Over the past decade, long-duration γ-ray bursts (GRBs)-including the subclass of X-ray flashes (XRFs)-have been revealed 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 with supernova SN 2006aj), the second-nearest 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 1048 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.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Despite a rich phenomenology, γ-ray bursts (GRBs) are divided 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 their origin to star-forming galaxies, and hence the death of massive stars, and indicating an energy release of about 1051 erg. While theoretical arguments 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 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 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 and providing support for coalescing compact object binaries as the progenitors.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We present a comprehensive study of the morphological properties of 42 7-ray burst (GRB) host galaxies imaged with the Hubble Space Telescope in the optical band. The purpose of this study is to ...understand the relation of GRBs to their macroenvironments and to compare the GRB-selected galaxies to other high-redshift samples. We perform both qualitative and quantitative analyses by categorizing the galaxies according to their visual properties and by examining their surface brightness profiles. We find that the majority of the galaxies have approximately exponential profiles, indicative of galactic disks, and have a median effective radius of about 1.7 kpc; 620% of the hosts are better fit with a bulge-dominated profile. Inspection of the visual morphologies reveals a high fraction of merging and interacting systems, with 630% showing clear signs of interaction and an additional 30% exhibiting irregular and asymmetric structure, which may be the result of recent mergers; these fractions are independent of redshift and galaxy luminosity. The fraction of mergers appears to be elevated compared to other high-redshift samples (i.e., the HDF), particularly for the low luminosities of GRB hosts (M sub(B) 6 -16 to -21 mag). Finally, we show mat GRB hosts clearly follow the size-luminosity relation present in other galaxy samples, but thanks to spectroscopic absorption redshifts they help to extend this relation to fainter luminosities.
Type Ia supernovae are key tools for measuring distances on a cosmic scale. They are generally thought to be the thermonuclear explosion of an accreting white dwarf in a close binary system. The ...nature of the mass donor is still uncertain. In the single-degenerate model it is a main-sequence star or an evolved star, whereas in the double-degenerate model it is another white dwarf. We show that the velocity structure of absorbing material along the line of sight to 35 type Ia supernovae tends to be blueshifted. These structures are likely signatures of gas outflows from the supernova progenitor systems. Thus, many type Ia supernovae in nearby spiral galaxies may originate in single-degenerate systems.
Over the past decade, our physical understanding of gamma-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; the short-duration (< or = 2 s) GRBs have a different origin, which has been argued to be the merger of two compact objects. Here we report optical observations of GRB 060614 (duration approximately 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.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Gaia16aye was a binary microlensing event discovered in the direction towards the northern Galactic disc and was one of the first microlensing events detected and alerted to by the
Gaia
space ...mission. Its light curve exhibited five distinct brightening episodes, reaching up to
I
= 12 mag, and it was covered in great detail with almost 25 000 data points gathered by a network of telescopes. We present the photometric and spectroscopic follow-up covering 500 days of the event evolution. We employed a full Keplerian binary orbit microlensing model combined with the motion of Earth and
Gaia
around the Sun to reproduce the complex light curve. The photometric data allowed us to solve the microlensing event entirely and to derive the complete and unique set of orbital parameters of the binary lensing system. We also report on the detection of the first-ever microlensing space-parallax between the Earth and
Gaia
located at L2. The properties of the binary system were derived from microlensing parameters, and we found that the system is composed of two main-sequence stars with masses 0.57 ± 0.05
M
⊙
and 0.36 ± 0.03
M
⊙
at 780 pc, with an orbital period of 2.88 years and an eccentricity of 0.30. We also predict the astrometric microlensing signal for this binary lens as it will be seen by
Gaia
as well as the radial velocity curve for the binary system. Events such as Gaia16aye indicate the potential for the microlensing method of probing the mass function of dark objects, including black holes, in directions other than that of the Galactic bulge. This case also emphasises the importance of long-term time-domain coordinated observations that can be made with a network of heterogeneous telescopes.
