Search

We investigate some key aspects of the “heartbeat” variability consisting of series of bursts with a slow rise and a fast decay, thus far detected only in GRS 1915+105, IGR J17091−3624, and MXB 1730−335. A previous analysis based on BeppoSAX data of GRS 1915+105 revealed a hard-X delay (HXD), that is a lag of the burst rise at higher energies with respect to lower ones; this leads to narrower pulse widths, w, at higher energies. We here use some light curves of Rossi-XTE observations of GRS 1915+105 for a deeper analysis of this effect and search for its presence in those extracted from some IGR J17091−3624 and MXB 1730−335 observations performed with the same satellite. Our results show that, at variance with GRS 1915+105, no HXD is evident in the light curves of MXB 1730−335 and only a marginal HXD may be argued for IGR J17091−3624. For GRS 1915+105 we find a decreasing trend of the pulse width with energy following a power law w = A ⋅ E−s with an index s ≈ 0.8. Furthermore, we confirm the increase of the HXD with the recurrence time Trec of the bursts in each series that was already found in previous works using BeppoSAX data. Based on a spectral analysis of these three sources we conclude that the differences highlighted in the properties of the “heartbeat” variability are probably related to the different accreting compact object and the eventual presence of a corona in these binary interacting systems.

We report the discovery of ROXA J081009.9+384757.0 = SDSS J081009.9+384757.0, a z = 3.95 blazar with a highly unusual Spectral Energy Distribution. This object was first noticed as a probable high fx/fr, high-luminosity blazar within the error region of a ≈10 −12 erg cm −2 s −1 ROSAT source which, however, also included a much brighter late-type star. We describe the results of a recent Swift observation that establishes beyond doubt that the correct counterpart of the X-ray source is the flat spectrum radio quasar. With a luminosity well in excess of 10 47 erg s −1 ROXA J081009.9+384757.0 is therefore one of the most luminous blazars known. We consider various possibilities for the nature of the electromagnetic emission from this source. In particular, we show that the SED is consistent with that of a blazar with synchrotron power peaking in the hard X-ray band. If this is indeed the case, the combination of high-luminosity and synchrotron peak in the hard-X-ray band contradicts the claimed anti-correlation between luminosity and position of the synchrotron peak usually referred to as the “blazar sequence”. An alternative possibility is that the X-rays are not due to synchrotron emission, in this case the very peculiar SED of ROXA J081009.9+384757.0 would make it the first example of a new class of radio loud AGN.

Context: We present the results of a set of observations of nine TeV detected BL Lac objects performed by the XRT and UVOT detectors on board the Swift satellite between March and December 2005. Aims: We are mainly interested in measuring the spectral parameters, and particularly the intrinsic curvature in the X-ray band. Methods: We perform X-ray spectral analysis of observed BL Lac TeV objects using either a log-parabolic or a simple power-law model . Results: We found that many of the objects in our sample do show significant spectral curvature, whereas those having the peak of the spectral energies distribution at energies lower than ~0.1 keV show power law spectra. In these cases, however, the statistics are generally low thus preventing a good estimate of the curvature. Simultaneous UVOT observations are important to verify how X-ray spectra can be extrapolated at lower frequencies and to search for multiple emission components. Conclusions: The results of our analysis are useful for the study of possible signatures of statistical acceleration processes predicting intrinsically curved spectra and for modelling the SED of BL Lacertae objects up to TeV energies where a corresponding curvature is likely to be present.

