are just but the best known examples. In these sources observations have convincingly shown that energy is produced by accretion onto black holes and neutron stars, a scenario in which matter flows close to the compact object gravitational radius at nearly light speed. Thanks to last-generation X-ray observatories, like Chandra-AX A F and XMM-Newton, the spectral and timing properties of black hole and neutron star sources have been probed to an unprecedented level of detail. This has provided theoreticians with a wealth of information on the structure of the accretion flow and made even more acute the necessity of adequate techniques to treat radiation transport under fully relativistic conditions. Actually, radiative transfer is just one of the aspects one is confronted with in the arena of relativistic astrophysics. The modelling of accretion flows (and relativistic jets, supernova explosions, the early unverse as well) often requires the solution of the full radiation hydrodynamical problem, of which radiation transport is but a part. It is, in fact, the (thermo)dynamical state of the matter (i.e. density, temperature, velocity) which fixes the emission and absorption coefficients. These determine the radiation field (intensity) which, through its moments (energy density, flux, pressure), enters the (magneto)hydrodynamical equations. It is Catch 22 once again: "You do not know the properties of matter unless you calculate the radiation field, but to calculate the radiation field you need the properties of matter". This is what makes the modelling of many astrophysical sources so challenging. In the following I will attempt to briefly summarize the contributions to this session, which cover both the issues outlined above, trying at the same time to put them in perspective; the references given below are by no means exhaustive. For

We briefly review selected results in astrophysics of neutron stars (NSs) obtained during the last two years, focusing on isolated radioquiet objects. We discuss in some details population synthesis of close-by isolated NSs, spectra of INSs, detection of spectral features in these sources (including cyclotron features), recent results on velocity distribution of NSs and accretion onto INSs.

The XMM-Newton spectra of the isolated neutron star RX J0720.4-3125 obtained over 4.5 years can be described by sinusoidal variations in the inferred blackbody temperature, the size of the emitting area and the depth of the absorption line with a period of 7.1 +/- 0.5 years, which we suggest to be the precession period of the neutron star. Precession of a neutron star with two hot spots of different temperature and size, probably not located exactly in antipodal positions, may account for the variations in the X-ray spectra, changes in the pulsed fraction, shape of the light curve and the phase-lag between soft and hard energy bands observed from RX J0720.4-3125. An independent sinusoidal fit to published and new pulse timing residuals from a coherent analysis covering ~12 years yields a consistent period of 7.7 +/- 0.6 years supporting the precession model., Comment: Accepted for publication in A&A Letters, 5 pages, 5 figures

Aims. We present a new optical observation of the X-ray dim isolated neutron star RX J1605.3+3249, performed in 2005 with the Wide Field Channel (WFC) of the Advanced Camera for Surveys (ACS) on board Hubble Space Telescope, aimed at measuring the neutron star proper motion.Methods. We have compared the target position with the one measured in a previous HST image of the field obtained in 2001.Results. We infer a proper motion of mu = 155.0 +/- 3.1 mas/yr, corresponding to a star velocity V-T similar to 73 km s(-1) at 100 pc distance. Our measurement refines the result recently reported by Motch et al. ( 2005, A&A, 429, 257), and confirms the optical source as the counterpart of the neutron star.

A ~50 ks XMM-Newton observation of SGR 1900+14 has been carried out in September 2005, after almost three years during which no bursts were detected from this soft gamma-ray repeater. The 0.8-10 keV spectrum was well fit by a power law plus blackbody model with photon index 1.9+/-0.1, temperature kT=0.47+/-0.02 keV and N_H = (2.12+/-0.08)x10^22 cm^-2, similar to previous observations of this source. The flux was ~5x10^{-12} erg/cm2/s, a factor 2 dimmer than the typical value and the smallest ever seen from SGR 1900+14. The long term fading of the persistent emission has been interrupted by the recent burst reactivation of the source. A target of opportunity XMM-Newton observation performed in April 2006 showed a flux ~15% higher. This variation was not accompanied by significant changes in the spectrum, pulsed fraction and light curve profile. We searched for emission and absorption lines in the spectra of the two observations, with negative results and setting tight upper limits of 50-200 eV (3 sigma), depending on the assumed line energy and width, on the equivalent width of lines in the 1-9 keV range., Comment: accepted for publication on The Astrophysical Journal

