The Swift X-ray telescope BURROWS, David N; HILL, J. E; CHINCARINI, G ...
Space science reviews,
10/2005, Volume:
120, Issue:
3-4
Journal Article
Peer reviewed
he Swift Gamma-Ray Explorer is designed to make prompt multiwavelength observations of gamma-ray bursts (GRBs) and GRB afterglows. The X-ray telescope (XRT) enables Swift to determine GRB positions ...with a few arcseconds accuracy within 100 s of the burst onset. The XRT utilizes a mirror set built for JET-X and an XMM-Newton/EPIC MOS CCD detector to provide a sensitive broad-band (0.2-10 keV) X-ray imager with effective area of > 120 cm^sup 2^ at 1.5 keV, field of view of 23.6 × 23.6 arcminutes, and angular resolution of 18 arcseconds (HPD). The detection sensitivity is 2×10^sup -14^ erg cm^sup -2^ s^sup -1^ in 10^sup 4^ s. The instrument is designed to provide automated source detection and position reporting within 5 s of target acquisition. It can also measure the redshifts of GRBs with Fe line emission or other spectral features. The XRT operates in an auto-exposure mode, adjusting the CCD readout mode automatically to optimize the science return for each frame as the source intensity fades. The XRT will measure spectra and lightcurves of the GRB afterglow beginning about a minute after the burst and will follow each burst for days or weeks.PUBLICATION ABSTRACT
Context. The Swift satellite is a multi-wavelength observatory specifically designed for gamma-ray burst (GRB) astronomy that is operational since 2004. Swift is also a very flexible multi-purpose ...facility that supports a wide range of scientific fields such as active galactic nuclei, supernovae, cataclysmic variables, Galactic transients, active stars and comets. The Swift X-ray Telescope (XRT) has collected more than 150 Ms of observations in its first seven years of operations. Aims. The purpose of this work is to present to the scientific community the list of all the X-ray point sources detected in XRT imaging data taken in photon counting mode during the first seven years of Swift operations. All these point-like sources, excluding the GRB, will be stored in a catalog publicly available (1SWXRT). Methods. We considered all the XRT observations with exposure time longer than 500 s taken in the period 2005−2011. Data were reduced and analyzed with standard techniques and a list of detected sources for each observation was produced. A careful visual inspection was performed to remove extended, spurious and piled-up sources. Finally, positions, count rates, fluxes, and the corresponding uncertainties were computed. Results. We have analyzed more than 35 000 XRT fields, with exposures ranging between 500 s and 100 ks, for a total exposure time of almost 140 Ms. The catalog includes approximately 89 000 entries, of which almost 85 000 are not affected by pile-up and are not GRBs. Considering that many XRT fields were observed several times, we have a total of ~36 000 distinct celestial sources. We computed count rates in three energy bands: 0.3−10 keV (Full, or F), 0.3−3 keV (Soft, or S) and 2−10 keV (Hard, or H). Each entry has a detection in at least one of these bands. In particular, we detect ~80 000, ~70 000 and ~25 500 in the F, S and H band, respectively. Count rates were converted into fluxes in the 0.5−10, 0.5−2 and 2−10 keV bands. The flux interval sampled by the detected sources is 7.4 × 10-15 − 9.1 × 10-11, 3.1 × 10-15 − 1.1 × 10-11 and 1.3 × 10-14 − 9.1 × 10-11 erg cm-2 s-1 for the F, S and H band, respectively. Some possible scientific uses of the catalog are also highlighted.
Aims. Long-duration gamma-ray bursts (GRBs) have been found to be associated with broad-lined type-Ic supernovae (SNe), but only a handful of cases have been studied in detail. Prompted by the ...discovery of the exceptionally bright, nearby GRB 130427A (redshift z = 0.3399), we aim at characterising the properties of its associated SN 2013cq. This is the first opportunity to test the progenitors of high-luminosity GRBs directly. Methods. We monitored the field of the Swift long-duration GRB 130427A using the 3.6 m TNG and the 8.2 m VLT during the time interval between 3.6 and 51.6 days after the burst. Photometric and spectroscopic observations revealed the presence of the type Ic SN 2013cq. Results. Spectroscopic analysis suggests that SN 2013cq resembles two previous GRB-SNe, SN 1998bw and SN 2010bh, associated with GRB 980425 and X-ray flash (XRF) 100316D, respectively. The bolometric light curve of SN 2013cq, which is significantly affected by the host galaxy contribution, is systematically more luminous than that of SN 2010bh (~2 mag at peak), but is consistent with SN 1998bw. The comparison with the light curve model of another GRB-connected SN 2003dh indicates that SN 2013cq is consistent with the model when brightened by 20%. This suggests a synthesised radioactive 56Ni mass of ~0.4M⊙. GRB 130427A/SN 2013cq is the first case of low-z GRB-SN connection where the GRB energetics are extreme (Eγ,iso ~ 1054 erg). We show that the maximum luminosities attained by SNe associated with GRBs span a very narrow range, but those associated with XRFs are significantly less luminous. On the other hand the isotropic energies of the accompanying GRBs span 6 orders of magnitude (1048 erg <Eγ,iso< 1054 erg), although this range is reduced when corrected for jet collimation. The GRB total radiated energy is in fact a small fraction of the SN energy budget.
