Two years of INTEGRAL result on X-ray binaries

INTEGRAL is the ESA lead International Gamma-Ray Astrophysical Laboratory, successfully launched the 17th October 2002 from Baikonur with a Proton vehicle. In view of the high sensitivity of the two γ-ray instruments IBIS and SPI and their capability to provide at the same time image, spectra and time profiles of all the sources in their wide field of view, a key project was approved as “Core Programme” to obtain deep observations of the Galactic Centre (GCDE) and to exploit regular scan of the whole Galaxy Plane since the beginning of the mission. This paper will briefly review the main astrophysical results obtained in the field of high energy Galactic sources with the INTEGRAL/IBIS γ-ray Imager onboard INTEGRAL, and make a comparison with the previous scenario depicted by the BeppoSAX and RXTE results.     

Can a double component outflow explain the X-ray and optical lightcurves of Swift Gamma-Ray Bursts?

An increasing sample of Gamma-Ray Bursts (GRBs) observed by Swift show evidence of ‘chromatic breaks’, i.e. breaks that are present in the X-ray but not in the optical. We find that in a significant fraction of these GRB afterglows the X-ray and the optical emission cannot be produced by the same component. We propose that these afterglow lightcurves are the result of a two-component jet, in which both components undergo energy injection for the whole observation and the X-ray break is due to a jet break in the narrow outflow. Bursts with chromatic breaks also explain another surprising finding, the paucity of late achromatic breaks. We propose a model that may explain the behaviour of GRB emission in both X-ray and optical bands. This model can be a radical and noteworthy alternative to the current interpretation for the ‘canonical’ XRT and UVOT lightcurves, and it bears fundamental implications for GRB physics.     

Probing the nature of short swift bursts via deep INTEGRAL monitoring of GRB 050925

We present results from Swift, XMM-Newton, and deep INTEGRAL monitoring in the region of GRB 050925. This short Swift burst is a candidate for a newly discovered soft gamma-ray repeater (SGR) with the following observational burst properties: (1) galactic plane (b = −0.1°) localization, (2) 150 ms duration, and (3) a blackbody rather than a simple power-law spectral shape (with a significance level of 97%). We found two possible X-ray counterparts of GRB 050925 by comparing the X-ray images from Swift XRT and XMM-Newton. Both X-ray sources show the transient behavior with a power-law decay index shallower than −1. We found no hard X-ray emission nor any additional burst from the location of GRB 050925 in ∼5 ms of INTEGRAL data. We discuss about the three BATSE short bursts which might be associated with GRB 050925, based on their location and the duration. Assuming GRB 050925 is associated with the H_II regions (W 58) at the galactic longitude of l = 70°, we also discuss the source frame properties of GRB 050925.     

Two years of INTEGRAL observation of the BHC IGR J17464-3213

On March 2003, IBIS, the γ-ray imager on board the INTEGRAL satellite, detected an outburst from a new source, IGR J17464-3213, that turned out to be an HEAO-1 transient, namely H1743-322. The spectral and temporal evolutions of the source were observed by INTEGRAL in different periods. Also RXTE observed the source for the first time on 2003 March 29 during a PCA Galactic bulge scan. The source flux decayed below the RXTE PCA sensitivity limit in November 2003, then in April 2004 it was again detected by INTEGRAL. On July 3, 2004 the source was again detected by RXTE/PCA at a 2–10 keV intensity of 16 mCrab and on July 7, reached 69 mCrab. Recently, a new outburst was observed on August 2005. We briefly summarise here the behaviour of the source observed by INTEGRAL from March 2003 to August 2005. The new outbursts of the source and the analysis of all the data collected (now public) give a global view of the spectral and time behaviour of this X-ray transient.     

The interplay between Gamma-Ray Bursts and their environment

Gamma-Ray Bursts (GRBs) are the most energetic and most relativistic phenomenon in the Universe. Understanding the nature of their progenitors has been one of the primary efforts of current research in high energy astrophysics, and their unmatched luminosity and other properties makes them ideal cosmological probes. In this contribution, I review the observational effects resulting from the interaction between the longer wavelength radiation accompanying GRBs and their close environment. In particular, it discusses signatures that, in addition to providing powerful clues on the GRB progenitors, can also shed light on the physical characteristics, such as metallicity and dust content, of the GRB host galaxies.     

