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[show abstract]
[hide abstract]
ABSTRACT: We present the first systematic study of X-ray flare candidates in short
gamma-ray bursts (SGRBs) exploiting the large 6-year Swift database with the
aim to constrain the physical nature of such fluctuations. We find that flare
candidates appear in different types of SGRB host galaxy environments and show
no clear correlation with the X-ray afterglow lifetime; flare candidates are
detected both in SGRBs with a bright extended emission in the soft gamma-rays
and in SGRBs which do not show such component. We furthermore show that SGRB
X-ray flare candidates only partially share the set of observational properties
of long GRB (LGRB) flares. In particular,the main parameter driving the
duration evolution of X-ray variability episodes in both classes is found to be
the elapsed time from the explosion, with very limited dependence on the
different progenitors, environments, central engine life-times, prompt
variability time-scales and energy budgets. On the contrary, SGRB flare
candidates significantly differ from LGRB flares in terms of peak luminosity,
isotropic energy, flare-to-prompt luminosity ratio and relative variability
flux. However, these differences disappear when the central engine time-scales
and energy budget are accounted for, suggesting that (i) flare candidates and
prompt pulses in SGRBs likely have a common origin; (ii) similar dissipation
and/or emission mechanisms are responsible for the prompt and flare emission in
long and short GRBs, with SGRBs being less energetic albeit faster evolving
versions of the long class. Finally, we show that in strict analogy to the SGRB
prompt emission, flares candidates fall off the lag-luminosity relation defined
by LGRBs, thus strengthening the SGRB flare-prompt pulse connection.
07/2011;
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A. Cucchiara,
A. J. Levan,
D. B. Fox,
N. R. Tanvir,
T. N. Ukwatta,
E. Berger,
T. Krühler,
A. Küpcü Yoldaş,
X. F. Wu,
K. Toma, [......],
A. S. Fruchter,
J. Rhoads,
R. E. Rutledge,
B. P. Schmidt,
M. A. Dopita,
P. Podsiadlowski,
R. Willingale,
C. Wolf,
S. R. Kulkarni,
and P. D'Avanzo
[show abstract]
[hide abstract]
ABSTRACT: Gamma-ray bursts (GRBs) serve as powerful probes of the early universe, with their luminous afterglows revealing the locations and physical properties of star-forming galaxies at the highest redshifts, and potentially locating first-generation (Population III) stars. Since GRB afterglows have intrinsically very simple spectra, they allow robust redshifts from low signal-to-noise spectroscopy, or photometry. Here we present a photometric redshift of z ~ 9.4 for the Swift detected GRB 090429B based on deep observations with Gemini-North, the Very Large Telescope, and the GRB Optical and Near-infrared Detector. Assuming a Small Magellanic Cloud dust law (which has been found in a majority of GRB sight lines), the 90% likelihood range for the redshift is 9.06 < z < 9.52, although there is a low-probability tail toward somewhat lower redshifts. Adopting Milky Way or Large Magellanic Cloud dust laws leads to very similar conclusions, while a Maiolino law does allow somewhat lower redshift solutions, though in all cases the most likely redshift is found to be z > 7. The non-detection of the host galaxy to deep limits (Y(AB) ~ 28, which would correspond roughly to 0.001L* at z = 1) in our late-time optical and infrared observations with the Hubble Space Telescope strongly supports the extreme-redshift origin of GRB 090429B, since we would expect to have detected any low-z galaxy, even if it were highly dusty. Finally, the energetics of GRB 090429B are comparable to those of other GRBs and suggest that its progenitor is not greatly different from those of lower redshift bursts.
