Swift/BAT Observations of X-Ray Flashes

An analysis of prompt gamma-rays of X-ray flashes (XRFs) observed with the Swift/BAT has been presented. Our sample includes 235 bursts. It is found that the BAT detection ratio of XRFs to typical Gamma-ray bursts (GRBs) is 42:193, for division at Γ=2 (roughly corresponding Ep～50 keV), Γ being the power law index of the BAT spectrum and Ep, the peak energy (Ep) of the νfν spectrum. This is consistent with the HETE-2 observations. For both XRFS and GRBs Γ are almost normally distributed in the range of 1 to 2.8, similar that observed with HETE-2. The distribution of Γ for the entire set of GRBs/XRFs is not available due to poor statistics on the peak at Γ>2.3. This result probably indicates that the BAT spectrum of a typical XRF could have a Γ of roughly 2.3, if they indeed are a distinct soft component of the GRB population. By comparing the fluence and the peak flux in different energy bands, it is found that the XRFs are ordinarily softer than the GRBs, but during the peak time the spectra of both GRBs and XRF are similar, showing that the dominant radiation mechanisms of both GRBs and XRFs are similar.     

GCN and VOEvent: A status report

The GRB Coordinates Network (GCN) has filled an important niche in the conduct and progress of research on GRB for more than 14 years. The methods used to collect and distribute the positions, lightcurves, spectra, and images on GRB in real time (a few seconds) will be reviewed. For the research on GRBs to continue to move forward and for other fields of transient astronomy to move forward, enhancements are needed to the GCN. VOEvents is one of those changes, and that addition will be described here. The enhanced system will be called VO‐GCN. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical afterglows of gamma-ray bursts: a bimodal distribution?

The luminosities of the optical afterglows of gamma-ray bursts, 12 h (rest-frame time) after the trigger, show a surprising clustering, with a minority of events being at a significantly smaller luminosity. If real, this dichotomy would be a crucial clue to understand the nature of optically dark afterglows, i.e. bursts that are detected in the X-ray band, but not in the optical. We investigate this issue by studying bursts of the pre- Swift era, both detected and undetected in the optical. The limiting magnitudes of the undetected ones are used to construct the probability that a generic burst is observed down to a given magnitude limit. Then, by simulating a large number of bursts with pre-assigned characteristics, we can compare the properties of the observed optical luminosity distribution with the simulated one. Our results suggest that the hints of bimodality present in the observed distribution reflect a real bimodality: either the optical luminosity distribution of bursts is intrinsically bimodal, or there exists a population of bursts with a quite significant grey absorption, i.e. wavelength-independent extinction. This population of intrinsically weak or grey-absorbed events can be associated with dark bursts.     

Gamma-ray bursts from the early Universe: predictions for present-day and future instruments

Long gamma-ray bursts (GRBs) are important for the study of the Universe near and beyond the epoch of reionization. In this paper, we describe the characteristics of an 'ideal' instrument that can be used to search for GRBs at z ≥ 6–10. We find that the detection of these objects requires soft-band detectors with high sensitivity and a moderately large field of view. In light of these results, we compare available and planned GRB missions, deriving conservative predictions of the number of high-redshift GRBs detectable by these instruments along with the maximum accessible redshift. We show that the Swift satellite will be able to detect various GRBs at z ≥ 6, and likely at z ≥ 10 if the trigger threshold is decreased by a factor of ∼2. Furthermore, we find that INTEGRAL and GLAST are not the best tools to detect bursts at z ≥ 6, the former being limited by the small field of view, and the latter by its hard energy band and relatively low sensitivity. Finally, future missions ( SVOM , EDGE and, in particular, EXIST ) will provide a good sample of GRBs at z ≥ 6 within a few years of operation.     

