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 X1036 erg s-1 in the 20-100 keV energy range. This low outburst luminosity and the long-term time-average accretion rate of ~5 X 10-12 M 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 t~ 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 X 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 (a~ 73) allow us to conclude that the burst fuel was composed by mixed hydrogen and helium with X >= 0.4.
After 25 years of quiescence, the microquasar V404 Cyg entered a new period of activity in June 2015. This X-ray source is known to undergo extremely bright and variable outbursts seen at all ...wavelengths. It is therefore an object of prime interest to understand the accretion-ejection connections. These can, however, only be probed through simultaneous observations at several wavelengths. We made use of the INTEGRAL instruments to obtain long, almost uninterrupted observations from 2015 June 20, 15:50 UTC to June 25, 4:05 UTC, from the optical V band up to the soft γ-rays. V404 Cyg was extremely variable in all bands, with the detection of 18 flares with fluxes exceeding 6 Crab (20–40 keV) within three days. The flare recurrence can be as short as ~20 min from peak to peak. A model-independent analysis shows that the >6 Crab flares have a hard spectrum. A simple 10–400 keV spectral analysis of the off-flare and flare periods shows that the variation in intensity is likely to be only due to variations of a cut-off power-law component. The optical flares seem to be at least of two different types: one occurring in simultaneity with the X-ray flares, the other showing a delay greater than 10 min. The former could be associated with X-ray reprocessing by either an accretion disk or the companion star. We suggest that the latter are associated with plasma ejections that have also been seen in radio.
We present the study of one year of INTEGRAL data on the neutron star low mass X-ray binary GX 5–1. Thanks to the excellent angular resolution and sensitivity of INTEGRAL, we are able to obtain a ...high quality spectrum of GX 5–1 from ~5 keV to ~100 keV, for the first time without contamination from the nearby black hole candidate GRS 1758-258 above 20 keV. During our observations, GX 5–1 was mostly found in the horizontal and normal branch of its hardness intensity diagram. A clear hard X-ray emission is observed above ~30 keV which exceeds the exponential cut-off spectrum expected from lower energies. This spectral flattening may have the same origin of the hard components observed in other Z sources as it shares the property of being characteristic to the horizontal branch. The hard excess is explained by introducing Compton up-scattering of soft photons from the neutron star surface due to a thin hot plasma expected in the boundary layer. The spectral changes of GX 5–1 downward along the “Z” pattern in the hardness intensity diagram can be well described in terms of monotonical decrease of the neutron star surface temperature. This may be a consequence of the gradual expansion of the boundary layer as the mass accretion rate increases.
INTEGRAL has observed the bright quasar 3C 273 on 3 epochs in January 2003 as one of the first observations of the open programme. The observation on January 5 was simultaneous with RXTE and ...XMM-Newton observations. We present here a first analysis of the continuum emission as observed by these 3 satellites in the band from $\simeq$3 keV to $\simeq$500 keV. The continuum spectral energy distribution of 3C 273 was observed to be weak and steep in the high energies during this campaign. We present the actual status of the cross calibrations between the instruments on the three platforms using the calibrations available in June 2003.
Seventeen years of hard X-ray observations with the instruments of the INTEGRAL observatory, with a focus on the Milky Way and in particular on the Galactic Centre region, have provided a unique ...database for exploration of the Galactic population of low-mass X-ray binaries (LMXBs). Our understanding of the diverse energetic phenomena associated with accretion of matter onto neutron stars and black holes has greatly improved. We review the large variety of INTEGRAL based results related to LMXBs. In particular, we discuss the spatial distribution of LMXBs over the Galaxy and their X-ray luminosity function as well as various physical phenomena associated with Atoll and Z sources, bursters, symbiotic X-ray binaries, ultracompact X-ray binaries and persistent black hole LMXBs. We also present an up-to-date catalogue of confirmed LMXBs detected by INTEGRAL, which comprises 166 objects. Last but not least, the long-term monitoring of the Galactic Centre with INTEGRAL has shed light on the activity of Sgr A* in the recent past, confirming previous indications that our supermassive black hole experienced a major accretion episode just ~ 100 years ago. This exciting topic is covered in this review too.
The remnant of the supernova of 1006 AD, the remnant first showing evidence for the presence of X-ray synchrotron emission from shock-accelerated electrons, was observed for 61000 ks with INTEGRAL in ...order to study electron acceleration to very high energies. The aim of the observation was to characterize the synchrotron emission and attempt to detect nonthermal bremsstrahlung using the combination of IBIS and JEM-X spatial and spectral coverage. The source was detected with JEM-X between the 2.4 and 8.4 keV bands and was not detected with either ISGRI or SPI above 20 keV. The ISGRI upper limit is about a factor of 4 above current model predictions, but confirms the presence of steepening in the power law extrapolated from lower energies (<4 keV).
We report the discovery with the Neutron Star Interior Composition Explorer (NICER) of narrow emission and absorption lines during photospheric radius expansion (PRE) X-ray bursts from the ...ultracompact binary 4U 1820 −30. NICER observed 4U 1820−30 in 2017 August during a low-flux, hard spectral state, accumulating about 60 ks of exposure. Five thermonuclear X-ray bursts were detected, of which four showed clear signs of PRE. We extracted spectra during the PRE phases and fit each to a model that includes a Comptonized component to describe the accretion-driven emission, and a blackbody for the burst thermal radiation. The temperature and spherical emitting radius of the fitted blackbody are used to assess the strength of PRE in each burst. The two strongest PRE bursts (burst pair 1) had blackbody temperatures of ≈0.6 keV and emitting radii of ≈100 km (at a distance of 8.4 kpc). The other two bursts (burst pair 2) had higher temperatures (≈0.67 keV) and smaller radii (≈75 km). All of the PRE bursts show evidence of narrow line emission near 1 keV. By coadding the PRE phase spectra of burst pairs 1 and, separately, 2, we find, in both coadded spectra, significant, narrow, spectral features near 1.0 (emission), 1.7, and 3.0 keV (both in absorption). Remarkably, all the fitted line centroids in the coadded spectrum of burst pair 1 appear systematically blueshifted by a factor of 1.046±0.006 compared to the centroids of pair 2, strongly indicative of a gravitational shift, a wind-induced blueshift, or more likely some combination of both effects. The observed shifts are consistent with this scenario in that the stronger PRE bursts in pair 1 reach larger photospheric radii, and thus have weaker gravitational redshifts, and they generate faster outflows, yielding higher blueshifts. We discuss possible elemental identifications for the observed features in the context of recent burst-driven wind models.
The Crab pulsar is the best studied rotation powered pulsar. We report the results obtained in the 3–35 keV energy band with the X-ray monitor, JEM–X, on ESAs recently launched γ-ray mission, ...INTEGRAL.
We summarize the inflight performance of JEM–X, the X–ray monitor on the INTEGRAL mission during the initial ten months of operations. The JEM–X instruments have now been tuned to stable operational ...conditions. The performance is found to be close to the pre-launch expectations. The ground calibrations and the inflight calibration data permit to determine the instruments characteristics to fully support the scientific data analysis.
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