Convincing evidence for long-term variations in the emission properties of the anomalous X-ray pulsar 1RXS J170849.0-400910 has been gathered in the last few years. In particular, and following the pulsar glitches of 1999 and 2001, XMM-Newton witnessed in 2003 a decline of the X-ray flux accompanied by a definite spectral softening. This suggested the existence of a correlation between the luminosity and the spectral hardness in this source, similar to that seen in the soft gamma-repeater SGR 1806-20. Here we report on new Chandra and Swift observations of 1RXS J170849.0-400910 performed in 2004 and 2005, respectively. These observations confirm and strengthen the proposed correlation. The trend appears to have now reversed: the flux increased and the spectrum is now harder. The consequences of these observations for the twisted magnetosphere scenario for anomalous X-ray pulsars are briefly discussed., Comment: 6 pages, 5 figures, accepted for publication on A&A

We present a detailed analysis of XMM archival data of two Ultraluminous X-ray Sources (ULXs) in the nearby spiral galaxy NGC 1313: NGC 1313 X-1 and X-2. The spectral continuum of these sources was modeled with a soft thermal component plus a power-law. If the soft component originates from an accretion disk, the inferred mass of the compact remnant is > 100 M_sun, making it an Intermediate Mass Black Hole (IMBH). A detailed analysis of the residuals of the XMM EPIC-pn spectrum shows some evidence for the presence of an Oxygen emission line in NGC 1313 X-1. The simultaneous presence of an excess in emission, although at a much reduced significance level, at different energies in the X-ray spectra of NGC 1313 X-1 and X-2 is suggestive of typical emission lines from young supernova remnants. An optical counterpart for NGC 1313 X-2 was also identified. On an ESO 3.6 m image, the Chandra error box embraces a R \~ 23 mag stellar-like object and excludes a previously proposed optical counterpart., 7 pages, 2 figures, to be published in Advances in Space Research

Recent Chandra and XMM-Newton observations of a number of X-ray ``dim'' pulsating neutron stars revealed quite unexpected features in the emission from these sources. Their soft thermal spectrum, believed to originate directly from the star surface, shows evidence for a phase-varying absorption line at some hundred eVs. The pulse modulation is relatively large (pulsed fractions in the range ~12%-35%), the pulse shape is often non-sinusoidal, and the hard X-ray color appears to be anti-correlated in phase with the total emission. Moreover, the prototype of this class, RX J0720.4-3125, has been found to undergo rather sensible changes both in its spectral and timing properties over a timescale of a few years. All these new findings seem difficult to reconcile with the standard picture of a cooling neutron star endowed with a purely dipolar magnetic field, at least if surface emission is produced in an atmosphere on top of the crust. In this paper we explore how a dipolar+quadrupolar star-centered field influence the properties of the observed lightcurves. The phase-resolved spectrum has been evaluated accounting for both radiative transfer in a magnetized atmosphere and general relativistic ray-bending. We computed over 78000 lightcurves varying the quadrupolar components and the viewing geometry. A comparison of the data with our model indicate that higher order multipoles are required to reproduce the observations., 17 pages, 9 figures. Figures 3 and 5 (top panel) are provided separately as .png files. Accepted for publication in MNRAS

We present archive ESO VLT photometric and spectroscopic data of the Ultraluminous X-ray source NGC 1313 X-2. The superb quality of the VLT images reveals that two distinct objects, with R magnitudes 23.7 and 23.6, are visible inside the Chandra error box. The two objects, separated by 0.75 arcsec, were unresolved in our previous ESO 3.6 m+EFOSC image. We show that both are stars in NGC 1313, the first a B0-O9 main sequence star of ~20 Msun, while the second a G supergiant of ~10 Msun. Irrespectively of which of the two objects the actual counterpart is, this implies that NGC 1313 X-2 is a high mass X-ray binary with a very massive donor., 7 pages, 4 figures, accepted for publication in ApJ Letters

