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An ultraviolet–optical flare from the tidal disruption of a helium-rich stellar core
The observation of a flare of radiation from the centre of an inactive galaxy fits a model of the tidal disruption of a helium-rich stellar core and its accretion onto a black hole of about three million solar masses.
A flare for black holes
Central supermassive black holes in distant galaxies are normally invisible to us, but sometimes their presence becomes evident in the form of flares produced by the tidal disruption of a star being accreted to the black hole. Such events are rare, and often we see only the later stages of the encounter — but here, Gezari
et al
. report detailed monitoring of an ultraviolet and optical flare from the nuclear region of an inactive galaxy at a redshift of 0.1696, which was first seen on 31 May 2010, peaked in July and was over by September. The observed continuum is cooler than expected for a simple accreting debris disk, but the well sampled rise and decline of the light curve follows the predicted mass-accretion rate. The black hole has about two million solar masses and the disrupted star had a helium-rich stellar core, as the authors deduced from the spectroscopic signature of ionized helium from the unbound debris.
The flare of radiation from the tidal disruption and accretion of a star can be used as a marker for supermassive black holes that otherwise lie dormant and undetected in the centres of distant galaxies
1
. Previous candidate flares
2
,
3
,
4
,
5
,
6
have had declining light curves in good agreement with expectations, but with poor constraints on the time of disruption and the type of star disrupted, because the rising emission was not observed. Recently, two ‘relativistic’ candidate tidal disruption events were discovered, each of whose extreme X-ray luminosity and synchrotron radio emission were interpreted as the onset of emission from a relativistic jet
7
,
8
,
9
,
10
. Here we report a luminous ultraviolet–optical flare from the nuclear region of an inactive galaxy at a redshift of 0.1696. The observed continuum is cooler than expected for a simple accreting debris disk, but the well-sampled rise and decay of the light curve follow the predicted mass accretion rate and can be modelled to determine the time of disruption to an accuracy of two days. The black hole has a mass of about two million solar masses, modulo a factor dependent on the mass and radius of the star disrupted. On the basis of the spectroscopic signature of ionized helium from the unbound debris, we determine that the disrupted star was a helium-rich stellar core.
Journal Article
Relativistic ejecta from X-ray flash XRF 060218 and the rate of cosmic explosions
X-Rated Supernova
A link between long γ-ray bursts (GRBs) and supernovae has been established, but whether there is a similar relationship between the weaker and softer X-ray flashes and supernovae is unclear. GRB/XRF 060218, spotted by the Swift satellite on 18 February this year, may supply that missing link. In the first of four papers on this novel burster, Campana
et al
. report the sighting of the X-ray signature of a shock break-out, possible evidence of a supernova in progress. Pian
et al
. report the optical discovery of a type Ic supernova 2006aj associated with GRB/XRF 060218. Soderberg
et al
. report radio and X-ray observations that show that XRF 060218 is 100 times less energetic than, but of a type that is ten times more common than cosmological GRBs. Mazzali
et al
. modelled the spectra and light curve of SN 2006aj to show that it had a much smaller explosion energy and ejected much less mass than other GRB-supernovae, suggesting that it was produced by a star with a mass was only about 20 times that of the Sun, leaving behind a neutron star, rather than a black hole.
Radio and X-ray observations of X-ray flash XRF 060218, which is associated with supernova SN 2006aj, show that this event is 100 times less energetic but ten times more common than cosmological gamma-ray bursts (GRBs). The production of relativistic ejecta seems to be the key physical distinction between GRBs/XRFs and ordinary supernovae.
Over the past decade, long-duration γ-ray bursts (GRBs)—including the subclass of X-ray flashes (XRFs)—have been revealed
1
,
2
,
3
to be a rare variety of type Ibc supernova. Although all these events result from the death of massive stars, the electromagnetic luminosities of GRBs and XRFs exceed those of ordinary type Ibc supernovae by many orders of magnitude. The essential physical process that causes a dying star to produce a GRB or XRF, and not just a supernova, is still unknown. Here we report radio and X-ray observations of XRF 060218 (associated
4
with supernova SN 2006aj), the second-nearest
5
,
6
GRB identified until now. We show that this event is a hundred times less energetic but ten times more common than cosmological GRBs. Moreover, it is distinguished from ordinary type Ibc supernovae by the presence of 10
48
erg coupled to mildly relativistic ejecta, along with a central engine (an accretion-fed, rapidly rotating compact source) that produces X-rays for weeks after the explosion. This suggests that the production of relativistic ejecta is the key physical distinction between GRBs or XRFs and ordinary supernovae, while the nature of the central engine (black hole or magnetar) may distinguish typical bursts from low-luminosity, spherical events like XRF 060218.
Journal Article
The afterglow of GRB 050709 and the nature of the short-hard γ-ray bursts
The final chapter in the long-standing mystery of the γ-ray bursts (GRBs) centres on the origin of the short-hard class of bursts, which are suspected on theoretical grounds to result from the coalescence of neutron-star or black-hole binary systems. Numerous searches for the afterglows of short-hard bursts have been made, galvanized by the revolution in our understanding of long-duration GRBs that followed the discovery in 1997 of their broadband (X-ray, optical and radio) afterglow emission. Here we present the discovery of the X-ray afterglow of a short-hard burst, GRB 050709, whose accurate position allows us to associate it unambiguously with a star-forming galaxy at redshift
z
= 0.160, and whose optical lightcurve definitively excludes a supernova association. Together with results from three other recent short-hard bursts, this suggests that short-hard bursts release much less energy than the long-duration GRBs. Models requiring young stellar populations, such as magnetars and collapsars, are ruled out, while coalescing degenerate binaries remain the most promising progenitor candidates.
