Slow and fast components in the X-ray light curves of Gamma-Ray Bursts

Gamma-ray burst light curves show quite different patterns: from very simple to extremely complex. We present a temporal and spectral study of the light curves in three energy bands (2-5, 5-10, 10-26 keV) of ten GRBs detected by the Wide Field Cameras on board BeppoSAX. For some events the time profiles are characterized by peaks superposed on a slowly evolving pedestal, which in some cases becomes less apparent at higher energies. We describe this behaviour with the presence of two components (slow and fast) having different variability time scales. We modelled the time evolution of slow components by means of an analytical function able to describe asymmetric rising and decaying profiles. The residual light curves, after the subtraction of the slow components, generally show structures more similar to the original curves in the highest energy band. Spectral study of these two components was performed evaluating their hardness ratios, used also to derive photon indices. Slow components are found generally softer than the fast ones suggesting that their origin is likely different. Being typical photon indices lower than those of the afterglows there is no evidence that the emission processes are similar. Another interesting possibility is that slow components can be related to the presence of a hot photosphere having a thermal spectrum with kT around a few keV superposed to a rapid variable non-thermal emission of the fast component.Comment: 16 pages, 20 figures (18 color, 2 B&W), accepted for publication in Astronomy and Astrophysic

Reflection nebulae in the Galactic Center: the case for soft X-ray imaging polarimetry

The origin of irradiation and fluorescence of the 6.4 keV bright giant molecular clouds surrounding Sgr A*, the central supermassive black hole of our Galaxy, remains enigmatic. Testing the theory of a past active period of Sgr A* requires X-ray polarimetry. In this paper, we show how modern imaging polarimeters could revolutionize our understanding of the Galactic Center. Through Monte Carlo modeling, we produce a 4-8 keV polarization map of the Galactic Center, focusing on the polarimetric signature produced by Sgr B1, Sgr B2, G0.11-0.11, Bridge E, Bridge D, Bridge B2, MC2, MC1, Sgr C3, Sgr C2, and Sgr C1. We estimate the resulting polarization, include polarized flux dilution by the diffuse plasma emission detected toward the GC, and simulate the polarization map that modern polarimetric detectors would obtain assuming the performances of a mission prototype. The eleven reflection nebulae investigated in this paper present a variety of polarization signatures, ranging from nearly unpolarized to highly polarized (about 77%) fluxes. A major improvement in our simulation is the addition of a diffuse, unpolarized plasma emission that strongly impacts soft X-ray polarized fluxes. The dilution factor is in the range 50% - 70%, making the observation of the Bridge structure unlikely even in the context of modern polarimetry. The best targets are the Sgr B and Sgr C complexes, and the G0.11-0.11 cloud. An exploratory observation of a few hundred kilo-seconds of the Sgr B complex would allow a significant detection of the polarization and be sufficient to derive hints on the primary source of radiation. A more ambitious program (few Ms) of mapping the giant molecular clouds could then be carried out to probe with great precision the turbulent history of Sgr A*, and place important constraints on the composition and three-dimensional position of the surrounding gas.Comment: 7 pages, 3 figures, 2 tables, accepted for publication in A&

Autocorrelation analysis of GRBM–Beppo-SAX burst data

An autocorrelation function (ACF) analysis was performed on 17 gamma-ray bursts with known redshift, using data from the GRBM on board Beppo-SAX. When corrected from the cosmic time dilation effect, the ACFs show a bimodal distribution at about half-maximum, in agreement with a previous study based on BATSE and Konus burst data. Although the results show more dispersion, the separation between the two classes is highly significant

A variable magnetic disc wind in the black hole X-ray binary GRS 1915+105?

Context. GRS 1915+105 being one of the brightest transient black hole binaries (BHBs) in the X-rays offers a unique testbed for the study of the connection between accretion and ejection mechanisms in BHBs. In particular, this source can be used to study the accretion disc wind and its dependence on the state changes in BHBs.Aims. Our aim is to investigate the origin and geometry of the accretion disc wind in GRS 1915+105. This study will provide a basis for planning future observations with the X-ray Imaging Spectroscopy Mission (XRISM), and may also provide important parameters for estimating the polarimetric signal with the upcoming Imaging X-ray Polarimetry Explorer (IXPE).Methods. We analysed the spectra of GRS 1915+105 in the soft and hard chi classes using the high-resolution spectroscopy offered by Chandra HETGS. In the soft state, we find a series of wind absorption lines that follow a non-linear dependence of velocity width, velocity shift, and equivalent width with respect to ionisation, indicating a multiple component or stratified outflow. In the hard state we find only a faint Fe XXVI absorption line. We model the absorption lines in both the states using a dedicated magneto-hydrodynamic (MHD) wind model to investigate a magnetic origin of the wind and to probe the cause of variability in the observed line flux between the two states.Conclusions. The MHD disc wind model provides a good fit for both states, indicating the possibility of a magnetic origin of the wind. The multiple ionisation components of the wind are well characterised as a stratification of the same magnetic outflow. We find that the observed variability in the line flux between soft and hard states cannot be explained by photo-ionisation alone but is most likely due to a large (three orders of magnitude) increase in the wind density. We find the mass outflow rate of the wind to be comparable to the accretion rate, suggesting an intimate link between accretion and ejection processes that lead to state changes in BHBs

The complete catalogue of gamma-ray bursts observed by the Wide Field Cameras on board BeppoSAX

We present the complete on-line catalogue of gamma-ray bursts observed by the two Wide Field Cameras on board \sax in the period 1996-2002. Our aim is to provide the community with the largest published data set of GRB's prompt emission X-ray light curves and other useful data. This catalogue (BS-GRBWFCcat) contains data on 77 bursts and a collection of the X-ray light curves of 56 GRB discovered or noticed shortly after the event and of other additional bursts detected in subsequent searches. Light curves are given in the three X-ray energy bands (2-5, 5-10, 10-26 keV). The catalogue can be accessed from the home web page of the ASI Science Data Center-ASDC (http://www.asdc.asi.it)Comment: 4 pages, 3 figure

The prompt emission of GRB990712 with BeppoSAX: evidence of a transient X-ray emission feature

We report on the prompt X- and gamma-ray observations of GRB990712 with the BeppoSAX Gamma-Ray Burst Monitor and Wide Field Camera No. 2. Due to Sun constraints, we could not perform a follow-up observation with the BeppoSAX Narrow Field Instruments. The light curve of the prompt emission shows two pulses and a total duration of about 40s in X-rays. In gamma-rays the event is even shorter. The 2-700 keV spectral emission with time shows a discontinuity in the peak energy Ep of the E F(E) spectrum: Ep is above our energy passband during the first pulse and goes down to ~10 keV during the second pulse. Another peculiarity is noted in this event for the first time: the possible evidence of a 2s duration emission feature during the tail of the first pulse. The feature is consistent with either a Gaussian profile with centroid energy of 4.5 keV or a blackbody spectrum with kTbb ~1.3 keV. We discuss the possible origin of the feature. The most attractive possibility is that we are observing the thermal emission of a baryon-loaded expanding fireball, when it becomes optically thin.Comment: 18 pages, 4 figures, published in The Astrophysical Journal Letters, relevant changes in the Discussion (section 4) with respect to previous versio