The Astrophysical Journal 06/2011; 736(1):7. · 6.02 Impact Factor
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B. A. Zauderer,
E. Berger, A. M. Soderberg,
A Loeb,
R. Narayan,
D. A. Frail,
G. R. Petitpas,
A. Brunthaler,
R. Chornock,
J. M. Carpenter, [......],
N. H. Volgenau,
T. L. Culverhouse,
M. F. Bietenholz,
M. P. Rupen,
W. Max-Moerbeck,
A. C. S. Readhead,
J. Richards,
M. Shepherd,
S. Storm,
C. L. H. Hull
[show abstract]
[hide abstract]
ABSTRACT: Active galactic nuclei (AGN), powered by long-term accretion onto central
supermassive black holes, produce relativistic jets with lifetimes of greater
than one million yr that preclude observations at birth. Transient accretion
onto a supermassive black hole, for example through the tidal disruption of a
stray star, may therefore offer a unique opportunity to observe and study the
birth of a relativistic jet. On 2011 March 25, the Swift {\gamma}-ray satellite
discovered an unusual transient source (Swift J164449.3+573451) potentially
representing such an event. Here we present the discovery of a luminous radio
transient associated with Swift J164449.3+573451, and an extensive set of
observations spanning centimeter to millimeter wavelengths and covering the
first month of evolution. These observations lead to a positional coincidence
with the nucleus of an inactive galaxy, and provide direct evidence for a
newly-formed relativistic outflow, launched by transient accretion onto a
million solar mass black hole. While a relativistic outflow was not predicted
in this scenario, we show that the tidal disruption of a star naturally
explains the high-energy properties, radio luminosity, and the inferred rate of
such events. The weaker beaming in the radio compared to {\gamma}-rays/X-rays,
suggests that radio searches may uncover similar events out to redshifts of z ~
6.
06/2011;
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G. Narayan,
R. J. Foley,
E. Berger,
M. T. Botticella,
R. Chornock,
M. E. Huber,
A. Rest,
D. Scolnic,
S. Smartt,
S. Valenti, [......],
E. A. Magnier,
J. S. Morgan,
P. A. Price,
A. G. Riess,
C. W. Stubbs,
W. E. Sweeney,
J. L. Tonry,
R. J. Wainscoat,
C. Waters,
and W. M. Wood-Vasey
[show abstract]
[hide abstract]
ABSTRACT: SN 2009ku, discovered by Pan-STARRS-1, is a Type Ia supernova (SN Ia), and a member of the distinct SN 2002cx-like class of SNe Ia. Its light curves are similar to the prototypical SN 2002cx, but are slightly broader and have a later rise to maximum in g. SN 2009ku is brighter (~0.6 mag) than other SN 2002cx-like objects, peaking at MV = –18.4 mag, which is still significantly fainter than typical SNe Ia. SN 2009ku, which had an ejecta velocity of ~2000 km s–1 at 18 days after maximum brightness, is spectroscopically most similar to SN 2008ha, which also had extremely low-velocity ejecta. However, SN 2008ha had an exceedingly low luminosity, peaking at MV = –14.2 mag, ~4 mag fainter than SN 2009ku. The contrast of high luminosity and low ejecta velocity for SN 2009ku is contrary to an emerging trend seen for the SN 2002cx class. SN 2009ku is a counterexample of a previously held belief that the class was more homogeneous than typical SNe Ia, indicating that the class has a diverse progenitor population and/or complicated explosion physics. As the first example of a member of this class of objects from the new generation of transient surveys, SN 2009ku is an indication of the potential for these surveys to find rare and interesting objects.
The Astrophysical Journal Letters 03/2011; 731(1):L11. · 5.53 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: The origin of ultra-high-energy cosmic rays (UHECRs) remains an enigma. They offer a window to new physics, including tests of physical laws at energies unattainable by terrestrial accelerators. They must be accelerated locally, otherwise, background radiations would severely suppress the flux of protons and nuclei, at energies above the Greisen-Zatsepin-Kuzmin (GZK) limit. Nearby, gamma ray bursts (GRBs), hypernovae, active galactic nuclei and their flares have all been suggested and debated as possible sources. A local sub-population of type Ibc supernovae (SNe) with mildly relativistic outflows have been detected as sub-energetic GRBs, X-ray flashes and recently as radio afterglows without detected GRB counterparts. Here, we measure the size-magnetic field evolution, baryon loading and energetics, using the observed radio spectra of SN 2009bb. We place such engine-driven SNe above the Hillas line and establish that they can readily explain the post-GZK UHECRs.
