Ada Paizis

A New Comptonization Model for Weakly Magnetized, Accreting Neutron Stars in Low-Mass X-Ray Binaries

We have developed a new model for the X-ray spectral fitting package XSPEC that takes into account the effects of both thermal and dynamical (i.e., bulk) Comptonization. The model consists of two components: one is the direct blackbody-like emission due to seed photons that are not subjected to effective Compton scattering, while the other is a convolution of the Greens function of the energy operator with a blackbody-like seed photon spectrum. When combined thermal and bulk effects are considered, the analytical form of the Greens function may be obtained as a solution of the diffusion equation describing Comptonization. Using data from the BeppoSAX, INTEGRAL, and RXTE satellites, we test our model on the spectra of a sample of six bright neutron star low-mass X-ray binaries with low magnetic fields, covering three different spectral states. Particular attention is given to the transient power-law-like hard X-ray (30 keV) tails, which we interpret in the framework of the bulk motion Comptonization process. We show that the values of the best-fit δ-parameter, which represents the importance of bulk with respect to thermal Comptonization, can be physically meaningful and can at least qualitatively describe the physical conditions of the environment in the innermost part of the system. Moreover, we show that in fitting the thermal Comptonization spectra to the X-ray spectra of these systems, the best-fit parameters of our model are in excellent agreement with those from compTT, a broadly used and well-established XSPEC model.

The discovery outburst of the X-ray transient IGR J17497-2821 observed with RXTE and ATCA

We report the results of a series of RXTE and ATCA observations of the recently discovered X-ray transient IGR J17497-2821. Our 3-200 keV PCA+HEXTE spectral analysis shows very little variations over a period of ~10 days around the maximum of the outburst. IGR J17497-2821 is found in a typical low-hard state (LHS) of X-ray binaries (XRBs), well represented by an absorbed Comptonized spectrum with an iron edge at about 7 keV. The high value of the absorption (~4 × 1022 cm-2) suggests that the source is located at a large distance, either close to the Galactic center or beyond. The timing analysis shows no particular features, while the shape of the power density spectra is also typical of the LHS of XRBs, with ~36% rms variability. No radio counterpart is found down to a limit of 0.21 mJy at 4.80 and 8.64 GHz. Although the position of IGR J17497-2821 in the radio to X-ray flux diagram is well below the correlation usually observed in the LHS of black holes, the comparison of its X-ray properties with those of other sources leads us to suggest that it is a black hole candidate.

The INTEGRAL Galactic bulge monitoring program: the first 1.5 years

Aims.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 the Galactic bulge region regularly and frequently, i.e., about every three days, with the instruments onboard 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. Methods: We describe the successful monitoring program and show the first results from the start of the monitoring up to 2006, April 21, i.e., for a period of about one and a half year, during three visibility seasons. We focus on the short (hour), medium (month) and long-term (year) variability in the hard X-ray bands, i.e., 20-60 keV and 60-150 keV. When available, we discuss the simultaneous observations in the soft X-ray, 3-10 keV and 10-25 keV, bands. Results: Almost all the sources in the Galactic bulge region we detect in the 20-60 keV and 60-150 keV bands are variable. During the last two and a half weeks of the third visibility season most of the known persistent (hard) X-ray sources in the Galactic Center region were not detected. Of our sample of sources, per visibility season we detect 32/33 sources in the 20-60 keV band and 8/9 sources in the 60-150 keV band above a signal to noise of 7. On average, we find per visibility season one active bright (âa†100 mCrab, 20-60 keV) black-hole candidate X-ray transient and three active weaker (âa‰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. During the course of our program 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. Conclusions: 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, as soon as possible after the observations have been performed, at http://isdc.unige.ch/Science/BULGE/. Appendices are only available in electronic form at http://www.aanda.org

Cygnus X-3 in the INTEGRAL era

Active throughout the entire electromagnetic spectrum, Cyg X-3 provides an excellent target for studying the multiwavelength behaviour of an accreting binary system. In this paper we present the results of the first INTEGRAL observations of the source in 2002 December, together with simultaneous RXTE/PCA-HEXTE observations and radio observations by Ryle and RATAN radio telescopes. The X-ray spectra were fitted with a thermal Comptonization model. The radio spectra from RATAN have the shape of a highly self-absorbed synchrotron spectrum with indications of a minor ejection event coinciding with the peak of an X-ray flare on the days before the INTEGRAL observations.

INTEGRAL observations of IGR J11215‐5952: the first Supergiant Fast X‐ray Transient displaying periodic outbursts

The hard X‐ray source IGR J11215‐5952, discovered with INTEGRAL during a brief outburst in 2005, has been proposed as a new member of the class of Supergiant Fast X‐ray Transients. Analysing archival INTEGRAL observations of the source field, we have discovered two previously unnoticed outbursts (in July 2003 and in May 2004), spaced by intervals of ∼330 days, suggesting a possible orbital period. The 5–100 keV spectrum is well described by a cut‐off power law, with a photon index of ∼0.5, and a cut‐off energy ∼15–20 keV, typical of High Mass X‐ray Binaries containing a neutron star. The luminosity is ∼3 × 1036 erg s−1 assuming 6.2 kpc, the distance of the likely optical counterpart, the blue supergiant HD 306414. A fourth outburst was discovered in 2006 with XTE/PCA, 329 days after the third one, confirming the periodic nature of the source outbursts. Follow‐up observations with Swift/XRT refined the source position and confirmed the association with HD 306414. The 5–100 keV spectrum, the recurrent natur...

A New Explanation for the Supergiant Fast X–ray Transients' Outbursts

The physical mechanism responsible for the short outbursts in a recently recognized class of High Mass X‐ray Binaries, the Supergiant Fast X‐ray Transients (SFXTs), is still unknown. Recent observations performed with Swift/XRT, XMM–Newton and INTEGRAL of the 2007 outburst from IGR J11215‐5952, the only SFXT known to exhibit periodic outbursts, suggest a new explanation for the outburst mechanism in this class of transients; the outbursts could be linked to the possible presence of a second wind component in the supergiant companion, in the form of an equatorial wind. The applicability of the model to the short outburst durations of all other Supergiant Fast X‐ray Transients, where a clear periodicity in the outbursts has not been found yet, is discussed. The scenario we are proposing also includes the persistently accreting supergiant High Mass X–ray Binaries.

The INTEGRAL Galactic Bulge monitoring program

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 γ‐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.