INTEGRAL Soft Gamma-Ray Imager Research Articles

Variability of the symbiotic X-ray binary GX 1+4

Context. GX 1+4 belongs to a rare class of X-ray binaries with red giant donors, symbiotic X-ray binaries. The system has a history of complicated variability on multiple timescales in the optical light and X-rays. The nature of this variability remains poorly understood. Aims. We study variability of GX 1+4 on long time-scale in X-ray and optical bands. Methods. The presented X-ray observations are from INTEGRAL Soft Gamma-Ray Imager and RXTE All Sky Monitor. The optical observations are from INTEGRAL Optical Monitoring Camera. Results. The variability of GX 1+4 both in optical light and hard X-ray emission (>17 keV) is dominated by ~50-70d quasi-periodic changes. The amplitude of this variability is highest during the periastron passage, while during the potential neutron star eclipse the system is always at minimum, which confirms the 1161d orbital period that has had been proposed for the system based on radial velocity curve. Neither the quasi-periodic variability or the orbital period are detected in soft X-ray emission (1.3-12.2 keV), where the binary shows no apparent periodicity.

Timing studies of X Persei and the discovery of its transient quasi-periodic oscillation feature

We present a timing analysis of X Persei (X Per) using observations made between 1998 and 2010 with the Proportional Counter Array (PCA) onboard the Rossi X-ray Timing Explorer (RXTE) and with the INTEGRAL Soft Gamma-Ray Imager (ISGRI). All pulse arrival times obtained from the RXTE-PCA observations are phase-connected and a timing solution is obtained using these arrival times. We update the long-term pulse frequency history of the source by measuring its pulse frequencies using RXTE-PCA and ISGRI data. From the RXTE-PCA data, the relation between the frequency derivative and X-ray flux suggests accretion via the companion's stellar wind. However, the detection of a transient quasi-periodic oscillation feature, peaking at ∼0.2 Hz, suggests the existence of an accretion disc. We find that double-break models fit the average power spectra well, which suggests that the source has at least two different accretion flow components dominating the overall flow. From the power spectrum of frequency derivatives, we measure a power-law index of ∼− 1, which implies that, on short time-scales, disc accretion dominates over noise, while on time-scales longer than the viscous time-scales, the noise dominates. From pulse profiles, we find a correlation between the pulse fraction and the count rate of the source.

A new survey technique at hard X-rays

In the view of the so-called Active Galactic Nuclei (AGN) unified model, the nuclear activity is powered by a super-massive black hole (SMBH) at the center of a galaxy. An optically thick gas-dust structure surrounding the SMBH absorbs efficiently the nuclear radiation. Current surveys of the INTEGRAL Soft-Gamma Ray Imager (IBIS/ISGRI) and of the Burst Alert Telescope (BAT) on board the Swift satellite have proven to be a powerful technique in detecting AGNs at soft gamma-ray energies. BAT and IBIS/ISGRI are coded-mask detectors having two major advantages: 1) they have a huge field of view, hence allowing to detect an large number of AGNs; 2) they operate at soft gamma-rays, hence collecting photons that are energetic enough to pierce efficiently through the gas-dust structure surrounding the SMBH. However, the sensitivity of coded-mask detectors is limited by heavy systematic uncertainties. Furthermore by design they block ∼50% of the incident photons causing an increase of statistical noise. This prevents the detection of the most absorbed AGNs. In this paper we will show that the BAT and the IBIS/ISGRI observations can be merged in order to produce a deep sky survey less susceptible to systematic and statistical noise. The survey of this virtual new mission extends over a wide sky area of 6200 deg2 sampling 113 sources. The scientific outcome from the study of the sample has been properly addressed to study the evolution of AGNs in the local Universe. Our results are compared to other missions and predictions on the results of the recently launched NuSTAR mission are made. We discuss the expected results when applying this survey to the entire extragalactic sky and first results of this survey on the Galactic center are shown.

INTEGRAL: In flight behavior of ISGRI and SPI

The payload of INTEGRAL, the space gamma-ray observatory launched in October 2002, features two gamma-ray telescopes that take advantage of the semiconductor technologies. The spectrometer SPI, is equipped with 19 high-purity germanium detectors cooled at 85 K. We will report on the SPI in-flight background, performance, the detector evolution and the annealings performed every 6 months. The INTEGRAL Soft Gamma-Ray Imager (ISGRI) is the low-energy camera of the IBIS telescope. It is the first large camera equipped with CdTe detectors. We will present some system aspects, in particular the noisy pixel handling and will report on its in-flight background, performance and their evolution.

Qualification model of the space ISGRI CdTe gamma-ray camera

A new generation of large area semiconductor based γ-ray camera for astrophysics space application has been developed through the γ-ray IBIS (Imager on Board INTEGRAL Satellite) telescope program, on board the ESA INTEGRAL satellite. This instrument features a coded-mask aperture imaging system, active and passive shields and two detector arrays. The first one (ISGRI : INTEGRAL Soft Gamma-Ray Imager) is made of 16384 CdTe detectors (4×4 mm large, 2 mm thick) operating at room temperature and will operate between 15 keV and 1 MeV. The second detector array (PICsIT : Pixelated CsI Telescope) is made of 4096 CsI scintillators (9.2×9.2 mm large, 30 mm high) coupled to silicon PIN photodiodes working between 150 keV and 10 MeV. IBIS combine broad band energy, spectroscopic capabilities and high spatial resolution imaging (12 arcmin) over a wide field of view (∼20°×20°, 50% sensitivity). A “flight-like” model of the detectors has been built, integrated and successfully tested. After a description of the IBIS imaging principle, we will report results obtained with the qualification model of the ISGRI camera.