In the last few years considerable observational resources have been devoted to study the thermal emission from isolated neutron stars. Detailed XMM and Chandra observations revealed a number of features in the X-ray pulse profile, like asymmetry, energy dependence, and possible evolution of the pulse profile over a time scale of months or years. Here we show that these characteristics may be explained by a patchy surface temperature distribution, which is expected if the magnetic field has a complex structure in which higher order multipoles contribute together with the dipole. We reconsider these effects from a theoretical point of view, and discuss their implications to the observational properties of thermally emitting neutron stars., 6 pages, 1 TeX file, 6 postscript figures; macro: elsart.cls. Accepted for publication in Advances in Space Research. Manuscript Number: JASR-D-04-00405R1

We report the results of a recent (July 2004) XMM-Newton Target of Opportunity observation of the Anomalous X-ray pulsar 1E 1048.1-5937, together with a detailed re-analysis of previous observations carried out in 2000 and 2003. In July 2004 the source had a 2-10 keV flux of 6.2$\times10^{-12}$ erg cm$^{-2}$ s$^{-1}$ and a pulsed fraction P$_F$=0.68. The comparison of the three data sets shows the presence of an anti-correlation between flux and pulsed fraction, implying that previous estimates of the source energetics based on the assumption of a large and constant pulsed fraction might be significantly underestimated. The source spectrum is well described by a power law plus blackbody model (kT~0.63 keV, photon index $\Gamma$~2.7-3.5) or, alternatively, by the sum of two blackbodies of which the hotter is Comptonized by relativistic electrons. In this case the temperatures are kT${_1}$~0.2-0.3 keV and kT${_2}$~0.4-0.5 keV and the emitting area of the cooler component is consistent with the whole neutron star surface. The long term luminosity variation of a factor >~2 is accompanied by relatively small variations in the spectral shape. Phase resolved spectroscopy indicates a harder spectrum in correspondence of the pulse maximum. No spectral features have been detected with 4$\sigma$ limits on the equivalent width in the range ~10-220 eV, depending on line energy and width., Comment: 10 pages, 6 figures; accepted for publication in A&A

GCRT J1745-3009 is a peculiar transient radio-source in the direction of the Galactic Center. It was observed to emit a series of ~ 1 Jy bursts at 0.33 GHz, with typical duration ~ 10 min and at apparently regular intervals of ~ 77 min. If the source is indeed at the distance of the Galactic Center as it seems likely, we show that its observational properties are compatible with those expected from a double neutron star binary, similar to the double pulsar system J0737-3039. In the picture we propose the (coherent) radio emission comes from the shock originating in the interaction of the wind of the more energetic pulsar with the magnetosphere of the companion. The observed modulation of the radio signal is the consequence of an eccentric orbit, along which the separation between the two stars varies. This cyclically drives the shock inside the light cylinder radius of the less energetic pulsar., Comment: 5 pages, 3 figures, accepted for publication in The Astrophysical Journal Letters, comment on geodetic precession added

We report on the results of a ~30 ks Chandra pointing of the soft gamma-ray repeater SGR 1806-20, the first X-ray observation with high spectral resolution performed after the 2004 December 27 Giant Flare. The source was found in a bursting active phase and with a significantly softer spectrum than that of the latest observations before the Giant Flare. The observed flux in the 2-10 keV range was ~2.2 x10^-11 erg/s/cm^2, about 20% lower than that measured three months before the event. This indicates that, although its giant flare was ~100 times more intense than those previously observed in two other soft gamma-ray repeaters, the post flare X-ray flux decay of SGR 1806-20 has been much faster. The pulsed fraction was about 3%, a smaller value than that observed before the flare. We discuss the different properties of the post-flare evolution of SGR 1806-20 in comparison to those of SGR 1900+14 and interpret the results as a strong evidence that a magnetospheric untwisting occurred (or is occurring) after the Giant Flare., Comment: accepted for publication on ApJ Letters