We present the results of a spectral analysis of 5 Swift XRT and UVOT observations of the BL Lac object PKS 0548-322 carried out over the period April-June 2005. The X-ray flux of this high energy peaked BL Lac (HBL) source was found to be approximately constant at a level of F(2-10 keV) ~ 4x10^-11 erg cm^-2 s^-1, a factor of 2 brighter than when observed by BeppoSAX in 1999 and close to the maximum intensity reported in the Einstein Slew Survey. The very good statistics obtained in the 0.3-10 keV Swift X-ray spectrum allowed us to detect highly significant deviations from a simple power law spectral distribution. A log-parabolic model describes well the X-ray data and gives a best fit curvature parameter of 0.18 and peak energy in the Spectral Energy Distribution of about 2 keV. The UV spectral data from Swift UVOT join well with a power law extrapolation of the soft X-ray data points suggesting that the same component is responsible for the observed emission in the two bands. The combination of synchrotron peak in the X-ray band and high intensity state confirms PKS 0548-322 as a prime target for TeV observations. X-ray monitoring and coordinated TeV campaigns are highly advisable., Accepted for publication in A&A (6 pages, 3 figures)

In this paper we present a broad-band study of the X-ray emission of the blazar PKS1830-211 based on Chandra and Integral observations. Notwithstanding the high redshift (z=2.507), it is a bright X-ray source (F(2-10 keV)~10^{-11} erg cm^{-2} s^{-1}), due to gravitational lensing by an intervening galaxy at z=0.89. Previous X-ray observations attribute the observed absorption at E, Comment: Accepted for publication in A&A

We report the results of new observations of the two BL Lac objects 3C 66A and ON 325 performed with Beppo SAX and of nearly simultaneous photometric measurements in the bandpasses from I to B . 3C 66A was observed in January 1999 and in July 2001: its X-ray flux was $F(2{-}10~{\rm keV}) = 2.0^{+0.3}_{-0.5}\times 10^{-12}$ erg cm -2 s -1 in January 1999 and $F(2{-}10~{\rm keV}) = (2.6 \pm 0.6)\times 10^{-12}$ erg cm -2 s -1 in July 2001. In both observations the X-ray spectrum can be fitted by a single power law with an energy index $1.2\pm 0.1$, steeper than that measured in the optical, equal to $0.86\pm 0.05$. The Beppo SAX observation of ON 325 was performed in December 1998 when its X-ray flux was $F(2{-}10~{\rm keV}) = 8^{+3}_{-4}\times10^{-13}$ erg cm -2 s -1 . The X-ray spectrum cannot be fitted by a single power law, but it is found to be well represented by a broken power law with a break energy at around 4 keV and energy indices equal to $1.48\pm0.12$ and $-0.57\pm0.63$. Both sources are characterised by a synchrotron-dominated emission at energies lower than a few keV, with a sharp flattening toward higher energies due to the inverse Compton component. We obtained satisfactory modelling of the overall spectral energy distributions, from the optical to low energy X-rays, with log-parabolic laws.

We present the results of the BeppoSAX observations of two BL Lacs, OQ 530 and S5 0716+714, as part of a ToO program for the simultaneous observation at radio, optical, X-ray and TeV energies. Both sources are detected in the LECS and MECS, with S5 0716+714 visible also in the PDS band, up to about 60 keV. The X-ray spectra of both sources are better fitted by a double power-law model, with a steep soft X-ray component flattening at harder energies, with breaks at 0.3 and 1.5 keV, respectively. The concave shape of the spectra in both objects is consistent with soft X-rays being produced by the synchrotron and harder X-rays by the inverse Compton processes. Also the X-ray variability properties confirm this scenario, in particular for S5 0716+714 our observation shows variations by about a factor 3 over one hour below 3 keV and no variability above. Their simultaneous broad band energy spectral distributions can be successfully interpreted within the frame of a homogeneous synchrotron and inverse Compton model, including a possible contribution from an external source of seed photons with the different spectral states of S5 0716+714 being reproduced by changing the injected power. The resulting parameters are fully consistent with the two sources being intermediate objects within the "sequence" scenario proposed for blazars., Comment: 10 pages, 8 figures, accepted by A&A