The tidal disruption of a solar-mass star around a supermassive black hole has been extensively studied analytically and numerically. In these events, the star develops into an elongated ...banana-shaped structure. After completing an eccentric orbit, the bound debris falls into the black hole, forming an accretion disk and emitting radiation. The same process may occur on planetary scales if a minor body passes too close to its star. In the Solar System, comets fall directly into our Sun or onto planets. If the star is a compact object, the minor body can become tidally disrupted. Indeed, one of the first mechanisms invoked to produce strong gamma-ray emission involved accretion of comets onto neutron stars in our Galaxy. Here we report that the peculiarities of the 'Christmas' gamma-ray burst (GRB 101225A) can be explained by a tidal disruption event of a minor body around an isolated Galactic neutron star. This would indicate either that minor bodies can be captured by compact stellar remnants more frequently than occurs in the Solar System or that minor-body formation is relatively easy around millisecond radio pulsars. A peculiar supernova associated with a gamma-ray burst provides an alternative explanation.
One of the most elusive features of gamma-ray bursts (GRBs) is the sporadic emission prior to the main prompt event observed in at least ~15% of cases. These precursors have spectral and temporal ...properties similar to the main prompt emission, and smaller, but comparable, energetics. They are separated from the main event by a quiescent time that may be extremely long, and, in some cases, more than one precursor has been observed in the same burst. Precursors are still a puzzle: despite many attempts, none of the proposed models can account for all the observed features. Based on the complete sample of bright long GRBs observed by Swift (BAT6), we propose a new scenario for which precursors are explained by assuming that the central GRB engine is a newly born magnetar. In this model the precursor and the prompt emission arise from accretion of matter onto the surface of the magnetar. The accretion process can be halted by the centrifugal drag exerted by the rotating magnetosphere onto the infalling matter, allowing for multiple precursors and very long quiescent times.
During its first observing run, in late 2015, the advanced Laser Interferometer Gravitational-wave Observatory facility announced three gravitational wave (GW) triggers to electromagnetic follow-up ...partners. Two of these have since been confirmed as being of astrophysical origin: both are binary black hole mergers at ∼500 Mpc; the other trigger was later found not to be astrophysical. In this paper, we report on the Swift follow-up observations of the second and third triggers, including details of 21 X-ray sources detected; none of which can be associated with the GW event. We also consider the challenges that the next GW observing run will bring as the sensitivity and hence typical distance of GW events will increase. We discuss how to effectively use galaxy catalogues to prioritize areas for follow-up, especially in the presence of distance estimates from the GW data. We also consider two galaxy catalogues and suggest that the high completeness at larger distances of the 2MASS Photometric Redshift catalogue makes it very well suited to optimize Swift follow-up observations.
We present new observations of the early X-ray afterglows of the first 27 gamma-ray bursts (GRBs) well observed by the Swift X-Ray Telescope (XRT). The early X-ray afterglows show a canonical ...behavior, where the light curve broadly consists of three distinct power-law segments: (1) an initial very steep decay (8t super(-a) with 3 a sub(1) 5), followed by (2) a very shallow decay (0.5 a sub(2) 1.0), and finally (3) a somewhat steeper decay (1 a sub(3) 1.5). These power-law segments are separated by two corresponding break times, t sub(break,1) 500 s and 10 super(3) s t sub(break,2) 10 super(4)s. On top of this canonical behavior, many events have superimposed X-ray flares, which are most likely caused by internal shocks due to long-lasting sporadic activity of the central engine, up to several hours after the GRB. We find that the initial steep decay is consistent with it being the tail of the prompt emission, from photons that are radiated at large angles relative to our line of sight. The first break in the light curve (t sub(break,1)) takes place when the forward shock emission becomes dominant, with the intermediate shallow flux decay (a sub(2)) likely caused by the continuous energy injection into the external shock. When this energy injection stops, a second break is then observed in the light curve (t sub(break,2))- This energy injection increases the energy of the afterglow shock by at least a factor of f 4 and augments the already severe requirements for the efficiency of the prompt gamma-ray emission.
