M. Del Santo

The INTEGRAL IBIS/ISGRI System Point Spread Function and Source Location Accuracy ?

The imager on board INTEGRAL (IBIS) presently provides the most detailed sky images ever obtained at energies above 30 keV. The telescope is based on a coded aperture imaging system which allows to obtain sky images in a large field of view (29 29) with an angular resolution of 12 0 . The System Point Spread Function of the telescope and its detailed charac- teristics are here described along with the specific analysis algorithms used to derive the accurate point-like source locations. The derived location accuracy is studied using the first in-flight calibration data on strong sources for the IBIS /ISGRI system. The dependence of the calibrated location accuracy with the signal to noise ratio of the sources is presented. These preliminary studies demonstrate that the IBIS/ISGRI telescope and the standard scientific analysis software allow source localizations with accuracy at 90% confidence level better than 1 0 for sources with signal to noise ratios >30 over the whole field of view, in agreement with the expected performances of the instrument.

INTEGRAL/RXTE high-energy observation of a state transition of GX 339–4

On 2004 August 15, we observed a fast (shorter than 10 h) state transition in the bright black hole transient GX 339-4 simultaneously with Rossi X-Ray Timing Explorer (RXTE) and INTEGRAL. This transition was evident both in timing and spectral properties. Combining the data from the Proportional Counter Array (PCA), the High-Energy X-ray Timing Experiment (HEXTE) and the Imager on Board the INTEGRAL Satellite (IBIS), we obtained good quality broad-band (3-200 keV) energy spectra before and after the transition. These spectra indicate that the hard component steepened. Also, the high-energy cut-off that was present at ∼70 keV before the transition was not detected after the transition. This is the first time that an accurate determination of the broad-band spectrum across such a transition has been measured on a short time-scale. It shows that, although some spectral parameters do not change abruptly through the transition, the high-energy cut-off increases/disappears rather fast. These results constitute a benchmark on which to test theoretical models for the production of the hard component in these systems.

Spectral variability of GX 339−4 in a hard-to-soft state transition★

We report on INTEGRAL observations of the bright black hole transient GX 339−4 performed during the period 2004 August–September, including the fast transition (10 h) observed simultaneously with INTEGRAL and RXTE on August 15 and previously reported. Our data cover three different spectral states, namely hard/intermediate state (HIMS), soft/intermediate state (SIMS) and high/soft state (HSS). We investigate the spectral variability of the source across the different spectral states. The hard X-ray spectrum becomes softer during the HIMSto-SIMS transition, but it hardens when reaching the HSS. A principal component analysis demonstrates that most of the variability occurs through two independent modes: a pivoting of the spectrum around 6 keV (responsible for 75 per cent of the variance) and an intensity variation of the hard component (responsible for 21 per cent). The pivoting is interpreted as due to changes in the soft cooling photon flux entering the corona, the second mode as fluctuations of the heating rate in the corona. These results are very similar to those previously obtained for Cygnus X-1. Our spectral analysis of the spectra of GX 339−4 shows a high energy excess with respect to pure thermal Comptonization models in the HIMS: a non-thermal power-law component seems to be requested by data. In all spectral states joint IBIS, SPI and JEM-X data are well represented by hybrid thermal/non-thermal Comptonization (EQPAIR). These fits allow us to track the evolution of each spectral component during the spectral transition. The spectral evolution seems to be predominantly driven by a reduction of the ratio of the electron heating rate to the soft cooling photon flux in the corona, lh/ls. The inferred accretion disc soft thermal emission increases by about two orders of magnitude, while the Comptonized luminosity decreases by at most a factor of 3. This confirms that the softening we observed is due to a major increase in the flux of soft cooling photons in the corona associated with a modest reduction of the electron heating rate.

GAMMA-RAY OBSERVATIONS OF CYGNUS X-1 ABOVE 100 MeV IN THE HARD AND SOFT STATES

We present the results of multi-year gamma-ray observations by the AGILE satellite of the black hole binary system Cygnus X-1. In a previous investigation we focused on gamma-ray observations of Cygnus X-1 in the hard state during the period mid-2007/2009. Here we present the results of the gamma-ray monitoring of Cygnus X-1 during the period 2010/mid-2012 which includes a remarkably prolonged “soft state” phase (2010 June–2011 May). Previous 1–10 MeV observations of Cyg X-1 in this state hinted at a possible existence of a nonthermal particle component with substantial modifications of the Comptonized emission from the inner accretion disk. Our AGILE data, averaged over the mid-2010/mid-2011 soft state of Cygnus X-1, provide a significant upper limit for gamma-ray emission above 100 MeV of Fsoft < 20 × 10 −8 photons cm −2 s −1 , excluding the existence of prominent non-thermal emission above 100 MeV during the soft state of Cygnus X-1. We discuss theoretical implications of our findings in the context of high-energy emission models of black hole accretion. We also discuss possible gamma-ray flares detected by AGILE. In addition to a previously reported episode observed by AGILE in 2009 October during the hard state, we report a weak but important candidate for enhanced emission which occurred at the end of 2010 June (2010 June 30 10:00–2010 July 2 10:00 UT) exactly coinciding with a hard-to-soft state transition and before an anomalous radio flare. An appendix summarizes all previous high-energy observations and possible detections of Cygnus X-1 above 1 MeV.