We present new optical and {\it Chandra} observations of the field containing the ultraluminous X-ray source NGC1313 X-2. On an ESO 3.6 m image, the {\it Chandra} error box embraces a $R=21.6$ mag stellar-like object and excludes a previously proposed optical counterpart. The resulting X-ray/optical flux ratio of NGC 1313 X-2 is $\sim 500$. The value of $f_X/f_{opt}$, the X-ray variability history and spectral distribution indicate a luminous X-ray binary in NGC 1313 as a likely explanation for NGC 1313 X-2. The inferred optical luminosity ($L\approx 10^5 L_\odot$) is consistent with that of a $\approx 10 M_\odot$ companion., 5 pages, 1 figure, to appear in Proc. of the BeppoSAX Symposium: "The Restless High-Energy Universe" (Amsterdam, May 5-8, 2003), E.P.J. van den Heuvel, J.J.M. in 't Zand, and R.A.M.J. Wijers (Eds), Nucl. Physics B. Suppl. Ser., Elsevier

The radiative acceleration of particles and the electrostatic potential fields that arise in low density plasmas hit by radiation produced by a transient, compact source are investigated. We calculate the dynamical evolution and asymptotic energy of the charged particles accelerated by the photons and the radiation-induced electric double layer in the full relativistic, Klein-Nishina regime. For fluxes in excess of $10^{27}$ ${\rm erg} {\rm cm}^{-2} {\rm s}^{-1}$, the radiative force on a diluted plasma ($n\la 10^{11}$ cm$^{-3}$) is so strong that electrons are accelerated rapidly to relativistic speeds while ions lag behind owing to their larger inertia. The ions are later effectively accelerated by the strong radiation-induced double layer electric field up to Lorentz factors $\approx 100$, attainable in the case of negligible Compton drag. The asymptotic energies achieved by both ions and electrons are larger by a factor 2--4 with respect to what one could naively expect assuming that the electron-ion assembly is a rigidly coupled system. The regime we investigate may be relevant within the framework of giant flares from soft gamma-repeaters., 14 pages, 7 figures, ApJ, in press (tentatively scheduled for the v. 592, 2003 issue)

We report the discovery of a new ultraluminous X-ray source (ULX) in the nearby galaxy NGC 4244 from Chandra archival data. The source, 1WGAJ1216.9+3743, is one of the least luminous and softest ULXs discovered so far. Its X-ray spectrum is the best available for a representative of the soft ULXs, a class of sources recently discovered by Chandra and XMM-Newton. 1WGAJ1216.9+3742 appears point-like in the Chandra image and has a complex spectral shape: a multicolor disk model, suitable for brighter ULXs, is inadequate for this source. 1WGAJ1216.9+3743 spectrum is heavily absorbed (NH ~ (1-4)E21 cm^-2),and very soft. The best-fit power-law model gives a photon index ~ 5 and implies a luminosity L(0.5-10 keV) ~ 1E39 erg/s. A comparison with previous detections shows that, despite the variability displayed by the source during the Chandra observation, 1WGAJ1216.9+3743 count rate, spectral shape and absorption are practically unchange over a 9-year period. We performed also deep optical imaging of the field containing the X-ray source, and found a possible R ~ 23.7 counterpart., 7 pages, 3 figures, emulateapj5 style, accepted version, scheduled for ApJ v582 n2 January 10, 2003 issue

The effects of bulk motion comptonization on the spectral formation in a converging flow onto a black hole are investigated. The problem is tackled by means of both a fully relativistic, angle-dependent transfer code and a semi-analytical, diffusion-approximation method. We find that a power-law high-energy tail is a ubiquitous feature in converging flows and that the two approaches produce consistent results at large enough accretion rates, when photon diffusion holds. Our semi-analytical approach is based on an expansion in eigenfunctions of the diffusion equation. Contrary to previous investigations based on the same method we find that, although the power-law tail at really large energies is always dominated by the flatter spectral mode, the slope of the hard X-ray portion of the spectrum is dictated by the second mode and it approaches Gamma=3 at large accretion rate, irrespective of the model parameters. The photon index in the tail is found to be largely independent on the spatial distribution of soft seed photons when the accretion rate is either quite low (< 5 in Eddington units) or sufficiently high (> 10). On the other hand, the spatial distribution of source photons controls the photon index at intermediate accretion rates, when Gamma switches from the first to the second mode. Our analysis confirms that a hard tail with photon index Gamma, to be published in the Astrophysical Journal, 16 pages and 9 figures