We present the results of recent BeppoSAX observations of the two BL Lac objects OJ 287 and MS 1458+22 in bright optical states and of simultaneous photometric measurements in the bandpasses from I to U . OJ 287 was observed in November 2001 and MS 1458+22 in February of the same year. The X-ray flux of OJ 287 was rather low, F (2-10 keV$) = (1.35\pm0.15)\times 10^{-12}$ erg cm -2 s -1 and its spectrum is well described by a single power law with an energy index $0.45\pm0.08$, flatter than that measured in the optical, equal to $1.53\pm0.03$. The X-ray emission of MS 1458+22 was characterized by a low flux level of F (2-10 keV$) = (7.5\pm0.6)\times 10^{-13}$ erg cm -2 s -1 , with a steeper spectral index of $1.71\pm0.15$, while in the optical it was $0.99\pm0.06$. We show that the broad-band SEDs of these two sources can be well represented by log-parabolic spectral laws and evaluate their parameters. This law, already observed for the synchrotron components of other BL Lac sources, can be explained if the emitting particles are accelerated by some statistical mechanism having a probability of energy gain that is a decreasing function of the energy itself.

We present the results of the optical monitoring of the BL Lac object ON 231 from 1997 to the spring 2001, including the large outburst of spring 1998. After this event the mean source luminosity declined slowly and continued to show changes greater than one magnitude. A simple analysis of the variability time scales in the best-sampled segments of the light curve reveals that the luminosity changes were faster during the burst and slowed down in the subsequent years.

We present the most continuous data base of optical multiband data ever published on the BL Lacertae object ON 231 (W Com). The data have been collected during an intensive and coordinated BVRcIc monitoring campaign carried out in the period from March 1994 to March 1997. During our campaign, the source brightness was at the highest level ever observed. The light curve shows a complex structure, characterized by the pres- ence of three major outbursts having the observed max- ima in March 1995, February 1996, and January 1997, when ON 231 reached its historical maximum (B' 14:2). Variability on time scales from a few hours up to a month have frequently been observed and the light curve seems to be the superposition of many flares with dierent ampli- tudes and time scales. The broad-band optical spectral en- ergy distribution is characterized by a spectral slope which correlates with the flux level. In particular, the higher is the flux the flatter is the spectrum.

We present an atlas of X and -ray pulse pro- les of PSR 0531+21 (Crab pulsar) measured since 1967. This atlas, containing 250 proles, is useful to study in detail the behaviour with time and energy of this impor- tant source and to test models of the emission geometrical pattern. The atlas database is also accessible from Mosaic. An analysis of the energy dependence and time changes of the Crab pulsar light curve (and in particular of the P 2=P 1 ratio), based on the atlas database, is presented.

The results of optical photometry of BL Lac over a time interval of two years in the B, V, R, I l- ters are presented. The source is found for most of the time in a low state. The mean colour indexes are similar to those observed in higher luminosity states, however a flattening of the spectral slope was present during three small amplitude outbursts. A brief comparison with the observations available in the literature and some consid- erations on the physical conditions driving the appearance of emission lines are made.

We present the results of our analysis devoted to the research of sources emitting in the energy bands surveyed by both the Swift-BAT and the Fermi-LAT telescopes. We cross-correlate the Fermi-LAT 1-year point source catalogue (1FGL) of {\gamma}-ray sources and the second Palermo BAT catalogue (2PBC) of hard X-ray sources, establishing a correspondence between sources when their error boxes overlap. We also extract the significance value in the BAT 15-150 keV map, obtained using a dedicated software for the reduction of BAT data, in the direction of the 1FGL sources and take into account those above the significance threshold {\sigma} = 3. We obtain a sample of common sources emitting in both the hard X- and the {\gamma}-ray energy bands and evaluate its content in galactic and extragalactic objects. We assess the fraction of unidentified sources and describe in greater detail the properties of two of them, 1FGL J0137.8+5814 and 1FGL J2056.7+4938, supporting their classification as blazars after the analysis of their broad-band spectral energy distribution. We discuss the blazar content of the collected 1FGL-2PBC sources: we build its redshift distibution and compare it with that of the whole blazar population as reported in the second edition of the BZCAT blazar catalogue., Comment: 12 pages, 5 figures, 5 tables