Using gamma-ray burst prompt emission to probe relativistic shock acceleration

It is widely accepted that the prompt transient signal in the 10 keV–10 GeV band from gamma-ray bursts (GRBs) arises from multiple shocks internal to the ultra-relativistic expansion. The detailed understanding of the dissipation and accompanying acceleration at these shocks is a currently topical subject. This paper explores the relationship between GRB prompt emission spectra and the electron (or ion) acceleration properties at the relativistic shocks that pertain to GRB models. The focus is on the array of possible high-energy power-law indices in accelerated populations, highlighting how spectra above 1 MeV can probe the field obliquity in GRB internal shocks, and the character of hydromagnetic turbulence in their environs. It is emphasized that diffusive shock acceleration theory generates no canonical spectrum at relativistic MHD discontinuities. This diversity is commensurate with the significant range of spectral indices discerned in prompt burst emission. Such system diagnostics are now being enhanced by the broad-band spectral coverage of bursts by the Fermi Gamma-Ray Space Telescope; while the Gamma-Ray Burst Monitor (GBM) provides key diagnostics on the lower energy portions of the particle population, the focus here is on constraints in the non-thermal, power-law regime of the particle distribution that are provided by the Large Area Telescope (LAT).     

Investigating γ- and X-ray properties of GRBs using multivariate statistics

We propose to study the interrelation between the γ- (Fluence, 1sec Peakflux, duration) and X-ray (early X flux, 24 h X flux, X decay index, X spectral index, X HI column density) properties using the canonical correlation method. Computing the canonical correlations and variables we show that there is a significant interrelation between the γ- and X-ray data. Using the canonical variables from the analysis, we computed their correlations (canonical loadings) with the original ones. The canonical loadings revealed that the γ-ray fluence and the early X-ray flux give the strongest contribution to the correlation in contrast to the X-ray decay index and spectral index. An interesting new result appears to be the strong contribution of the HI column density to the correlation. Accepting the collapsar model of long GRBs this effect may be interpreted as an indication for the ejection of an HI envelope by the progenitor in the course of producing the GRB.     

Modeling the spectral evolution in the decaying tail of gamma-ray bursts observed by Swift

The detailed study of the spectral evolution during the steep decay phase of early X-ray light curves of gamma-ray bursts (GRBs) is a very important task that can give us information on different emission components contributing to the prompt-to-afterglow transition and help to understand the link between these two stages. Time resolved spectral analysis of bright GRBs detected by Swift has shown that a good modeling of the early X-ray light curves can be obtained with Band or cut-off power-law broad band spectra with evolving parameters (e.g., decaying peak energy). We developed a procedure to simultaneously fit the temporal evolution of all the spectral parameters of a GRB during the prompt-to-afterglow transition based on the analysis of the Swift Burst Alert Telescope (BAT) and the Swift X-ray Telescope (XRT) count rate and hardness ratio light curves. The procedure has been tested on GRB 060614 and GRB 090618, two very peculiar bright GRB detected by Swift that show a long exponentially decaying tail with strong softening and peak energy crossing the XRT energy band.     

GRB observed by IBIS/PICsIT in the MeV energy range

We present the preliminary results of a systematic search for GRB and other transients in the publicly available data for the IBIS/PICsIT (0.2–10 MeV) detector on board INTEGRAL. Lightcurves in 2–8 energy bands with time resolution from 1 to 62.5 ms have been collected and an analysis of spectral and temporal characteristics has been performed. This is the nucleus of a forthcoming first catalog of GRB observed by PICsIT.     

Gamma ray bursts: Short vs. long

Short and long GRBs are thought to be two distinct classes based on their different duration and spectrum. Through the spectral analysis of two similarly selected samples of BATSE short and long GRBs, we show that short GRBs are harder than long events, confirming what found from the comparison of their hardness ratio. However, this spectral diversity seems to be due to a harder low energy spectral component of short GRBs, rather than a (slightly higher) peak energy. Interestingly short GRBs have a spectrum which is similar to the spectrum of the emission of the first 1–2 s of long events. We find evidence that short GRBs are inconsistent with the E_peak–E_iso correlation defined by long bursts while they follow the same E_peak–L_iso correlation of long GRBs. These results, coupled to the similar variability timescale of short events and the first seconds of long ones, suggest that a common (or similar) dissipation mechanism could operate in both classes. The difference in the duration would then be due mainly to the central engine lifetime.     