Transonic discs with accretion rates relevant to intrinsically bright Galactic X-ray sources ($L\approx 10^{38}$-$10^{39} {\rm erg s}^{-1}$) exhibit a time dependent cyclic behaviour due to the onset of a thermal instability driven by radiation pressure. In this paper we calculate radiation spectra emitted from thermally-unstable discs to provide detailed theoretical predictions for observationally relevant quantities. The emergent spectrum has been obtained by solving self-consistently the vertical structure and radiative transfer in the disc atmosphere. We focus on four particular stages of the disc evolution, the maximal evacuation stage and three intermediate stages during the replenishment phase. The disc is found to undergo rather dramatic spectral changes during the evolution, emitting mainly in the 1-10 keV band during outburst and in the 0.1-1 keV band off-outburst. Local spectra, although different in shape from a blackbody at the disc effective temperature, may be characterized in terms of a hardening factor $f$. We have found that $f$ is rather constant both in radius and in time, with a typical value $\sim 1.65$., 10 pages Latex with 11 ps figures. Accepted for publication in MNRAS

We suggest a simple thermal Comptonization model to account for the observed phase lags-hardness ratio correlation of GRS 1915+105.

We report the results of X-ray (2-10 keV) observations of the giant molecular clouds SgrB, SgrC and SgrD in the Galactic Center region, together with the discovery of the point-like source SAXJ1748.2-2808. The data have been obtained with the MECS instrument on the BeppoSAX satellite. The core of SgrB2 has an X-ray luminosity of 6x10^34 erg/s and its spectrum is characterized by a strong Fe emission line at 6.5 keV with an equivalent width of 2 keV. Faint diffuse X-ray emission is detected from SgrC and from the SNR G1.05-0.15 (SgrD). A new, unresolved source with a strong Fe line has been discovered in the SgrD region. This source, SAXJ1748.2-2808, is probably associated with a SiO and OH maser source at the Galactic Center distance. If so, its luminosity is 10^34 erg/s. We propose that the X-ray emission from SAX J1748.2-2808 is produced either by protostars or by a giant molecular cloud core. Emission from sources similar to SAX J1748.2-2808 could have an impact on the expected contribution on the observed Fe line emission from the Galactic ridge., 12 pages, 8 figures, accepted for publication in Astronomy & Astrophysics Main Journal

Thermal instability driven by radiation pressure might be relevant for intrinsically bright accreting sources. The most promising candidate where this instability seems to be at work is one of the two known galactic superluminal sources, GRS 1915+105 (Belloni et al. 1997). In spite of being of relevance, this scenario has not yet been confirmed by proper time-dependent modelling. Non-linear time-dependent calculations performed by (1998) show that thermally unstable discs undergo limit-cycle behaviour with successive evacuation and refilling of the central parts of the disc. This evolution is very similar to the one proposed by (1997) in their phenomenological model. Further investigations are needed to confirm the thermal instability being operational in this source. First of all the spectra emitted from the disc during its evolution should be calculated and compared with observations. Here such spectra are computed assuming local blackbody emission from the best studied transonic disc model.

In this Letter we investigate the properties of advection-dominated accretion flows (ADAFs) fed by the evaporation of a Shakura-Sunyaev accretion disk (SSD). In our picture the ADAF fills the central cavity evacuated by the SSD and extends beyond the transition radius into a coronal region. We find that, because of global angular momentum conservation, a significant fraction of the hot gas flows away from the black hole forming a transsonic wind, unless the injection rate depends only weakly on radius (if $r^2\dot\sigma\propto r^{-\xi}$, $\xi< 1/2$). The Bernoulli number of the inflowing gas is negative if the transition radius is $\lesssim 100$ Schwarzschild radii, so matter falling into the hole is gravitationally bound. The ratio of inflowing to outflowing mass is $\approx 1/2$, so in these solutions the accretion rate is of the same order as in standard ADAFs and much larger than in advection-dominated inflow/outflow models (ADIOS). The possible relevance of evaporation-fed solutions to accretion flows in black hole X-ray binaries is briefly discussed., Comment: 5 pages Latex with 2 ps figures. Accepted for publication in ApJ Letters