UNVEILING THE HIGH ENERGY TAIL OF 1E 1740.7-2942 WITH INTEGRAL *

The microquasar 1E 1740.7-2942 is observed with INTEGRAL since Spring 2003. Here, we report on the source high-energy behavior by using the first three years of data collected with SPI and IBIS telescopes, taking advantage of the instruments complementarity. Light curves analysis showed two main states for 1E 1740.7-2942: the canonical low/hard state of black hole candidates (BHCs) and a dim state, characterized by an ~20 times fainter emission, detected only below 50 keV and when summing more than 1 Ms of data. For the first time the continuum of the low/hard state has been measured up to ~600 keV with a spectrum that is well represented by a thermal Comptonization plus an additional component necessary to fit the data above 200 keV. This high-energy component could be related to nonthermal processes as already observed in other BHCs. Alternatively, we show that a model composed of two thermal Comptonizations provides an equally representative description of the data: the temperature of the first population of electrons results as (kT e)1~ 30 keV while the second, (kT e)2, is fixed at 100 keV. Finally, searching for 511 keV line showed no feature, either narrow or broad, transient or persistent.

First results from the INTEGRAL galactic plane scans

Scans of the Galactic plane performed at regular intervals constitute a key element of the guaranteed time obser- vations of the INTEGRAL observing programme. These scans are done for two reasons: frequent monitoring of the Galactic plane in order to detect transient sources, and time resolved mapping of the Galactic plane in continuum and diuse line emission. This paper describes first results obtained from the Galactic plane scans executed so far during the early phase (Dec. 2002-May 2003) of the nominal mission.

3-200 keV spectral states and variability of the INTEGRAL black hole binary IGR J17464-3213

On March 2003, IBIS, the gamma-ray imager on board the INTEGRAL satellite, detected an outburst from a new source, IGR J17464-3213, that turned out to be a HEAO 1 transient, H1743-322. In this paper we report on the high-energy behavior of this black hole candidate (BHC) studied with the three main instruments on board INTEGRAL. The data, collected with unprecedented sensitivity in the hard X-ray range, show a quite hard Comptonized emission from 3 up to 150 keV during the rising part of the source outburst, with no thermal emission detectable. A few days later, a prominent soft-disk multicolor component appears, with the hard tail luminosity almost unchanged: ~5 × 10-9 ergs cm-2 s-1. Two months later, during a second monitoring campaign near the end of the outburst, the observed disk component was unchanged. Conversely, the Comptonized emission from the central hot part of the disk reduced by a factor of ~10. We present here its long-term behavior in different energy ranges and the combined JEM-X, SPI, and IBIS wideband spectral evolution of this source.

High-energy observations of the state transition of the X-ray nova and black hole candidate XTE J1720-318

We report the results of extensive high-energy observations of the X-ray transient and black hole candidate XTE J1720-318 performed with INTEGRAL, XMM-Newton and RXTE. The source, which underwent an X-ray outburst in 2003 January, was observed in February in a spectral state dominated by a soft component with a weak high-energy tail. The XMM-Newton data provided a high column density Nh of 1.2*e22 cm^{-2} which suggests that the source lies at the Galactic Centre distance. The simultaneous RXTE and INTEGRAL Target of Opportunity observations allowed us to measure the weak and steep tail, typical of a black-hole binary in the so-called High/Soft State. We then followed the evolution of the source outburst over several months using the INTEGRAL Galactic Centre survey observations. The source became active again at the end of March: it showed a clear transition towards a much harder state, and then decayed to a quiescent state after April. In the hard state, the source was detected up to 200 keV with a power law index of 1.9 and a peak luminosity of 7*e36 erg s^{-1} in the 20-200 keV band, for an assumed distance of 8 kpc. We conclude that XTE J1720-318 is indeed a new member of the black hole X-ray novae class which populate our galactic bulge and we discuss its properties in the frame of the spectral models used for transient black hole binaries.

The magnetic field in the X-ray corona of Cygnus X-1

The different electron distributions in the hard and soft spectral states (HS and SS) of BH binaries could be caused by kinetic processes and changing because of varying physical conditions in the corona. In presence of a magnetic field in the corona, the electron distribution can appear thermal, even when acceleration mechanisms would produce non thermal distributions. This is due to fast and efficient thermalization through synchrotron self-absorption. We have analyzed data from 6 years of observations of Cygnus X-1 with the INTEGRAL observatory and produced 12 high-quality, stacked broad-band hard X-ray spectra representative of the whole range of spectral shapes observed. We then fit these spectra with hybrid thermal/non-thermal Comptonization models and study the evolution of the physical parameters of the accretion flow across the spectral transition. In particular, we use the BELM model to constrain the magnetic field in the corona through its effects on the coronal emission. Indeed, the hot electrons of the X-ray corona produce soft (optical-UV) synchrotron radiation which is then Comptonized and may affect the temperature of the electrons through Compton cooling. We find that in the SS, the emission is dominated by Comptonization of the disc photons and the magnetic field is at most of the order of 1E+06 G. In the hard states, the data are consistent with a pure synchrotron self-Compton model. If the non-thermal excess observed above a few hundred keV in the HS is produced in the same region as the bulk of the thermal Comptonization, we obtain an upper limit on the coronal magnetic field of about 1E+05 G. If, on the other hand, the non-thermal excess is produced in a different location, the constraints on the magnetic field in the HS are somewhat relaxed and the upper limit rises to 1E+07 G. We discuss these constraints in the context of current accretion flow models.

Two years of INTEGRAL monitoring of the Soft Gamma-Ray Repeater SGR 1806-20: from quiescence to frenzy

SGR 1806-20 has been observed for more than 2 years with the INTEGRAL satellite. In this period the source went from a quiescent state into a very active one culminating in a giant flare on December 27, 2004. Here we report on the properties of all the short bursts detected with INTEGRAL before the giant flare. We derive their number-intensity distribution and confirm the hardness-intensity correlation for the bursts found by Gotz et al. (2004a, A&A, 417, L45). Our sample includes a very bright outburst that occurred on October 5, 2004, during which over one hundred bursts were emitted in 10 minutes, involving an energy release of 3x1042 erg. We present a detailed analysis of it and discuss our results in the framework of the magnetar model.