Antonella Tarana

Simultaneous Multiwavelength Observations of the Low/Hard State of the X-Ray Transient Source SWIFT J1753.5–0127

We report the results of simultaneous multiwavelength observations of the X-ray transient source SWIFT J1753.5-0127 performed with INTEGRAL, RXTE, NTT, REM, and VLA on 2005 August 10-12. The source, which underwent an X-ray outburst since 2005 May 30, was observed during the INTEGRAL Target of Opportunity program dedicated to new X-ray novae located in the Galactic halo. Broadband spectra and fast timing variability properties of SWIFT J1753.5-0127 are analyzed together with the optical, near-infrared, and radio data. We show that the source was significantly detected up to 600 keV with Comptonization parameters and timing properties typical of the so-called low/hard state of black hole candidates. We build a spectral energy distribution and show that SWIFT J1753.5-0127 does not follow the usual radio/X-ray correlation of X-ray binaries in the low/hard state. We give estimates of distance and mass. We conclude that SWIFT J1753.5-0127 belongs to the X-ray nova class and that it is likely a black hole candidate transient source of the Galactic halo that remained in the low/hard state during its main outburst. We discuss our results in the context of Comptonization and jet models.

Spectral evolution of the Atoll source 4U 1728-34 with RXTE and INTEGRAL: evidence for hard X-ray tail

We report the temporal and spectral results on the INTEGRAL and RXTE 2006–2007 observation campaign of the Atoll source 4U 1728–34 (GX 354‐0). The source shows, more than once, spectral evolution as revealed by the hardness intensity diagram. The soft state is well described by a Comptonization with an electron temperature of 3 keV and a high optical depth of 6. In the hard spectral state the emission extends to above 100 keV and it can be described by Comptoniziation (with higher temperature of 10 keV) plus a power law component with Γ of 1.8, which is evidence for non‐thermal emission from the source.

Spectral States of the X-Ray Binary IGR J17091–3624 Observed by INTEGRAL and RXTE

IGR J17091-3624 was discovered in 2003 April by IBIS, the gamma-ray imager on board the INTEGRAL satellite, during its Galactic Center Deep Exposure program. The source was initially detectable only in the 40-100 keV range but after 2 days was also detected in the 15-40 keV range. Its flux had by then increased to 40 and 25 mcrab in the 15-40 and 40-100 keV bands, respectively. RXTE observed the source simultaneously on 2003 April 20, with an effective exposure of 2 ks. We report here the spectral and temporal evolution of the source, which shows a transition between the hard and soft states. We analyze in detail the RXTE/INTEGRAL Comptonized spectrum of the hard state, as well as the JEM-X detection of a blackbody component during the source softening. Even though the source spectral behavior and time variability show a similarity with the outburst of the black hole candidate IGR J17464-3213 (=H1743-322), observed by INTEGRAL in 2003, the nature of its compact object (BH vs. NS) remains controversial.

THE TWO INTEGRAL X-RAY TRANSIENTS IGR J17091–3624 AND IGR J17098–3628: A MULTIWAVELENGTH LONG-TERM CAMPAIGN

IGR J17091–3624 and IGR J17098–3628 are two X-ray transients discovered by INTEGRAL and classified as possible black hole candidates. We present here the results obtained from the analysis of multiwavelength data sets collected by different instruments from 2005 until the end of 2007 on both sources. IGR J17098–3628 has been regularly detected by INTEGRAL and RXTE over the entire period of the observational campaign; it was also observed with pointed observations by XMM-Newton and Swift/X-ray Telescope (XRT) in 2005 and 2006 and exhibited flux variations not linked with the change of any particular spectral features. IGR J17091 – 3624 was initially in quiescence (after a period of activity between 2003 April and 2004 April) and it was then detected again in outburst in the XRT field of view during a Swift observation of IGR J17098–3628 on 2007 July 9. The observations during quiescence provide an upper limit to the 0.2-10 keV luminosity, while the observations in outburst cover the transition from the hard to the soft state. Moreover, we obtain a refined X-ray position for IGR J17091–3624 from the Swift/XRT observations during the outburst in 2007. The new position is inconsistent with the previously proposed radio counterpart. We identify in VLA archive data a compact radio source consistent with the new X-ray position and propose it as the radio counterpart of the X-ray transient.

INTEGRAL* Spectral Variability Study of the Atoll 4U 1820–30: First Detection of Hard X-Ray Emission

We study the 4-200 keV spectral and temporal behavior of the low-mass X-ray binary 4U 1820-30 with INTEGRAL during 2003-2005. This source as been observed in both the soft (banana) and hard (island) spectral states. A high-energy tail, above 50 keV, in the hard state has been observed for the first time. This places the source in the category of X-ray bursters showing high-energy emission. The tail can be modeled as a soft power-law component, with the photon index of 2.4, on top of thermal Comptonization emission from a plasma with electron temperature kTe 6 keV and optical depth τ 4. Alternatively, but at a poorer goodness of fit, the hard-state broadband spectrum can be accounted for by emission from a hybrid, thermal-nonthermal, plasma. During this monitoring the source spent most of the time in the soft state, usual for this source, and the 4 keV spectra are described by thermal Comptonization with kTe 3 keV and τ 6-7.