Nature Communications 02/2011; 2:175. · 7.40 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We present extensive radio and X-ray observations of the nearby Type Ic SN 2007gr in NGC 1058 obtained with the Very Large Array (VLA) and the Chandra X-ray Observatory and spanning 5 to 150 days after explosion. Through our detailed modeling of these data, we estimate the properties of the blast wave and the circumstellar environment. We find evidence for a freely expanding and non-relativistic explosion with an average blast wave velocity, , and a total internal energy for the radio emitting material of E 2 × 1046 erg assuming equipartition of energy between electrons and magnetic fields ( e = B = 0.1). The temporal and spectral evolution of the radio emission points to a stellar wind-blown environment shaped by a steady progenitor mass loss rate of (wind velocity, vw = 103 km s–1). These parameters are fully consistent with those inferred for other SNe Ibc and are in line with the expectations for an ordinary, homologous SN explosion. Our results are at odds with those of Paragi et al. who recently reported evidence for a relativistic blast wave in SN 2007gr based on their claim that the radio emission was resolved away in a low signal-to-noise Very Long Baseline Interferometry (VLBI) observation. Here we show that the exotic physical scenarios required to explain the claimed relativistic velocity—extreme departures from equipartition and/or a highly collimated outflow—are excluded by our detailed VLA radio observations. Moreover, we present an independent analysis of the VLBI data and propose that a modest loss of phase coherence provides a more natural explanation for the apparent flux density loss which is evident on both short and long baselines. We conclude that SN 2007gr is an ordinary Type Ibc supernova.
The Astrophysical Journal 11/2010; 725(1):922. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We report on VLBI as well as Very Large Array radio observations of the Type I b/c supernova 2009bb. The high radio luminosity of this supernova seems to require relativistic outflow, implying that the early radio emission was "engine-driven," that is, driven by collimated outflow from a compact object, even though no gamma-ray emission was seen. The radio light curve shows a general decline, with a "bump" near t = 52 d, seen most prominently at 5 GHz. The light-curve bump could be either engine-driven or it might represent the turn-on of the normal radio emission from a supernova, driven by interaction with the circumstellar material rather than by the engine. We undertook VLBI observations to resolve SN 2009bb's relativistic outflow. Our observations constrain the angular outer radius at an age of 85 d to be <0.64 mas, corresponding to <4 × 1017 cm and an average apparent expansion speed of <1.74 c. This result is consistent with the moderately relativistic ejecta speeds implied by the radio luminosity and spectrum.
The Astrophysical Journal 11/2010; 725(1):4. · 6.02 Impact Factor
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S. Gezari,
A. Rest,
M. E. Huber,
G. Narayan,
K. Forster,
J. D. Neill,
D. C. Martin,
S. Valenti,
S. J. Smartt,
R. Chornock, [......],
J. S. Morgan,
P. M. Onaka,
P. A. Price,
P. H. Rhoads,
W. A. Siegmund,
C. W. Stubbs,
J. L. Tonry,
R. J. Wainscoat,
M. F. Waterson,
and C. G. Wynn-Williams
[show abstract]
[hide abstract]
ABSTRACT: We present the early UV and optical light curve of Type IIP supernova (SN) 2010aq at z = 0.0862, and compare it to analytical models for thermal emission following SN shock breakout in a red supergiant star. SN 2010aq was discovered in joint monitoring between the Galaxy Evolution Explorer (GALEX) Time Domain Survey (TDS) in the NUV and the Pan-STARRS1 Medium Deep Survey (PS1 MDS) in the g, r, i, and z bands. The GALEX and Pan-STARRS1 observations detect the SN less than 1 day after the shock breakout, measure a diluted blackbody temperature of 31, 000 ± 6000 K 1 day later, and follow the rise in the UV/optical light curve over the next 2 days caused by the expansion and cooling of the SN ejecta. The high signal-to-noise ratio of the simultaneous UV and optical photometry allows us to fit for a progenitor star radius of 700 ± 200R ☉, the size of a red supergiant star. An excess in UV emission two weeks after shock breakout compared with SNe well fitted by model atmosphere-code synthetic spectra with solar metallicity is best explained by suppressed line blanketing due to a lower metallicity progenitor star in SN 2010aq. Continued monitoring of PS1 MDS fields by the GALEX TDS will increase the sample of early UV detections of Type II SNe by an order of magnitude and probe the diversity of SN progenitor star properties.
