The Swift GRB MIDEX mission

Swift is a first-of-its-kind multiwavelength transient observatory for γ-ray burst astronomy. It has the optimum capabilities for the next breakthroughs in determining the origin of γ-ray bursts and their afterglows, as well as for using bursts to probe the early Universe. Swift will also monitor the soft gamma repeaters and perform the first sensitive hard X-ray survey of the sky. The mission is being developed by an international collaboration and consists of three instruments, the Burst Alert Telescope (BAT), the X-ray Telescope (XRT), and the Ultraviolet and Optical Telescope (UVOT). The BAT, a wide-field γ-ray detector, will detect >100 γ-ray bursts per year with a sensitivity 5× that of BATSE. The sensitive narrow-field XRT and UVOT will be autonomously slewed to the burst location within 20–70 s to determine 0.3–5.0″ positions and perform optical, UV, and X-ray spectrophotometry. Strong education/public outreach and follow-up programs will help to engage the public and the astronomical community. Swift launch is planned for late 2004.     

Investigating gamma-ray burst data reduction techniques with Swift’s instruments

The amount of data on gamma-ray bursts (GRBs) and the detected afterglows observed by the Swift satellite contributed significantly to the understanding of the phenomenon. The behavior of the early afterglow rises some interesting questions. With the early afterglow localizations of gamma-ray burst positions made by Swift, the clear delimitation of the prompt phase and the afterglow is not so obvious any more. There are hints of a canonical X-ray afterglow lightcurve with segments of different slopes. Not all bursts necessarily show all the segments. It is important to see if the prompt phase and the afterglow has the same origin or they stem from different parts of the progenitor system. We will combine the of gamma-ray burst data from BAT and XRT and compare the extrapolated gamma-ray flux to the X-ray in a sample of bursts and find that there is a good agreement between the two measurements. This indicates that the physical process shaping burst and the early afterglow are the same.     

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.     

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).     

The INTEGRAL view of Gamma-Ray Bursts

After more than six and half years in orbit, the ESA space observatory INTEGRAL has provided new, exciting results in the soft gamma-ray energy range (from a few keV to a few MeV). With the discovery of about 700 hard X-Ray sources, it has changed our previous view of a sky composed of peculiar and “monster” sources. The new high energy sky is in fact full of a large variety of normal, very energetic emitters, characterized by new accretion and acceleration processes (see also IBIS cat4 (Bird et al., 2010). At the same time, about one GRB/month is detected and imaged by the two main gamma-ray instruments on board: IBIS and SPI. In this paper, we review the major achievements of the INTEGRAL observatory in the field of Gamma-Ray Bursts. We summarize the global properties of Gamma-Ray Bursts detected by INTEGRAL, with respect to their duration, spectral index, and peak flux distributions. We recall INTEGRAL results on the spectral lag analysis, showing how long-lag GRBs appear to form a separate population at low peak fluxes. We review the outcome of polarisation studies performed by using INTEGRAL data. Finally, concerning single GRB studies, we highlight the properties of particularly interesting Gamma-Ray Bursts in the INTEGRAL sample.     

The origin of the prompt optical emission in GRB 060111B

The detection of a bright optical emission measured with good temporal resolution during the prompt phase makes GRB 060111B a rare event that is especially useful for constraining theories of the prompt optical emission. Comparing this burst with other GRBs with evidence of optical peaks, we find that the optical peak epoch (t_p) is anti-correlated with the high energy burst energetic assuming an isotropic energy release (E_iso) in agreement with Liang et al. (2009), and that the steeper is the post-peak afterglow decay, the less is the agreement with the correlation. GRB 060111B is among the latters and it does not match the correlation. The Cannonball scenario is also discussed and we find that this model cannot be excluded for GRB 060111B.     

Time-dependent thermal X-ray afterglows from GRBS

Time-dependent thermal X-ray spectra are calculated from physically plausible conditions around GRB. It is shown that account for time-dependent ionization processes strongly affects the observed spectra of hot rarefied plasma. These calculations may provide an alternative explanation to the observed X-ray lines of early GRBs afterglows (such as GRB 011211). Our technique will allow one to obtain independent constraints on the GRB collimation angle and on the clumpiness of circumstellar matter.     

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.     

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.     

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.     

Testing the blast wave model with Swift GRBs

The complex structure of the light curves of Swift GRBs (e.g. superimposed flares and shallow decay) has made their interpretation and that of the blast wave caused by the burst, more difficult than in the pre-Swift era. We aim to constrain the blast wave parameters: electron energy distribution, p, density profile of the circumburst medium, k, and the continued energy injection index, q. We do so by comparing the observed multi-wavelength light curves and X-ray spectra of a Swift sample to the predictions of the blast wave model.     

