S. Covino

Relativistic jet activity from the tidal disruption of a star by a massive black hole

Supermassive black holes have powerful gravitational fields with strong gradients that can destroy stars that get too close, producing a bright flare in ultraviolet and X-ray spectral regions from stellar debris that forms an accretion disk around the black hole. The aftermath of this process may have been seen several times over the past two decades in the form of sparsely sampled, slowly fading emission from distant galaxies, but the onset of the stellar disruption event has not hitherto been observed. Here we report observations of a bright X-ray flare from the extragalactic transient Swift J164449.3+573451. This source increased in brightness in the X-ray band by a factor of at least 10,000 since 1990 and by a factor of at least 100 since early 2010. We conclude that we have captured the onset of relativistic jet activity from a supermassive black hole. A companion paper comes to similar conclusions on the basis of radio observations. This event is probably due to the tidal disruption of a star falling into a supermassive black hole, but the detailed behaviour differs from current theoretical models of such events.D. N. Burrows , J. A. Kennea , G. Ghisellini , V. Mangano , B. Zhang , K. L. Page , M. Eracleous , P. Romano , T. Sakamoto , A. D. Falcone , J. P. Osborne , S. Campana , A. P. Beardmore , A. A. Breeveld , M. M. Chester , R. Corbet , S. Covino , J. R. Cummings , P. D’Avanzo , V. D’Elia , P. Esposito , P. A. Evans , D. Fugazza, J. M. Gelbord , K. Hiroi , S. T. Holland , K. Y. Huang , M. Im, G. Israel , Y. Jeon , Y.-B. Jeon , N. Kawai , H. A. Krimm , P. Mészáros , H. Negoro , N. Omodei , W.K. Park , J. S. Perkins , M. Sugizaki , H.-I. Sung , G. Tagliaferri , E. Troja , Y. Ueda, Y. Urata, R. Usui , L. A. Antonelli , S. D. Barthelmy , G. Cusumano , P. Giommi , F. E. Marshall , A. Melandri , M. Perri , J. L. Racusin , B. Sbarufatti , M. H. Siegel , & N. Gehrels 21

Relativistic jet activity from the tidal disruption of a star by a massive black hole [Discovery of the onset of rapid accretion by a dormant massive black hole]

Supermassive black holes have powerful gravitational fields with strong gradients that can destroy stars that get too close, producing a bright flare in ultraviolet and X-ray spectral regions from stellar debris that forms an accretion disk around the black hole. The aftermath of this process may have been seen several times over the past two decades in the form of sparsely sampled, slowly fading emission from distant galaxies, but the onset of the stellar disruption event has not hitherto been observed. Here we report observations of a bright X-ray flare from the extragalactic transient Swift J164449.3+573451. This source increased in brightness in the X-ray band by a factor of at least 10,000 since 1990 and by a factor of at least 100 since early 2010. We conclude that we have captured the onset of relativistic jet activity from a supermassive black hole. A companion paper comes to similar conclusions on the basis of radio observations. This event is probably due to the tidal disruption of a star falling into a supermassive black hole, but the detailed behaviour differs from current theoretical models of such events.D. N. Burrows , J. A. Kennea , G. Ghisellini , V. Mangano , B. Zhang , K. L. Page , M. Eracleous , P. Romano , T. Sakamoto , A. D. Falcone , J. P. Osborne , S. Campana , A. P. Beardmore , A. A. Breeveld , M. M. Chester , R. Corbet , S. Covino , J. R. Cummings , P. D’Avanzo , V. D’Elia , P. Esposito , P. A. Evans , D. Fugazza, J. M. Gelbord , K. Hiroi , S. T. Holland , K. Y. Huang , M. Im, G. Israel , Y. Jeon , Y.-B. Jeon , N. Kawai , H. A. Krimm , P. Mészáros , H. Negoro , N. Omodei , W.K. Park , J. S. Perkins , M. Sugizaki , H.-I. Sung , G. Tagliaferri , E. Troja , Y. Ueda, Y. Urata, R. Usui , L. A. Antonelli , S. D. Barthelmy , G. Cusumano , P. Giommi , F. E. Marshall , A. Melandri , M. Perri , J. L. Racusin , B. Sbarufatti , M. H. Siegel , & N. Gehrels 21