The Astrophysical Journal Letters 08/2010; 720(1):L77. · 5.53 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We report on VLBI, as well as VLA radio observations of the Type Ib/c supernova 2009bb. The high radio luminosity of this supernova seems to require relativistic outflow, implying that the early radio emission was "engine-driven", that is driven by collimated outflow from a compact object, even though no gamma-ray emission was seen. The radio light curve shows a general decline, with a "bump" near t = 52 d, seen most prominently at 5 GHz. The lightcurve bump could be either engine-driven, or it might represent the turn-on of the normal radio emission from a supernova, driven by interaction with the CSM rather than by the engine. We undertook VLBI observations to resolve SN 2009bb's relativistic outflow. Our observations constrain the angular outer radius at an age of 85 d to be <0.64 mas, corresponding to <4 x 10^17 cm and an average apparent expansion speed of <1.74c. This result is consistent with the moderately relativistic ejecta speeds implied by the radio luminosity and spectrum. Comment: 7 pages; 3 figures, accepted for publication in ApJ
06/2010;
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H B Perets,
A Gal-Yam,
P A Mazzali,
D Arnett,
D Kagan,
A V Filippenko,
W Li,
I Arcavi,
S B Cenko,
D B Fox, [......],
E O Ofek,
L Bildsten,
G Nelemans,
K J Shen,
N N Weinberg,
B D Metzger,
A L Piro,
E Quataert,
M Kiewe,
D Poznanski
[show abstract]
[hide abstract]
ABSTRACT: Supernovae are thought to arise from two different physical processes. The cores of massive, short-lived stars undergo gravitational core collapse and typically eject a few solar masses during their explosion. These are thought to appear as type Ib/c and type II supernovae, and are associated with young stellar populations. In contrast, the thermonuclear detonation of a carbon-oxygen white dwarf, whose mass approaches the Chandrasekhar limit, is thought to produce type Ia supernovae. Such supernovae are observed in both young and old stellar environments. Here we report a faint type Ib supernova, SN 2005E, in the halo of the nearby isolated galaxy, NGC 1032. The 'old' environment near the supernova location, and the very low derived ejected mass ( approximately 0.3 solar masses), argue strongly against a core-collapse origin. Spectroscopic observations and analysis reveal high ejecta velocities, dominated by helium-burning products, probably excluding this as a subluminous or a regular type Ia supernova. We conclude that it arises from a low-mass, old progenitor, likely to have been a helium-accreting white dwarf in a binary. The ejecta contain more calcium than observed in other types of supernovae and probably large amounts of radioactive (44)Ti.