The low energy gamma-ray experiments on GRO in perspective

The NASA Gamma-Ray Observatory, GRO, will carry two instruments for low energy gamma-ray astronomy. The ‘Oriented Scintillation Spectrometer Experiment - OSSE’ represents the latest step in the evolution of collimated detectors. A large detection area, simultaneous source and background observation and rigorous control over systematic errors yield significant improvements in sensitivity over earlier instruments. The ‘Imaging Compton Telescope - COMPTEL’ brings the proven concept of the Compton telescope to the state of the art level. Position sensitive scintillation detectors make it possible to generate sky images with a resolution of about 2° over a f.o.v. of about 1 sr. The complementary nature of these two experiments promises a first in-depth exploration of the sky in a wavelength range which covers the transition from the X-ray sky to the apparently unrelated high energy gamma-ray sky. Possible directions of further evolution of these experiments will be discussed.     

Gamma ray astrophysics and signatures of axion-like particles

We propose that axion-like particles (ALPs) with a two-photon vertex, consistent with all astrophysical and laboratory bounds, may lead to effects in the spectra of high-energy gamma-ray sources detectable by satellite or ground-based telescopes. We discuss two kinds of signatures: (i) a peculiar spectral depletion due to gamma rays being converted into ALPs in the magnetic fields of efficient astrophysical accelerators according to the “Hillas criterion”, such as jets of active galactic nuclei or hot spots of radio galaxies; (ii) an appearance of otherwise invisible sources in the GeV or TeV sky due to back-conversion of an ALP flux (associated with gamma-ray emitters suffering some attenuation) in the magnetic field of the Milky Way. These two mechanisms might also provide an exotic way to avoid the exponential cutoff of very high energy gamma-rays expected due to the pair production onto the extragalactic background light.     

Progress on SVOM Satellite Development

SVOM (Space-based multiband astronomical Variable Objects Monitor) is a Chinese-French space mission dedicated to studying Gamma-Ray Bursts. The satellite has four instruments to detect and localize the prompt GRB emission and measure the evolution of the afterglow in the visible band and in X-rays, and a VHF communication system enabling the fast transmission of SVOM alerts to the ground. The ground segment includes an array of wide-angle cameras and two follow-up telescopes. It was planned to be in orbit in 2021, and now has to be delayed about six months because of COVID-19 epidemic.      

Results of a search for a new class of GRBS in the SMM data

We present here the results of a search for the soft and short Gamma-Ray Bursts (GRBs) in the data of the Hard X-Ray Burst Spectrometer (HXRBS) on the Solar Maximum Mission (SMM) that constitute a new event series discovered by Hurley /1/ and Laros /2/. Data for four events are presented, including their time profiles and spectral characteristics. In one case the instrument time resolution reveals a total burst duration of 55 ms with rise and decay times of ⩽ 5 ms.     

Gamma-ray burst and spectroscopy instrumentation development at the goddard space flight center

This paper summarizes the activities within the Low-Energy Gamma-Ray Group of the Laboratory for High-Energy Astrophysics at the Goddard Space Flight Center that are specifically related to the development of instrumentation for gamma-ray astronomy. Three programs are described: 1) the Gamma-Ray Imaging Spectrometer (GRIS), a balloon-borne array of seven germanium detectors for high-resolution spectrographic studies of persistent gamma-ray sources, 2) the Transient Gamma-Ray Spectrometer (TGRS), a single radiatively-cooled germanium detector for the spectrographic study of gamma-ray bursts, and 3) the Rapidly Moving Telescope (RMT), a ground-based optical telescope for the detection and study of short-lived optical transients, particularly those that occur in coincidence with gamma-ray bursts.     

Sigma: A new gamma-ray space observatory with high angular resolution for 1985 and beyond

The SIGMA mission is designed to obtain images of the sky in the hard X-ray/low energy gamma-ray domain 30 keV - 2 MeV, with an angular accuracy a few arc min., and a sensitivity for point sources down to a level of a few UFU. The principal scientific objectives of the SIGMA mission are described, as well as the anticipated performance of the instrument.     