An enigmatic long-lasting γ-ray burst not accompanied by a bright supernova

Gamma-ray bursts (GRBs) are short, intense flashes of soft γ-rays coming from the distant Universe. Long-duration GRBs (those lasting more than ∼2 s) are believed to originate from the deaths of massive stars, mainly on the basis of a handful of solid associations between GRBs and supernovae. GRB 060614, one of the closest GRBs discovered, consisted of a 5-s hard spike followed by softer, brighter emission that lasted for ∼100 s (refs 8, 9). Here we report deep optical observations of GRB 060614 showing no emerging supernova with absolute visual magnitude brighter than MV = -13.7. Any supernova associated with GRB 060614 was therefore at least 100 times fainter, at optical wavelengths, than the other supernovae associated with GRBs. This demonstrates that some long-lasting GRBs can either be associated with a very faint supernova or produced by different phenomena.

THE AFTERGLOWS OF SWIFT-ERA GAMMA-RAY BURSTS. I. COMPARING PRE-SWIFT AND SWIFT-ERA LONG/SOFT (TYPE II) GRB OPTICAL AFTERGLOWS

We have gathered optical photometry data from the literature on a large sample of Swift-era gamma-ray burst (GRB) afterglows including GRBs up to 2009 September, for a total of 76 GRBs, and present an additional three pre-Swift GRBs not included in an earlier sample. Furthermore, we publish 840 additional new photometry data points on a total of 42 GRB afterglows, including large data sets for GRBs 050319, 050408, 050802, 050820A, 050922C, 060418, 080413A, and 080810. We analyzed the light curves of all GRBs in the sample and derived spectral energy distributions for the sample with the best data quality, allowing us to estimate the host-galaxy extinction. We transformed the afterglow light curves into an extinction-corrected z = 1 system and compared their luminosities with a sample of pre-Swift afterglows. The results of a former study, which showed that GRB afterglows clustered and exhibited a bimodal distribution in luminosity space, are weakened by the larger sample. We found that the luminosity distribution of the two afterglow samples (Swift-era and pre-Swift) is very similar, and that a subsample for which we were not able to estimate the extinction, which is fainter than the main sample, can be explained by assuming a moderate amount of line-of-sight host extinction. We derived bolometric isotropic energies for all GRBs in our sample, and found only a tentative correlation between the prompt energy release and the optical afterglow luminosity at 1 day after the GRB in the z = 1 system. A comparative study of the optical luminosities of GRB afterglows with echelle spectra (which show a high number of foreground absorbing systems) and those without, reveals no indication that the former are statistically significantly more luminous. Furthermore, we propose the existence of an upper ceiling on afterglow luminosities and study the luminosity distribution at early times, which was not accessible before the advent of the Swift satellite. Most GRBs feature afterglows that are dominated by the forward shock from early times on. Finally, we present the first indications of a class of long GRBs, which form a bridge between the typical high-luminosity, high-redshift events and nearby low-luminosity events (which are also associated with spectroscopic supernovae) in terms of energetics and observed redshift distribution, indicating a continuous distribution overall.

SN 2003lw and GRB 031203: A Bright Supernova for a Faint Gamma-Ray Burst

Optical and near-infrared observations of the gamma-ray burst GRB 031203, at z = 0.1055, are reported. A very faint afterglow is detected superposed onto the host galaxy in our first infrared JHK observations, carried out ~9 hr after the burst. Subsequently, a rebrightening is detected in all bands, peaking in the R band about 18 rest-frame days after the burst. The rebrightening closely resembles the light curve of a supernova like SN 1998bw, assuming that the GRB and the SN went off almost simultaneously, but with a somewhat slower evolution. Spectra taken close to the maximum of the rebrightening show extremely broad features as in SN 1998bw. The determination of the absolute magnitude of this SN (SN 2003lw) is difficult owing to the large and uncertain extinction, but likely this event was brighter than SN 1998bw by 0.5 mag in the VRI bands, reaching an absolute magnitude MV = -19.75 ± 0.15.

An origin for short gamma-ray bursts unassociated with current star formation.