Nature 05/2010; 465(7296):322-5. · 36.28 Impact Factor
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S. B. Cenko,
D. A. Frail,
F. A. Harrison,
S. R. Kulkarni,
E. Nakar,
P. C. Chandra,
N. R. Butler,
D. B. Fox,
A. Gal-Yam,
M. M. Kasliwal, [......],
H. I. Teplitz,
M. W. Werner,
D. C.-J. Bock,
J. S. Bloom,
D. A. Starr,
A. V. Filippenko,
R. A. Chevalier,
N. Gehrels,
J. N. Nousek,
and T. Piran
[show abstract]
[hide abstract]
ABSTRACT: Long-duration gamma-ray bursts (GRBs) are widely believed to be highly collimated explosions (bipolar conical outflows with half-opening angle θ 1°-10°). As a result of this beaming factor, the true energy release from a GRB is usually several orders of magnitude smaller than the observed isotropic value. Measuring this opening angle, typically inferred from an achromatic steepening in the afterglow light curve (a "jet" break), has proven exceedingly difficult in the Swift era. Here, we undertake a study of five of the brightest (in terms of the isotropic prompt γ-ray energy release, E γ,iso) GRBs in the Swift era to search for jet breaks and hence constrain the collimation-corrected energy release. We present multi-wavelength (radio through X-ray) observations of GRBs 050820A, 060418, and 080319B, and construct afterglow models to extract the opening angle and beaming-corrected energy release for all three events. Together with results from previous analyses of GRBs 050904 and 070125, we find evidence for an achromatic jet break in all five events, strongly supporting the canonical picture of GRBs as collimated explosions. The most natural explanation for the lack of observed jet breaks from most Swift GRBs is therefore selection effects. However, the opening angles for the events in our sample are larger than would be expected if all GRBs had a canonical energy release of ~1051 erg. The total energy release we measure for the "hyper-energetic" (E tot 1052 erg) events in our sample is large enough to start challenging models with a magnetar as the compact central remnant.
The Astrophysical Journal 02/2010; 711(2):641. · 6.02 Impact Factor
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A M Soderberg,
S Chakraborti,
G Pignata,
R A Chevalier,
P Chandra,
A Ray,
M H Wieringa,
A Copete,
V Chaplin,
V Connaughton, [......],
M Hamuy,
C Fransson,
O Fox,
E M Levesque,
J E Grindlay,
P Challis,
R J Foley,
R P Kirshner,
P A Milne,
M A P Torres
[show abstract]
[hide abstract]
ABSTRACT: Long duration gamma-ray bursts (GRBs) mark the explosive death of some massive stars and are a rare sub-class of type Ibc supernovae. They are distinguished by the production of an energetic and collimated relativistic outflow powered by a central engine (an accreting black hole or neutron star). Observationally, this outflow is manifested in the pulse of gamma-rays and a long-lived radio afterglow. Until now, central-engine-driven supernovae have been discovered exclusively through their gamma-ray emission, yet it is expected that a larger population goes undetected because of limited satellite sensitivity or beaming of the collimated emission away from our line of sight. In this framework, the recovery of undetected GRBs may be possible through radio searches for type Ibc supernovae with relativistic outflows. Here we report the discovery of luminous radio emission from the seemingly ordinary type Ibc SN 2009bb, which requires a substantial relativistic outflow powered by a central engine. A comparison with our radio survey of type Ibc supernovae reveals that the fraction harbouring central engines is low, about one per cent, measured independently from, but consistent with, the inferred rate of nearby GRBs. Independently, a second mildly relativistic supernova has been reported.