Oxygen line mapping of SN 1006 with Suzaku

SN 1006 is one of the supernova remnants (SNRs) with relatively low-temperature electrons, considering the young age of just 1000 years. We carried out SN 1006 mapping observations with the X-ray Imaging Spectrometers (XIS) and the Hard X-ray Detector (HXD) onboard Suzaku, the fifth Japanese X-ray satellite. Thanks to the excellent spectral resolution of XIS in the soft X-ray band, H-like and He-like oxygen emission lines were clearly detected, and we could make a map of the line intensity, and as well as a flux and the photon index of nonthermal component. We found that these parameters have spatial dependences from region to region in the SNR; the north region is bright in nonthermal, while dim in thermal; the east region is bright in both nonthermal and thermal; the inner region shows dim nonthermal and bright thermal emission. The photon index is the smallest in the north region.     

Active galactic nuclei in the ROSAT survey

One of the main objectives of the ROSAT mission turned out to be the study of active galactic nuclei. The soft energy range of the X-Ray Telescope combined with the good energy resolution of the PSPC detector allows an investigation of the spectral properties of sources in this energetically important energy band. The high sensitivity of the instrument in the All Sky Survey will yield more than 25000 previously unknown X-ray AGN for the statistical and morphological exploration of these objects.

A short overview is given of the actual ongoing research which ranges from the detailed spectral study of some well known objects and correlations of samples of AGN with existing catalogues at other wavelengths to identification programmes for large numbers of suspected AGN candidates.     

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.     

High Energy X-ray Telescope Data Analysis Method

The science analysis of the data from the High Energy X-ray Telescope （HE） on the Hard X-ray Modulation Telescope （HXMT） satellite is organized in three stages: calibration, screening and extraction of high-level scientific products. At the first stage, the raw PHA value of each event is converted to PI value accounting for temporal changes in gain and energy offset. At the second stage, the calibrated events are screened by applying cleaning criteria. At the third stage, scientific products, i.e. spectra, light curves and redistribution matrix files, are extracted. This work will introduce the three stages as well as the screening criteria and the data combining method.      

Broad band X-ray properties of GRO J1948+32/KS 1947+300

We present here results obtained from three BeppoSAX observations of the transient X-ray pulsar GRO J1948+32 carried out during the declining phase of its 2000 November–2001 June outburst. Timing analysis of the data clearly shows a 18.7 s pulsation in the X-ray light curves in 0.1–100 keV energy band. The pulse profile of GRO 1948+32 is characterized by a broad peak with a sharp rise followed by a narrow dip. The dip in the pulse profile shows very strong energy dependence. Phase-averaged spectroscopy obtained with three of the BeppoSAX instruments shows that the 0.1–100 keV energy spectrum is described by a Comptonized component, a weak blackbody component (7% of the total emission) for soft X-rays, a narrow and weak iron emission line at 6.7 keV and low column density of material in the line of sight. The results obtained from the analysis are discussed in this paper.     

GRB host galaxies: An unbiased sample

We describe the current status and recent results from our Swift/VLT legacy survey, a VLT Large Programme aimed at characterizing the host galaxies of a homogeneously selected sub-sample of Swift   GRBs. The immediate goals are to determine the host luminosity function, study the effects of reddening, determine the fraction of Lyα$\mathrm{Ly}\mathrm{\alpha }$ emitters in the hosts, and obtain redshifts for targets without a reported one. The main effort so far has been the definition of a very carefully selected sample, obeying strict and well-defined criteria: 68 targets in total. Among the preliminary results is a large optical detection rate, the lack of extremely red objects (only one possible case in the sample) and an update of the Swift   GRB redshift distribution with 〈z〉∼2.0$〈z〉\sim 2.0$.     

The ROSAT view of the cataclysmic variable sky

The ROSAT All-Sky Survey has for the first time permitted a synoptic view of the soft X-ray sky with high sensitivity. In this paper, we discuss the X-ray properties of known cataclysmic variables (CVs) as observed in the Survey and present a status report on programs to identify CVs among the newly discovered ROSAT X-ray sources. Of 170 CVs with known orbital period, 92 were detected in the Survey and 22 of these fall in the bright-source category with more than 0.5 PSPC cts/s. Among the new bright sources, so far 19 have been identified as CVs and 3 as CV candidates, about doubling the census. We present spectra and light curves of known and new systems and discuss the origin of the X-ray emission in the different subclasses of CVs.