Two short (< 2 s) γ-ray bursts (GRBs) have recently been localized and fading afterglow counterparts detected. The combination of these two results left unclear the nature of the host galaxies of the bursts, because one was a star-forming dwarf, while the other was probably an elliptical galaxy. Here we report the X-ray localization of a short burst (GRB 050724) with unusual γ-ray and X-ray properties. The X-ray afterglow lies off the centre of an elliptical galaxy at a redshift of z = 0.258 (ref. 5), coincident with the position determined by ground-based optical and radio observations. The low level of star formation typical for elliptical galaxies makes it unlikely that the burst originated in a supernova explosion. A supernova origin was also ruled out for GRB 050709 (refs 3, 31), even though that burst took place in a galaxy with current star formation. The isotropic energy for the short bursts is 2–3 orders of magnitude lower than that for the long bursts. Our results therefore suggest that an alternative source of bursts—the coalescence of binary systems of neutron stars or a neutron star-black hole pair—are the progenitors of short bursts.

An unexpectedly rapid decline in the X-ray afterglow emission of long gamma-ray bursts.

‘Long’ γ-ray bursts (GRBs) are commonly accepted to originate in the explosion of particularly massive stars, which give rise to highly relativistic jets. Inhomogeneities in the expanding flow result in internal shock waves that are believed to produce the γ-rays we see. As the jet travels further outward into the surrounding circumstellar medium, ‘external’ shocks create the afterglow emission seen in the X-ray, optical and radio bands. Here we report observations of the early phases of the X-ray emission of five GRBs. Their X-ray light curves are characterised by a surprisingly rapid fall-off for the first few hundred seconds, followed by a less rapid decline lasting several hours. This steep decline, together with detailed spectral properties of two particular bursts, shows that violent shock interactions take place in the early jet outflows.

The First Survey of X-Ray Flares from Gamma-Ray Bursts Observed by Swift: Temporal Properties and Morphology

We present the first systematic investigation of the morphological and timing properties of flares in GRBs observed by Swift XRT. We consider a large sample drawn from all GRBs detected by Swift, INTEGRAL, and HETE-2 prior to 2006 January 31, which had an XRT follow-up and which showed significant flaring. Our sample of 33 GRBs includes long and short, at low and high redshift, and a total of 69 flares. The strongest flares occur in the early phases, with a clear anticorrelation between the flare peak intensity and the flare time of occurrence. Fitting each X-ray flare with a Gaussian model, we find that the mean ratio of the width and peak time is --> ? t/t = 0.13 ? 0.10, albeit with a large scatter. Late flares at times >2000 s have long durations, -->? t > 300 s, and can be very energetic compared to the underlying continuum. We further investigated whether there is a clear link between the number of pulses detected in the prompt phase by BAT and the number of X-ray flares detected by XRT, finding no correlation. However, we find that the distribution of intensity ratios between successive BAT prompt pulses and that between successive XRT flares is the same, an indication of a common origin for gamma-ray pulses and X-ray flares. All evidence indicates that flares are indeed related to the workings of the central engine and, in the standard fireball scenario, originate from internal shocks rather than external shocks. While all flares can be explained by long-lasting engine activity, 29/69 flares may also be explained by refreshed shocks. However, 10 can only be explained by prolonged activity of the central engine.

GRB 090423 at a redshift of z ≈ 8.1

Gamma-ray bursts (GRBs) are produced by rare types of massive stellar explosion. Their rapidly fading afterglows are often bright enough at optical wavelengths that they are detectable at cosmological distances. Hitherto, the highest known redshift for a GRB was z = 6.7 (ref. 1), for GRB 080913, and for a galaxy was z = 6.96 (ref. 2). Here we report observations of GRB 090423 and the near-infrared spectroscopic measurement of its redshift, z = . This burst happened when the Universe was only about 4 per cent of its current age. Its properties are similar to those of GRBs observed at low/intermediate redshifts, suggesting that the mechanisms and progenitors that gave rise to this burst about 600,000,000 years after the Big Bang are not markedly different from those producing GRBs about 10,000,000,000 years later.

Broadband observations of the naked-eye gamma-ray burst GRB 080319B

Long-duration γ-ray bursts (GRBs) release copious amounts of energy across the entire electromagnetic spectrum, and so provide a window into the process of black hole formation from the collapse of massive stars. Previous early optical observations of even the most exceptional GRBs (990123 and 030329) lacked both the temporal resolution to probe the optical flash in detail and the accuracy needed to trace the transition from the prompt emission within the outflow to external shocks caused by interaction with the progenitor environment. Here we report observations of the extraordinarily bright prompt optical and γ-ray emission of GRB 080319B that provide diagnostics within seconds of its formation, followed by broadband observations of the afterglow decay that continued for weeks. We show that the prompt emission stems from a single physical region, implying an extremely relativistic outflow that propagates within the narrow inner core of a two-component jet.