Abstract
We report the discovery of the unusually bright long-duration gamma-ray burst (GRB), GRB 221009A, as observed by the Neil Gehrels Swift Observatory (Swift), Monitor of All-sky X-ray Image, ...and Neutron Star Interior Composition Explorer Mission. This energetic GRB was located relatively nearby (
z
= 0.151), allowing for sustained observations of the afterglow. The large X-ray luminosity and low Galactic latitude (
b
= 4.°3) make GRB 221009A a powerful probe of dust in the Milky Way. Using echo tomography, we map the line-of-sight dust distribution and find evidence for significant column densities at large distances (≳10 kpc). We present analysis of the light curves and spectra at X-ray and UV–optical wavelengths, and find that the X-ray afterglow of GRB 221009A is more than an order of magnitude brighter at
T
0
+ 4.5 ks than that from any previous GRB observed by Swift. In its rest frame, GRB 221009A is at the high end of the afterglow luminosity distribution, but not uniquely so. In a simulation of randomly generated bursts, only 1 in 10
4
long GRBs were as energetic as GRB 221009A; such a large
E
γ
,iso
implies a narrow jet structure, but the afterglow light curve is inconsistent with simple top-hat jet models. Using the sample of Swift GRBs with redshifts, we estimate that GRBs as energetic and nearby as GRB 221009A occur at a rate of ≲1 per 1000 yr—making this a truly remarkable opportunity unlikely to be repeated in our lifetime.
A peculiar aspect of Cherenkov telescopes is that they are designed to detect atmospheric light flashes on the time scale of nanoseconds, being almost blind to stellar sources. As a consequence, the ...pointing calibration of these instruments cannot be done in general exploiting the standard astrometry of the focal plane. In this paper we validate a procedure to overcome this problem for the case of the innovative ASTRI telescope, developed by INAF, exploiting sky images produced as an ancillary output by its novel Cherenkov camera. In fact, this instrument implements a statistical technique called “Variance method” (VAR) owning the potentiality to image the star field (angular resolution
∼
1
1
′
). We demonstrate here that VAR images can be exploited to assess the alignment of the Cherenkov camera with the optical axis of the telescope down to
∼
1
′
′
. To this end, we evaluate the position of the stars with sub-pixel precision thanks to a deep investigation of the convolution between the point spread function and the pixel distribution of the camera, resulting in a transformation matrix that we validated with simulations. After that, we considered the rotation of the field of view during long observing runs, obtaining light arcs that we exploited to investigate the alignment of the Cherenkov camera with high precision, in a procedure that we have already tested on real data. The strategy we have adopted, inherited from optical astronomy, has never been performed on Variance images from a Cherenkov telescope until now, and it can be crucial to optimize the scientific accuracy of the incoming MiniArray of ASTRI telescopes.
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 65.5, a second phase characterized by a flat decay slope of 60.54, and a third phase with a decay slope of 61.14. 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 discovered XRF 050416A with the Burst Alert Telescope and began observing it with its narrow-field instruments only 64.5 s after the burst onset. Its very soft spectrum classifies this event as ...an X-ray flash. The afterglow X-ray emission was monitored up to 74 days after the burst. The X-ray light curve initially decays very fast (decay slope a 6 2.4), subsequently flattens (a 6 0.44), and eventually steepens again (a 6 0.88), similar to many X-ray afterglows. The first and second phases end 6172 and 61450 s after the burst onset, respectively. We find evidence of spectral evolution from a softer emission with photon index 6 3.0 during the initial steep decay, to a harder emission with 6 2.0 during the following evolutionary phases. The spectra show intrinsic absorption in the host galaxy with column density of 66.8 x 10 super(21) cm super(-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 a possible description of the XRF 050416A X-ray afterglow for an opening angle larger than 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.
We report the results of Swift observations of the gamma-ray burst GRB 050603. With a V magnitude V = 18.2 about 10 hr after the burst, the optical afterglow was the brightest thus far detected by ...Swift and one of the brightest optical afterglows ever seen. The Burst Alert Telescope (BAT) light curves show three fast-rise exponential-decay spikes with T sub(90) = 12 s and a fluence of 7.6 x 10 super(-6) ergs cm super(-2) in the 15-150 keV band. With Eg sub(,iso) = 1.26 x 10 super(54) ergs, it was also one of the most energetic bursts of all times. The Swift spacecraft began observation of the afterglow with the narrow-field instruments about 10 hr after the detection of the burst. The burst was bright enough to be detected by the Swift UV/Optical telescope (UVOT) for almost 3 days and by the X-Ray Telescope (XRT) for a week after the burst. The X-ray light curve shows a rapidly fading afterglow with a decay index a =1.76 +0.15-0.07. The X-ray energy spectral index was b sub(X) = 0.71 c 0.10 with the column density in agreement with the Galactic value. The spectral analysis does not show an obvious change in the X-ray spectral slope over time. The optical UVOT light curve decays with a slope of a = 1.8 c 0.2. The steepness and the similarity of the optical and X-ray decay rates suggest that the afterglow was observed after the jet break. We estimate a jet opening angle of about 1-2.
A peculiar aspect of Cherenkov telescopes is that they are designed to detect atmospheric light flashes on the time scale of nanoseconds, being almost blind to stellar sources. As a consequence, the ...pointing calibration of these instruments cannot be done in general exploiting the standard astrometry of the focal plane. In this paper we validate a procedure to overcome this problem for the case of the innovative ASTRI telescope, developed by INAF, exploiting sky images produced as an ancillary output by its novel Cherenkov camera. In fact, this instrument implements a statistical technique called "Variance method" (VAR) owning the potentiality to image the star field (angular resolution \(\sim 11'\)). We demonstrate here that VAR images can be exploited to assess the alignment of the Cherenkov camera with the optical axis of the telescope down to \(\sim 1''\). To this end, we evaluate the position of the stars with sub-pixel precision thanks to a deep investigation of the convolution between the point spread function and the pixel distribution of the camera, resulting in a transformation matrix that we validated with simulations. After that, we considered the rotation of the field of view during long observing runs, obtaining light arcs that we exploited to investigate the alignment of the Cherenkov camera with high precision, in a procedure that we have already tested on real data. The strategy we have adopted, inherited from optical astronomy, has never been performed on Variance images from a Cherenkov telescope until now, and it can be crucial to optimize the scientific accuracy of the incoming MiniArray of ASTRI telescopes.
The Swift Gamma-Ray Burst Explorer performed its first autonomous, X-ray follow-up to a newly detected GRB on 2005 January 17, within 193 s of the burst trigger by the Swift Burst Alert Telescope. ...While the burst was still in progress, the X-Ray Telescope (XRT) obtained a position and an image for an uncataloged X-ray source simultaneously with the gamma-ray observation. The XRT observed flux during the prompt emission was 1.1 x 10 super(-8) ergs cm super(-2) s super(-1) in the 0.5-10 keV energy band. The emission in the X-ray band decreased by 3 orders of magnitude within 700 s, following the prompt emission. This is found to be consistent with the gamma-ray decay when extrapolated into the XRT energy band. During the following 6.3 hr, the XRT observed the afterglow in an automated sequence for an additional 947 s, until the burst became fully obscured by the Earth limb. A faint, extremely slowly decaying afterglow, a = -0.21, was detected. Finally, a break in the light curve occurred and the flux decayed with a < -1.2. The X-ray position triggered many follow-up observations: no optical afterglow could be confirmed, although a candidate was identified 3" from the XRT position.
PDF