On 2006 May 5, a 4 s duration, low-energy, similar to 10 super(49) erg, gamma-ray burst (GRB) was observed, spatially associated with a z = 0.0804 galaxy. Here we report the discovery of the GRB ...optical afterglow and observations of its environment using Gemini South, the Hubble Space Telescope (HST), Chandra, Swift, and the Very Large Array. The optical afterglow of this GRB is spatially associated with a prominent star-forming region in the Sc-type galaxy 2dFGRS S173Z112. Its proximity to a star-forming region suggests that the progenitor delay time, from birth to explosion, is smaller than similar to 10 Myr. Our HST deep imaging rules out the presence of a supernova brighter than an absolute magnitude of about -11 (or -12.6 in the case of maximal extinction) at about 2 weeks after the burst and limits the ejected mass of radioactive super(56)Ni to be less than about 2 x 10 super(-4) M unk (assuming no extinction). Although it was suggested that GRB 060505 may belong to a new class of long-duration GRBs with no supernova, we argue that the simplest interpretation is that the physical mechanism responsible for this burst is the same as that for short-duration GRBs.
We present optical spectroscopy of the afterglow of GRB 050505 obtained with the Keck 110 m telescope. The spectrum exhibits three absorption systems with the highest, at z = 4.2748, arising in the ...host galaxy. The host absorption is marked by a DLA with log N(H super(I)) = 22.05 c0.10, higher than that of any QSO-DLA detected to date but similar to several other recent measurements from GRB spectra. We further deduce a metallicity of Z - 0.06 Z sub( ), with a depletion pattern that is similar to that of the Galactic warm halo or warm disk. More importantly, we detect strong absorption from Si II* indicating a dense environment, n sub(H) 10 super(2) cm super(-3), in the vicinity of the burst, with a size of 64 pc. The C IV absorption system spans a velocity range of about 10 super(3) km s super(-1), most likely arising in the progenitor stellar wind. In this context the lack of corresponding Si IV absorption indicates that the progenitor had a mass 25 M sub( )and a metallicity 0.1 Z sub( ), and therefore required a binary companion to eject its hydrogen envelope prior to the GRB explosion. Finally, by extending the GRB-DLA sample to z - 4.3 we show that these objects appear to follow a similar metallicity-redshift relation as in QSO-DLAs, but with systematically higher metallicities. It remains to be seen whether this trend is simply due to the higher neutral hydrogen columns in GRB-DLAs and/or sight lines which probe star-forming regions, or if it is a manifestation of different star formation properties in GRB-DLAs.
We present observations of the unusually bright and long g-ray burst GRB 050820A, one of the best sampled broadband data sets in the Swift era. The g-ray light curve is marked by a soft precursor ...pulse some 200 s before the main event; the lack of any intervening emission suggests that it is due to a physical mechanism distinct from the GRB itself. The large time lag between the precursor and the main emission enabled simultaneous observations in the g-ray, X-ray, and optical bandpasses, something only achieved for a handful of events to date. While the contemporaneous X-rays are the low-energy tail of the prompt emission, the optical does not directly track the g-ray flux. Instead, the early-time optical data appear consistent with the forward shock synchrotron peak passing through the optical and are therefore likely the beginning of the afterglow. On hour timescales after the burst, the X-ray and optical light curves are inconsistent with an adiabatic expansion of the shock into the surrounding region, but rather indicate that there is a period of energy injection. Observations at late times allow us to constrain the collimation angle of the relativistic outflow to 6:8 f 9:3. Our estimates of both the kinetic energy of the afterglow (E sub(KE) = 5.2 super(+7.9-4.1) x 10 super(51) ergs) and the prompt g-ray energy release (E sub(g) = 7.5 super(+6.7-2.4) x 10 super(51) ergs) make GRB 050820A one of the most energetic events for which such values could be determined.
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