Spectral states evolution of 4U 1728-34 observed by INTEGRAL and RXTE: non-thermal component detection

We report results of a one-year monitoring of the low-mass X-ray binary (LMXB) source (atoll type) 4U 1728-34 with INTEGRAL and RXTE. Three time intervals were covered by INTEGRAL, during which the source showed strong spectral evolution. We studied the broad-band X-ray spectra in detail by fitting several models in the different sections of the hardness-intensity diagram. The soft states are characterized by prominent blackbody emission plus a contribution from a Comptonized emission. The hard states are characterized by the presence of an excess flux with respect to the Comptonization model above 50 keV, while the soft component is fainter. To obtain an acceptable fit to the data, this excess is modelled either with a power law with photon index Gamma similar to 2 or a Comptonization (CompPS) spectrum implying the presence of hybrid thermal and non-thermal electrons in a corona. This makes 4U 1728-34 one of the few LMXBs of atoll type showing non-thermal emission at high energy. From our analysis, it is also apparent that the presence of the hard tail is more prominent as the overall spectrum becomes harder. We also discuss alternative models which can describe these hard states.

INTEGRAL and BeppoSAX Observations of the Transient Atoll Source 4U 1608–522: From Quiescent to Hard Spectral State

We report on the spectral evolution of 4U 1608–522 using observations performed as part of the long-term Galactic bulge monitoring program with INTEGRAL. The data set includes the 2005 April outburst. BeppoSAX archival data (two observations, from 1998 and 2000) are also analyzed and compared with the INTEGRAL data. Three different spectral states have been identified from the hardness-intensity diagram derived from INTEGRAL: the canonical hard and soft states, as well as an intermediate state. The hard-state spectrum is well described by a weak blackbody component plus a Comptonized plasma with high electron temperature (kTe 60 keV) extending up to 200 keV without any additional cutoff. The soft spectra are characterized by a cold Comptonized plasma (kTe = 2-3 keV; 7 keV for the intermediate state) and a strong disk blackbody component. A reflection component, indicating reflection of the X-rays from the accretion disk, is also present in the soft state seen by BeppoSAX in 1998. The 2000 BeppoSAX observation reveals the source to have been in its quiescent state, which can be modeled with a neutron star atmosphere (assuming a neutron star radius of 10 km and mass 1.4 M☉) with an effective temperature kTeff of 0.1 keV, plus a power-law component with Γ ~ 3, detected for the first time in this source. This spectrum can also be modeled with a simple blackbody compatible with emission originating from a small fraction of the neutron stars surface, of radius 0.4 km.

INTEGRAL high-energy monitoring of the X-ray burster KS 1741−293★

KS 1741-293, discovered in 1989 by the X-ray camera TTM on the Kvant module of the Mir space station and identified as an X-ray burster, had not been detected in the hard X-ray band until the advent of the INTEGRAL observatory. Moreover, this source has recently been the object of scientific discussion, being also associated with a nearby extended radio source that in principle could be the supernova remnant produced by the accretion-induced collapse in the binary system. Our long-term monitoring with INTEGRAL, covering the period from 2003 February to 2005 May, confirms that KS 1741-293 is transient in the soft and hard X-ray bands. When the source is active, from a simultaneous JEM-X and IBIS data analysis, we provide a wide-band spectrum from 5 to 100 keV, which can be fitted by a two-component model: a multiple blackbody for the soft emission and a Comptonized or a cut-off power-law model for the hard component. Finally, by the detection of two X-ray bursters with JEM-X, we confirm the bursting nature of KS 1741-293, including this source in the class of hard-tailed X-ray bursters.

Renewed activity from the X-ray transient SAXJ 1810.8-2609 with INTEGRAL

We report on the results of International Gamma-Ray Astrophysics Laboratory (INTEGRAL) observations of the neutron star low-mass X-ray binary SAX J1810.8-2609 during its latest active phase in 2007 August. The current outburst is the first one since 1998 and the derived luminosity is 1.1-2.6 ×1036 erg s-1 in the 20-100 keV energy range. This low outburst luminosity and the long-term time-average accretion rate of ~5 × 10-12 M sun yr-1 suggest that SAX J1810.8-2609 is a faint soft X-ray transient. During the flux increase, spectra are consistent with a thermal Comptonization model with a temperature plasma of kT e~ 23-30 keV and an optical depth of τ~ 1.2-1.5, independent of the luminosity of the system. This is a typical low hard spectral state for which the X-ray emission is attributed to the upscattering of soft seed photons by a hot, optically thin electron plasma. During the decay, spectra have a different shape, the high energy tail being compatible with a single power law. This confirm similar behavior observed by BeppoSAX during the previous outburst, with the absence of visible cutoff in the hard X-ray spectrum. INTEGRAL/JEM-X instrument observed four X-ray bursts in Fall 2007. The first one has the highest peak flux (≈3.5 crab in 3-25 keV) giving an upper limit to the distance of the source of about 5.7 kpc, for a L Edd ≈ 3.8 × 1038 erg s-1. The observed recurrence time of ~ 1.2 days and the ratio of the total energy emitted in the persistent flux to that emitted in the bursts (α~ 73) allow us to conclude that the burst fuel was composed by mixed hydrogen and helium with X >= 0.4. INTEGRAL is an ESA project with Instruments and Science Data Center funded by ESA member states, especially the PI countries: Denmark, France, Germany, Italy, Switzerland, Spain; Czech Republic and Poland; and with the participation of Russia and USA.

Sub‐luminous X‐ray Bursters Unveiled with INTEGRAL

In 2005 March 22nd, the INTEGRAL satellite caught a type‐I X‐ray burst from the unidentified source XMMU J174716.1–281048, serendipitously discovered with XMM‐Newton in 2003. Based on the type‐I X‐ray burst properties, we derived the distance of the object and suggested that the system is undergoing a prolonged accretion episode of many years. We present new data from a Swift/XRT campaign which strengthen this suggestion. AX J1754.2–2754 was an unclassified source reported in the ASCA catalogue of the Galactic Centre survey. INTEGRAL observed a type‐I burst from it in 2005, April 16th. Recently, a Swift ToO allowed us to refine the source position and establish its persistent nature.