Valentina La Parola

Swift XRT Observations of the Afterglow of GRB 050319

Swift discovered the high-redshift GRB 050319 with the Burst Alert Telescope (BAT) and began observing with its narrow-field instruments only 225 s after the burst onset. The afterglow X-ray emission was monitored by the XRT up to 28 days after the burst. The light curve shows a decay with three different phases, each characterized by a distinct slope: an initial steep decay with a power-law index of ~5.5, a second phase characterized by a flat decay slope of ~0.54, and a third phase with a decay slope of ~1.14. During the first phase the spectral energy distribution is softer than in the following two phases, and the photon index is consistent with the GRB prompt spectrum. The extrapolation of the BAT light curve to the XRT band suggests that the initial fast-decaying phase of the XRT afterglow might be the low-energy tail of the prompt emission. The second break in the afterglow light curve occurs about 27,000 s after the burst. The spectral energy distribution before and after the second break does not change, and it can be tentatively interpreted as a jet break or the end of a delayed or continuous energy injection phase.

SWIFT XRT OBSERVATIONS OF THE AFTERGLOW OF XRF 050416A

Swift discovered XRF 050416Awith the Burst Alert Telescope and began observing it with its narrow-field instrumentsonly64.5saftertheburst onset.Itsverysoftspectrumclassifiesthisevent asanX-rayflash.TheafterglowX-ray emissionwasmonitoredupto74daysaftertheburst.TheX-raylightcurveinitiallydecaysveryfast(decayslope � � 2:4), subsequently flattens (� � 0:44), and eventually steepens again (� � 0:88), similar to many X-ray afterglows. The first and second phases end � 172 and � 1450 s after the burst onset, respectively. We find evidence of spectral evolution from a softer emission with photon index � � 3:0 during the initial steep decay, to a harder emission with � � 2:0 during the following evolutionary phases. The spectra show intrinsic absorption in the host galaxy with column density of � 6:8 ; 10 21 cm � 2 . The consistency of the initial photon index with the high-energy BAT photon index suggests that the initial fast decaying phase of the X-ray light curve may be the low-energy tail of the prompt emission. The lack of jet break signatures in the X-ray afterglow light curve is not consistent with empirical relations between the source rest-frame peak energy and the collimation-corrected energy of the burst. The standard uniform jet model can give apossible descriptionof the XRF 050416A X-ray afterglow for anopeningangle largerthan a few tens of degrees, although numerical simulations show that the late-time decay is slightly flatter than expected from on-axis viewing of a uniform jet. A structured Gaussian-type jet model with uniform Lorentz factor distribution and viewing angle outside the Gaussian core is another possibility, although a full agreement with data is not achieved with the numerical models explored. Subject headingg gamma rays: bursts — X-rays: individual (XRF 050416A)

The Swift Supergiant Fast X-ray Transients Project: recent results

We present an overview of our Supergiant Fast X-ray Transients (SFXT) project, that started in 2007, by highlighting the unique observational contribution Swift is giving to this exciting new field. By means of outburst detection with Swift/BAT and follow-up with Swift/XRT, we demonstrated that while the brightest phase of the outburst only lasts a few hours, further significant activity is observed at lower fluxes for a considerably longer (weeks) time. After intense monitoring with Swift/XRT, we now have a firm estimate of the time SFXTs spend in each phase. The 4 SFXTs we monitored for 1-2 years spend between 3 and 5 % of the time in bright outbursts. The most most probable flux level at which a random observation will find these sources, when detected, is F(2-10 keV) ~ 1-2E-11 erg cm^{-2} s^{-1} (unabsorbed), corresponding to luminosities of a few 10^{33} to a few 10^{34} erg s^{-1}. Finally, the duty-cycle of inactivity ranges between 19 and 55 %.

Using muon rings for the optical calibration of the ASTRI telescopes for the Cherenkov Telescope Array

High-energy muons constitute a very useful tool to calibrate the total optical throughput of any telescope of the Cherenkov Telescope Array (CTA). Differences in precision and efficiency can however be present due to the variety of telescope types and sizes. In this contribution we present some preliminary results on simulated muon ring images collected by the ASTRI small sized dual-mirror (SST-2M) telescope in the basic configuration installed in Italy at the Serra La Nave observing station. ASTRI SST-2M is able, using 6% of the detected muon events, to calibrate with muons the optical throughput down to a degradation of the optical efficiency of 30%. Moreover, its precision in reconstructing the muon arrival direction is about one camera pixel, and its error on the reconstructed ring radius is ~ 6.3%. The adopted procedures will be tested and validated with real data acquired by the prototype after the commissioning phase. The nine telescopes that will form the ASTRI mini-array, proposed to be installed at the final CTA southern site during the pre-production phase, will improve these results thanks to the higher detection efficiency and the lower optical cross-talk and after-pulse of their updated silicon photomultipliers.

GRB 080319B: the prompt emission of the “Naked Eye Burst”

We briefly review the main results of the observations carried out during the prompt emission of the exceptionally bright GRB 080319B.