Nature 01/2010; 463(7280):513-5. · 36.28 Impact Factor
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E. M. Levesque, A. M. Soderberg,
R. J. Foley,
E. Berger,
L. J. Kewley,
S. Chakraborti,
A. Ray,
M. A. P. Torres,
P. Challis,
R. P. Kirshner, [......],
V. Connaughton,
A. Copete,
O. Fox,
C. Fransson,
J. E. Grindlay,
M. A. Hamuy,
P. A. Milne,
G. Pignata,
M. D. Stritzinger,
and M. H. Wieringa
[show abstract]
[hide abstract]
ABSTRACT: We investigate the environment of the nearby (d 40 Mpc) broad-lined Type Ic supernova (SN) 2009bb. This event was observed to produce a relativistic outflow likely powered by a central accreting compact object. While such a phenomenon was previously observed only in long-duration gamma-ray bursts (LGRBs), no LGRB was detected in association with SN 2009bb. Using an optical spectrum of the SN 2009bb explosion site, we determine a variety of interstellar medium properties for the host environment, including metallicity, young stellar population age, and star formation rate. We compare the SN explosion site properties to observations of LGRB and broad-lined SN Ic host environments on optical emission line ratio diagnostic diagrams. Based on these analyses, we find that the SN 2009bb explosion site has a metallicity between 1.7 Z ☉ and 3.5 Z ☉, in agreement with other broad-lined SN Ic host environments and at odds with the low-redshift LGRB host environments and recently proposed maximum metallicity limits for relativistic explosions. We consider the implications of these findings and the impact that SN 2009bb's unusual explosive properties and environment have on our understanding of the key physical ingredient that enables some SNe to produce a relativistic outflow.
The Astrophysical Journal Letters 12/2009; 709(1):L26. · 5.53 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: We present our analysis of a sample of type II supernova light curves
measured by the Caltech Core Collapse Project (CCCP). CCCP is a large
observational program which made use of the robotic 60" and the Hale
200" telescopes to obtain optical photometry, spectroscopy and IR
photometry of 50 nearby core-collapse supernovae. It provides a
complete sample of core-collapse events, with well-defined selection
criteria, and uniform, high-quality optical/IR observations.
Our goal is to characterize the little-studied properties of
core-collapse supernovae as a population. We attempt to describe the
light curves of type II SNe of all subtypes using only a few empirically
derived parameters, with the aspiration of explaining most of our light
curves with a single tunable formula. Eventually, associating a physical
meaning to each of the parameters may help to better understand the
mechanisms underlying the diversity in type II SN light curves.
11/2009; 214.
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N R Tanvir,
D B Fox,
A J Levan,
E Berger,
K Wiersema,
J P U Fynbo,
A Cucchiara,
T Krühler,
N Gehrels,
J S Bloom, [......],
B P Schmidt, A M Soderberg,
J Sollerman,
A W Stephens,
G Stratta,
T N Ukwatta,
D Watson,
E Westra,
T Wold,
C Wolf
[show abstract]
[hide abstract]
ABSTRACT: Long-duration gamma-ray bursts (GRBs) are thought to result from the explosions of certain massive stars, and some are bright enough that they should be observable out to redshifts of z > 20 using current technology. Hitherto, the highest redshift measured for any object was z = 6.96, for a Lyman-alpha emitting galaxy. Here we report that GRB 090423 lies at a redshift of z approximately 8.2, implying that massive stars were being produced and dying as GRBs approximately 630 Myr after the Big Bang. The burst also pinpoints the location of its host galaxy.