We present a detailed investigation of atmospheres around accreting neutron stars with high magnetic—eld G) and low luminosity ergs s(BZ1012 (L [1033 ~1). We compute the atmospheric structure, inten-sity, and emergent spectrum for a plane-parallel, pure hydrogen medium by solving the transfer equa-tions for the normal modes coupled to the hydrostatic and energy balance equations. The hard tailfound in previous investigations for accreting, nonmagnetic neutron stars with comparable luminosity issuppressed, and the X-ray spectrum, although still harder than a blackbody at the star eƒective tem-perature, is nearly Planckian in shape. Spectra from accreting atmospheres, both with high and low—elds, are found to exhibit a signi—cant excess at optical wavelengths above the Rayleigh-Jeans tail of theX-ray continuum.Subject headings: accretion, accretion disks ¨ radiative transfer ¨ stars: magnetic —elds ¨stars: neutron ¨ X-rays: stars 1 . INTRODUCTION The problem of investigating the properties of radiationemitted by neutron stars (NSs) accreting at low rates, M0B1010¨1014gs~1,becameofinterestafteritwasrealizedthatthe Galaxy may contain a large population of low-luminosity magnetic accretors (see, e.g., Nelson et al. 1995).TheGalaxyshouldharbormorethan103Be/X-raybinarieswith an accreting NS shining at B1032¨1034 ergs s~1(Rappaport & van den Heuvel 1982; van den Heuvel RBlaesMseeTrevesetal.1999forareview).At variance with neutron stars accreting at high rates,e.g., in X-ray pulsators, in low-luminosity sources the inter-action of the escaping radiation with the inNowing materialis of little importance, so they provide a much simpler casefor investigating the physics of accretion in a strongly mag-netized environment. For luminosities far below theEddington limit, the accretion problem becomes germaneto that of calculating the spectrum emerging from staticatmospheres around cooling NSs. Spectra from coolingNSs have been widely investigated by a number of authorsin connection with the X-ray emission from young, milli-second pulsars and isolated NSs, both for low and highmagnetic—eldsandfordiƒerentchemicalcompositions(see,e.g., Romani 1987; Shibanov et al. 1992; Rajagopal R Pavlov et al. 1996b). Emerging spectra arenot very diƒerent from a blackbody at the star eƒectivetemperature, the distinctive hardening present at low —elds(B[109 G) becoming less pronounced when the magnetic—eldisD1012¨1013G.Similarconclusionswerereachedforthespectrumemittedbylow-luminosity,low-—eld,accretingNSs by Zampieri et al. (1995, hereafter ZTZT) for a purehydrogenatmosphericcomposition.The search of isolated neutron stars with ROSAT pro-duced in recent years half a dozen promising candidates(Walter, Wolk, & 1996; Haberl et al. 1997;Neu”hauserHaberl, Motch, & Pietsch 1998; Schwope et al. 1999;Motch et al. 1999; Haberl, Pietsch, & Motch 1999). All ofthem show a soft, thermal X-ray spectrum, with typicalenergies D100 eV, and have an exceedingly large X-ray tooptical Nux ratio, Although their association withZ104.isolated neutron stars is —rmly established, their interpreta-tion in terms of an accreting or a cooling object is still amatter of lively debate. Present models predict rathersimilar spectral distributions in both cases, especially in theX-ray band. It is therefore of particular importance toimprove our theoretical understanding of these two classesofsources,looking,inparticular,forspectralsignaturesthatcanenableustodiscriminatebetweenthem.In this paper we present a —rst detailed calculation ofspectra emitted by strongly magnetized G), ac-(BZ1012creting neutron stars, focusing our attention on low lumi-nosities, L D1030¨1033 ergs s~1, such as those expectedfrom old neutron stars accreting the interstellar medium.Spectral distributions are computed by solving the transferequations for the normal modes coupled to the hydrostaticequilibrium and the energy balance for diƒerent values oftheaccretionrateandthemagnetic—eld.We—ndthatspectraemergingfrommagnetizedaccretionatmospheres are blackbody-like in the X-ray band, in closeagreement with the known results for cooling, magnetizedatmospheres. However, accretion spectra show a new anddistinctivefeatureatlowenergies,beingcharacterizedbyanexcess over the Raleigh-Jeans tail of the X-ray continuum387