A Monte Carlo template based analysis for air-Cherenkov arrays

We present a high-performance event reconstruction algorithm: an Image Pixel-wise fit for Atmospheric Cherenkov Telescopes (ImPACT). The reconstruction algorithm is based around the likelihood fitting of camera pixel amplitudes to an expected image template. A maximum likelihood fit is performed to find the best-fit shower parameters. A related reconstruction algorithm has already been shown to provide significant improvements over traditional reconstruction for both the CAT and H.E.S.S. experiments. We demonstrate a significant improvement to the template generation step of the procedure, by the use of a full Monte Carlo air shower simulation in combination with a ray-tracing optics simulation to more accurately model the expected camera images. This reconstruction step is combined with an MVA-based background rejection.Examples are shown of the performance of the ImPACT analysis on both simulated and measured (from a strong VHE source) gamma-ray data from the H.E.S.S. array, demonstrating an improvement in sensitivity of more than a factor two in observation time over traditional image moments-fitting methods, with comparable performance to previous likelihood fitting analyses. ImPACT is a particularly promising approach for future large arrays such as the Cherenkov Telescope Array (CTA) due to its improved high-energy performance and suitability for arrays of mixed telescope types.     

AMICA, an astro-mapper for AMS

The alpha magnetic spectrograph (AMS) is a composite particle detector to be accommodated on the International Space Station (ISS). AMS is mainly devoted to galactic, charged cosmic rays studies, antimatter and dark matter searches. Besides the main, classical physics goals, capabilities in the field of GeV and multi-GeV gamma astrophysics have been established and are under investigation by a number of groups. Due to the unsteadiness of the ISS platform, a star-mapper device is required in order to fully exploit the intrinsic arc-min angular resolution provided by the silicon tracker. A star-mapper is conceptually an imaging, optical instrument able to autonomously recognize a stellar field and to calculate its own orientation with respect to an inertial reference frame. AMICA (Astro Mapper for Instruments Check of Attitude) on AMS is responsible for providing real-time information that is going to be used off-line for compensating the large uncertainties in the ISS flight attitude and the structural degrees of freedom. In this paper, we describe in detail the AMICA sub-system, the accommodation/integration issues and the in-flight alignment procedure adopting identified galactic (Pulsars) and extra-galactic (AGNs) sources.     

Analysis of TTE/BATSE time profiles for short gamma-ray bursts

The TTE/BATSE time profiles for short gamma-ray bursts (GRBs) are analyzed. A sample of 287 short GRBs and a sample of 143 background regions are studied. Bursts similar to BRBs with precursors and bursts with time profiles that are not encountered among the bursts whose time profiles were investigated by using a combination of DISCSC and PREB data. In addition, there are fewer events with single-peak time profiles among short GRBs than among long GRBs (many double and triple bursts). A fractal analysis of the TTE time profiles for short GRBs is performed. According to the TTE data, the range of fractal dimensions for short bursts is 0.80≤D≤2.25. The derived fractal-dimension distribution exhibits two peaks that correspond to a similar distribution obtained previously by reducing the DISCSC data for short GRBs (D=1.44±0.02 and D=1.90±0.03) and a third peak (D=1.05±0.03). The bursts with 〈D〉=1.90±0.03 correspond to events in whose sources the processes with long-term variations and (or) quasi-periodicity take place, while the event time profiles with a fractal dimension 〈D〉=1.05±0.03 correspond to events in whose sources many extremely short random processes take place. A more detailed analysis of a subgroup of bursts with D=1.44±0.02 shows that its fractal dimension distribution is broader than that for a group with the same (within the limits of the measurement errors) D. At least two more GRB subgroups can be distinguished in this subgroup: (1) bursts with 〈D〉=1.51±0.04; according to the TTE data, their fractal dimensions correspond to those of the background; i.e., these are events with smooth time profiles without any variability in the sources on the time scales on which the fractal dimension is analyzed; and (2) bursts with 〈D〉=1.31±0.05, whose time profiles can correspond to those of the events obtained from the fireball model with internal shock waves. We present time profiles for the events obtained by using this model. The range of fractal dimensions for the modeled time profiles is 1.213≤D≤1.400, with the fractal dimensions for such an event and for the real GRB 990208 being equal, within the error limits, for some model parameters. A study of the TTE and DISCSC fractal-dimension distributions for the background indicates that the fractal dimension distributions obtained by analyzing these two types of data can be processed simultaneously.