Nature 10/2009; 461(7268):1254-7. · 36.28 Impact Factor
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E. Berger, A. M. Soderberg,
R. A. Chevalier,
C. Fransson,
R. J. Foley,
D. C. Leonard,
J. H. Debes,
A. M. Diamond-Stanic,
A. K. Dupree,
I. I. Ivans,
J. Simmerer,
I. B. Thompson,
and C. A. Tremonti
[show abstract]
[hide abstract]
ABSTRACT: We present multi-epoch high-resolution optical spectroscopy, UV/radio/X-ray imaging, and archival Hubble and Spitzer observations of an intermediate luminosity optical transient recently discovered in the nearby galaxy NGC 300. We find that the transient (NGC 300 OT2008-1) has a peak absolute magnitude of M bol –11.8 mag, intermediate between novae and supernovae, and similar to the recent events M85 OT2006-1 and SN 2008S. Our high-resolution spectra, the first for this event, are dominated by intermediate velocity (~200-1000 km s–1) hydrogen Balmer lines and Ca II emission and absorption lines that point to a complex circumstellar environment, reminiscent of the yellow hypergiant IRC+10420. In particular, we detect asymmetric Ca II H&K absorption with a broad red wing extending to ~103 km s–1, indicative of gas inflow at high velocity (possibly the wind of a massive binary companion). The low luminosity, intermediate velocities, and overall similarity to a known eruptive star indicate that the event did not result in a complete disruption of the progenitor. We identify the progenitor in archival Spitzer observations, with deep upper limits from Hubble data. The spectral energy distribution points to a dust-enshrouded star with a luminosity of about 6 × 104 L ☉, indicative of a ~10-20 M ☉ progenitor (or binary system). This conclusion is in good agreement with our interpretation of the outburst and circumstellar properties. The lack of significant extinction in the transient spectrum indicates that the dust surrounding the progenitor was cleared by the outburst. We thus predict that the progenitor should be eventually visible with Hubble if the transient event marks an evolutionary transition to a dust-free state, or with Spitzer if the event marks a cyclical process of dust formation.
The Astrophysical Journal 06/2009; 699(2):1850. · 6.02 Impact Factor
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N. R. Tanvir,
D. B. Fox,
A. J. Levan,
E. Berger,
K. Wiersema,
J. P. U. Fynbo,
A. Cucchiara,
T. Kruehler,
N. Gehrels,
J. S. Bloom, [......],
B. P. Schmidt, A. M. Soderberg,
J. Sollerman,
A. W. Stephens,
G. Stratta,
T. N. Ukwatta,
D. Watson,
E. Westra,
T. Wold,
C Wolf
[show abstract]
[hide abstract]
ABSTRACT: It is thought that the first generations of massive stars in the Universe were an important, and quite possibly dominant, source of the ultra-violet radiation that reionized the hydrogen gas in the intergalactic medium (IGM); a state in which it has remained to the present day. Measurements of cosmic microwave background anisotropies suggest that this phase-change largely took place in the redshift range z=10.8 +/- 1.4, while observations of quasars and Lyman-alpha galaxies have shown that the process was essentially completed by z=6. However, the detailed history of reionization, and characteristics of the stars and proto-galaxies that drove it, remain unknown. Further progress in understanding requires direct observations of the sources of ultra-violet radiation in the era of reionization, and mapping the evolution of the neutral hydrogen fraction through time. The detection of galaxies at such redshifts is highly challenging, due to their intrinsic faintness and high luminosity distance, whilst bright quasars appear to be rare beyond z~7. Here we report the discovery of a gamma-ray burst, GRB 090423, at redshift z=8.26 -0.08 +0.07. This is well beyond the redshift of the most distant spectroscopically confirmed galaxy (z=6.96) and quasar (z=6.43). It establishes that massive stars were being produced, and dying as GRBs, ~625 million years after the Big Bang. In addition, the accurate position of the burst pinpoints the location of the most distant galaxy known to date. Larger samples of GRBs beyond z~7 will constrain the evolving rate of star formation in the early universe, while rapid spectroscopy of their afterglows will allow direct exploration of the progress of reionization with cosmic time. Comment: Submitted to Nature
06/2009;
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D. A. Perley,
S. B. Cenko,
J. S. Bloom,
H. -W. Chen,
N. R. Butler,
D. Kocevski,