Abstract
We investigate whether there is any correlation between the X-ray afterglow luminosity and the prompt emission properties of a carefully selected sub-sample of bright Swift long Gamma-ray ...bursts (GRBs) nearly complete in redshift (∼90 per cent). Being free of selection effects (except flux limit), this sample provides the possibility to compare the rest frame physical properties of GRB prompt and afterglow emission in an unbiased way. The afterglow X-ray luminosities are computed at four different rest frame times (5 min, 1 h, 11 h and 24 h after trigger) and compared with the prompt emission isotropic energy E
iso, the isotropic peak luminosity L
iso and the rest frame peak energy E
peak. We find that the rest frame afterglow X-ray luminosity do correlate with these prompt emission quantities, but the significance of each correlation decreases over time. This result is in agreement with the idea that the GRB X-ray light curve can be described as the result of a combination of different components whose relative contribution and weight change with time, with the prompt and afterglow emission dominating at early and late time, respectively. In particular, we found evidence that the plateau and the shallow decay phase often observed in GRB X-ray light curves are powered by activity from the central engine. The existence of the L
X - E
iso correlation at late times () suggests a similar radiative efficiency among different bursts with on average about 6 per cent of the total kinetic energy powering the prompt emission.
We present a detailed analysis of Swift multiwavelength observations of GRB 070110 and its remarkable afterglow. The early X-ray light curve, interpreted as the tail of the prompt emission, displays ...a spectral evolution already seen in other gamma-ray bursts. The optical afterglow shows a shallow decay up to similar to 2 days after the burst, which is not consistent with standard afterglow models. The most intriguing feature is a very steep decay in the X-ray flux at similar to 2 x 10 super(-1) s after the burst, ending an apparent plateau. The abrupt drop of the X-ray light curve rules out an external shock as the origin of the plateau in this burst and implies long-lasting activity of the central engine. The temporal and spectral properties of the plateau phase point toward a continuous central engine emission rather than the episodic emission of X-ray flares. We suggest that the observed X-ray plateau is powered by a spinning-down central engine, possibly a millisecond pulsar, which dissipates energy at an internal radius before depositing energy into the external shock.
Context. Our knowledge of short gamma-ray bursts (GRBs) has significatively improved in the Swift era. Rapid multiband observations from the largest ground-based observatories led to the discovery of ...the optical afterglows and host galaxies of these events. In spite of these advancements, the number of short GRBs with secure detections in the optical is still fairly small. Short GRBs are commonly thought to originate from the merging of double compact object binaries but direct evidence for this scenario is still missing. Aims. Optical observations of short GRBs allow us to measure redshifts, firmly identify host galaxies, characterize their properties, and accurately localize GRBs within them. Multiwavelength observations of GRB afterglows provide useful information on the emission mechanisms at work. These are all key issues that allow one to discriminate among different models of these elusive events. Methods. We carried out photometric observations of the short/hard GRB 051227, GRB 061006, and GRB 071227 with the ESO-VLT starting from several hours after the explosion down to the host galaxy level several days later. For GRB 061006 and GRB 071227 we also obtained spectroscopic observations of the host galaxy. We compared the results obtained from our optical observations with the available X-ray data of these bursts. Results. For all the three above bursts, we discovered optical afterglows and firmly identified their host galaxies. About half a day after the burst, the optical afterglows of GRB 051227 and GRB 061006 present a decay significatly steeper than in the X-rays. In the case of GRB 051227, the optical decay is so steep that it likely indicates different emission mechanisms in the two wavelength ranges. The three hosts are blue star forming galaxies at moderate redshifts and with metallicities comparable to the Solar one. The projected offsets of the optical afterglows from their host galaxy center span a wide range, but all afterglows lie within the light of their hosts and present evidence for local absorption in their X-ray spectra. We discuss our findings in light of the current models of short GRB progenitors.
PDF