Short gamma-ray bursts
Gamma-ray bursts (GRBs) are either ‘long and soft’, or ‘short and hard’. The long-duration type leave a strong afterglow and have been extensively studied. So we have a good idea of what causes them: explosions of massive stars in distant star-forming galaxies. Short GRBs, with no strong afterglow, were harder to pin down. The Swift satellite, launched last November, is designed to study bursts as soon as they happen. Having shown its worth with long GRBs (reported in the 18 August issue of
Nature
), Swift has now bagged a short burst, GRB 050509B, precisely measured its location and detected the X-ray afterglow. Four papers this week report on this and another recent short burst. Now, over 20 years after they were first recognized, the likely origin of the short GRBs is revealed as a merger between neutron stars of a binary system and the instantaneous production of a black hole.
Journal Article
The afterglow and elliptical host galaxy of the short γ-ray burst GRB 050724
Hard evidence
Gamma-ray bursts (GRBs) are either ‘long and soft’, or ‘short and hard’. It is now clear that the long-duration type are caused by explosions of massive stars in distant star-forming galaxies. Only in recent months, with the Swift satellite latching onto bursts as soon as they happen, has it been possible to collect data on short bursts that may lead to similar certainty as to their cause. GRB 050724 burst onto the scene on 24 July, and has all the properties needed to solve the mystery of short GRBs. The new evidence supports the merging compact object model of short GRBs, involving either a neutron star–neutron star merger, or a neutron star–black hole binary system as progenitor.
Despite a rich phenomenology, γ-ray bursts (GRBs) are divided
1
into two classes based on their duration and spectral hardness—the long-soft and the short-hard bursts. The discovery of afterglow emission from long GRBs was a watershed event, pinpointing
2
their origin to star-forming galaxies, and hence the death of massive stars, and indicating
3
an energy release of about 10
51
erg. While theoretical arguments
4
suggest that short GRBs are produced in the coalescence of binary compact objects (neutron stars or black holes), the progenitors, energetics and environments of these events remain elusive despite recent
5
,
6
,
7
,
8
localizations. Here we report the discovery of the first radio afterglow from the short burst GRB 050724, which unambiguously associates it with an elliptical galaxy at a redshift
9
z
= 0.257. We show that the burst is powered by the same relativistic fireball mechanism as long GRBs, with the ejecta possibly collimated in jets, but that the total energy release is 10–1,000 times smaller. More importantly, the nature of the host galaxy demonstrates that short GRBs arise from an old (> 1 Gyr) stellar population, strengthening earlier suggestions
5
,
6
and providing support for coalescing compact object binaries as the progenitors.
Journal Article
A novel explosive process is required for the γ-ray burst GRB 060614
The long and the short of it
The tidy classification system that divided γ-ray bursts (GRBs) into long-duration busts (lasting more than two seconds) and short may have had its day. The final nail in its coffin may be GRB 060614. Discovered on 14 June 2006 by the Burst Alert Telescope on-board the Swift satellite, this burst was long, at 102 seconds, but as reported in a clutch of papers in this issue, it has a number of properties, including the absence of an accompanying supernova, that were previously considered diagnostic of a 'short' GRB. The hunt is now on for a classification system to take account of the diversity now apparent in GRBs. In the accompanying News & Views, Bing Zhang suggests that the answer may be to adopt a Type I/Type II classification similar to that used for supernovae.
Optical observations of γ-ray burst (GRB) 060614 (duration ∼100s) rule out the presence of an associated supernova. This would seem to require a new explosive process: either a massive 'collapsar' that powers a GRB without any associated supernova, or a new type of engine, as long-lived as the collapsar but without a massive star.
Over the past decade, our physical understanding of γ-ray bursts (GRBs) has progressed rapidly, thanks to the discovery and observation of their long-lived afterglow emission. Long-duration (≳2 s) GRBs are associated with the explosive deaths of massive stars (‘collapsars’, ref.
1
), which produce accompanying supernovae
2
,
3
,
4
,
5
; the short-duration (≲2 s) GRBs have a different origin, which has been argued to be the merger of two compact objects
6
,
7
,
8
,
9
. Here we report optical observations of GRB 060614 (duration ∼100 s, ref.
10
) that rule out the presence of an associated supernova. This would seem to require a new explosive process: either a massive collapsar that powers a GRB without any associated supernova, or a new type of ‘engine’, as long-lived as the collapsar but without a massive star. We also show that the properties of the host galaxy (redshift
z
= 0.125) distinguish it from other long-duration GRB hosts and suggest that an entirely new type of GRB progenitor may be required.
Journal Article
A common origin for cosmic explosions inferred from calorimetry of GRB030329
Past studies
1
,
2
,
3
have suggested that long-duration γ-ray bursts have a ‘standard’ energy of
E
γ
≈ 10
51
erg in the ultra-relativistic ejecta, after correcting for asymmetries in the explosion (‘jets’). But a group of sub-energetic bursts, including the peculiar GRB980425 associated
4
with the supernova SN1998bw (
E
γ
≈ 10
48
erg), has recently been identified
2
,
3
. Here we report radio observations of GRB030329 that allow us to undertake calorimetry of the explosion. Our data require a two-component explosion: a narrow (5° opening angle) ultra-relativistic component responsible for the γ-rays and early afterglow, and a wide, mildly relativistic component that produces the radio and optical afterglow more than 1.5 days after the explosion. The total energy release, which is dominated by the wide component, is similar
1
,
2
,
3
,
5
to that of other γ-ray bursts, but the contribution of the γ-rays is energetically minor. Given the firm link
6
,
7
of GRB030329 with SN2003dh, our result indicates a common origin for cosmic explosions in which, for reasons not yet understood, the energy in the highest-velocity ejecta is extremely variable.
Journal Article