Low and medium energy gamma-ray astronomy — present status and future aspects

During the last few years quite some progress has been achieved in the field of low and medium energy gamma-ray astronomy below about 30 MeV. Gamma rays from the galactic center and anti-center region have been detected, which require a high interstellar electron flux in the 100 MeV range, if they are predominantly diffuse in nature. Though the Crab pulsar and its nebula are still the only galactic gamma-ray sources which definitely have been detected, some recently determined upper limits to the gamma-ray fluxes of other radio pulsars are close to the theoretically expected values. Active galaxies seem to have a maximum of luminosity in the range between several 100 keV and a few MeV and, therefore, are of special interest. First observational results have been reported on the Seyfert galaxies NGC 4151 and MCG 8-11-11, and the radio galaxy CenA. The nature of the diffuse cosmic gamma-ray component at low gamma-ray energies is not yet solved. Unresolved active galaxies are good candidates for its origin.Considering the present status of gamma ray astronomy the study of galactic sources like radio pulsars and the unidentified high energy gamma-ray sources, the Milky Way as a whole, active galaxies and the diffuse cosmic sky seem to be the prime targets for broad band observations below 30 MeV in the GRO area. An unexplored field like that of low energy gamma-ray astronomy, however, is always open for surprises.     

Spectral evolution in gamma-ray bursts

The Hard X-ray Burst Spectrometer (HXRBS) and the Gamma-Ray Spectrometer (GRS) on NASA's Solar Maximum Mission satellite (SMM) have independently monitored cosmic gamma-ray bursts since launch in February 1980. Several bursts with relatively simple pulse structure and sufficient intensity have been analyzed for evidence of spectral variability on timescales shorter than the pulse durations. In many of these bursts we find pulse structures, ranging in duration from 1 to 10 seconds, which exhibit a trend of hard-to-soft spectral evolution. No significant evidence for soft-to-hard evolution has been found, although the possibility of weak, extended low-energy emission is suggested in a few bursts. The HXRBS data above 100 keV and the GRS data above 1 MeV indicate that the spectral evolution generally is not due to time-varying absorption features at energies below 100 keV.     

Gamma-ray blazars: The view from AGILE

During the first 3 years of operation the Gamma-Ray Imaging Detector onboard the AGILE satellite detected several blazars in a high γ-ray activity: 3C 279, 3C 454.3, PKS 1510-089, S5 0716+714, 3C 273, W Comae, Mrk 421, PKS 0537-441 and 4C +21.35. Thanks to the rapid dissemination of our alerts, we were able to obtain multiwavelength data from other observatories such as Spitzer, Swift, RXTE, Suzaku, INTEGRAL, MAGIC, VERITAS, and ARGO as well as radio-to-optical coverage by means of the GASP Project of the WEBT and the REM Telescope. This large multifrequency coverage gave us the opportunity to study the variability correlations between the emission at different frequencies and to obtain simultaneous Spectral Energy Distributions of these sources from radio to γ-ray energy bands, investigating the different mechanisms responsible for their emission and uncovering in some cases a more complex behavior with respect to the standard models. We present a review of the most interesting AGILE results on these γ-ray blazars and their multifrequency data.     

GRASP — A gamma-ray astronomy mission dedicated to the fine spectroscopy and positioning of celestial gamma-ray sources

The GRASP Mission - Gamma Ray Astronomy with Spectroscopy and Positioning - will be the first high resolution spectral imager to operate in the gamma-ray region of the spectrum. The instrument covers the photon energy range from approximately 15 keV to more than 100 MeV. A combination of discrete germanium solid state devices and scintillation counters form a position sensitive gamma-ray detection matrix which is operated in conjunction with a coded aperture mask to create arc minute images of the gamma-ray sky with a spectral resolution of typically λ/Δλ ∼1000. The use of a coded mask with a ‘zoom’ facility will permit the combination of field of view and angular resolution to be adjusted to suit the scientific aims of each observation. The respective continuum and line sensitivities will be typically 10⁻⁸ph cm⁻² s⁻¹ keV⁻¹ and 3 10⁻⁶ cm⁻² s⁻¹ for point sources of gamma-rays with photon energies close to 1 MeV.