J. X. Prochaska,
M. Brodwin,
K. Glazebrook,
M. M. Kasliwal,
S. R. Kulkarni,
S. Lopez,
E. O. Ofek,
M. Pettini, A. M. Soderberg,
D. Starr
[show abstract]
[hide abstract]
ABSTRACT: In this work we present the first results of our imaging campaign at Keck Observatory to identify the host galaxies of "dark" gamma-ray bursts (GRBs), events with no detected optical afterglow or with detected optical flux significantly fainter than expected from the observed X-ray afterglow. We find that out of a uniform sample of 29 Swift bursts rapidly observed by the Palomar 60-inch telescope through March 2008 (14 of which we classify as dark), all events have either a detected optical afterglow, a probable optical host-galaxy detection, or both. Our results constrain the fraction of Swift GRBs coming from very high redshift (z > 7), such as the recent GRB 090423, to between 0.2-7 percent at 80% confidence. In contrast, a significant fraction of the sample requires large extinction columns (host-frame A_V > 1 mag, with several events showing A_V > 2-6 mag), identifying dust extinction as the dominant cause of the dark GRB phenomenon. We infer that a significant fraction of GRBs (and, by association, of high-mass star formation) occurs in highly obscured regions. However, the host galaxies of dark GRBs seem to have normal optical colors, suggesting that the source of obscuring dust is local to the vicinity of the GRB progenitor or highly unevenly distributed within the host galaxy. Comment: 21 pages, 10 figures. Published in AJ
05/2009;
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D. A. Howell,
A. Conley,
M. Della Valle,
P. E. Nugent,
S. Perlmutter,
G. H. Marion,
K. Krisciunas,
C. Badenes,
P. Mazzali,
G. Aldering, [......],
R. Quimby,
A. Rest,
A. Riess,
M. Sako, A. M. Soderberg,
L. Strolger,
R Thomas,
M. Turatto,
S. Van Dyk,
W. M. Wood-Vasey
[show abstract]
[hide abstract]
ABSTRACT: In the next decade Type Ia supernovae (SNe Ia) will be used to test theories predicting changes in the Dark Energy equation of state with time. Ultimately this requires a dedicated space mission like JDEM. SNe Ia are mature cosmological probes --- their limitations are well characterized, and a path to improvement is clear. Dominant systematic errors include photometric calibration, selection effects, reddening, and population-dependent differences. Building on past lessons, well-controlled new surveys are poised to make strides in these areas: the Palomar Transient Factory, Skymapper, La Silla QUEST, Pan-STARRS, the Dark Energy Survey, LSST, and JDEM. They will obviate historical calibrations and selection biases, and allow comparisons via large subsamples. Some systematics follow from our ignorance of SN Ia progenitors, which there is hope of determining with SN Ia rate studies from 0<z<4. Aside from cosmology, SNe Ia regulate galactic and cluster chemical evolution, inform stellar evolution, and are laboratories for extreme physics. Essential probes of SNe Ia in these contexts include spectroscopy from the UV to the IR, X-ray cluster and SN remnant observations, spectropolarimetry, and advanced theoretical studies. While there are an abundance of discovery facilities planned, there is a deficit of follow-up resources. Living in the systematics era demands deep understanding rather than larger statistics. NOAO ReSTAR initiative to build 2-4m telescopes would provide necessary follow-up capability. Finally, to fully exploit LSST, well-matched wide-field spectroscopic capabilities are desirable.
04/2009;
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Matthew McQuinn,
Joshua S. Bloom,
Jonathan Grindlay,
David Band,
S. D. Barthelmy,
E. Berger,
A Corsi,
S. Covino,
G. J. Fishman,
Steven R. Furlanetto, [......],
A. S. Kutyrev,
Abraham Loeb,
S. Harvey Moseley,
Tsvi Piran,
L. Piro,
J. X. Prochaska,
R. Salvaterra,
P. Schady, A. M. Soderberg,
G. Tagliaferri
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ABSTRACT: The first structures in the Universe formed at z>7, at higher redshift than all currently known galaxies. Since GRBs are brighter than other cosmological sources at high redshift and exhibit simple power-law afterglow spectra that is ideal for absorption studies, they serve as powerful tools for studying the early universe. New facilities planned for the coming decade will be able to obtain a large sample of high-redshift GRBs. Such a sample would constrain the nature of the first stars, galaxies, and the reionization history of the Universe.
03/2009;