As many as $10^9$ neutron stars populate the Galaxy, but only $\approx 10^3$ are directly observed as pulsars or as accreting sources in X-ray binaries. In principle also the accretion of the interstellar medium may make isolated neutron stars shine, and their weak luminosity could be detected in soft X-rays. Recent ROSAT observations have convincingly shown that neutron stars accreting from the interstellar medium are extremely rare, if observed at all, in contrast with earlier theoretical predictions. Until now two possible explanations for their elusiveness have been proposed: their velocity distribution may peak at $\sim 200-400 {\rm km s}^{-1}$, as inferred from pulsar statistics, and this would severely choke accretion; the magnetic field may decay on timescales $\sim 10^8-10^9$ yr, preventing a large fraction of neutron stars from entering the accretor stage. The search for accreting neutron stars has produced up to now a handful of promising candidates. While little doubt is left that these objects are indeed isolated neutron stars, the nature of their emission is still controversial. In particular accreting objects can be confused with much younger, cooling neutron stars. However, a combination of observations and theoretical modeling may help in discriminating between the two classes., 22 pages Latex, 3 Figures, Invited Review to be published to PASP

In this Letter we propose a simple thermal comptonization model to account for the observed properties of the phase lags associated to the ``plateau'' intervals of GRS 1915+105. By invoking a temperature stratification in a corona and assuming that the optical depth of the comptonizing region increases as the disk inner radius moves inward, we are able to reproduce both the observed colors and time lags in the continuum., Comment: 6 pages Latex with 2 ps figures. Accepted for publication in ApJ Letters

The paucity of old isolated accreting neutron stars in ROSAT observations is used to derive a lower limit on the mean velocity of neutron stars at birth. The secular evolution of the population is simulated following the paths of a statistical sample of stars for different values of the initial kick velocity, drawn from an isotropic Gaussian distribution with mean velocity $0\leq < V>\leq 550$ ${\rm km s^{-1}}$. The spin--down, induced by dipole losses and the interaction with the ambient medium, is tracked together with the dynamical evolution in the Galactic potential, allowing for the determination of the fraction of stars which are, at present, in each of the four possible stages: Ejector, Propeller, Accretor, and Georotator. Taking from the ROSAT All Sky Survey an upper limit of $\sim 10$ accreting neutron stars within $\sim 140$ pc from the Sun, we infer a lower bound for the mean kick velocity, $ < V>\gtrsim 200-300$ ${\rm km s^{-1}},$ corresponding to a velocity dispersion $��_V\gtrsim 125-190$ km s$^{-1}$. The same conclusion is reached for both a constant magnetic field ($B\sim 10^{12}$ G) and a magnetic field decaying exponentially with a timescale $\sim 10^9$ yr. Such high velocities are consistent with those derived from radio pulsar observations. Present results, moreover, constrain the fraction of low velocity stars, which could have escaped pulsar statistics, to less than 1%., 13 pages, 6 PostScript figures, accepted to ApJ

We consider a black hole accreting spherically from the surrounding medium. If accretion produces a luminosity close to the Eddington limit the hole acquires a net charge so that electrons and ions can fall with the same velocity. The condition for the electrostatic field to be large enough to break the vacuum near the hole horizon translates into an upper limit for the hole mass, $M\sim 6.6\times 10^{20} {\rm g}.$ The astrophysical conditions under which this phaenomenon can take place are rather extreme, but in principle they could be met by a mini black hole residing at the center of a star., 6 pages, accepted for publication in the Astrophysical Journal