231 research outputs found
Burst of the Century? A Case Study of the Afterglow of Nearby Ultra-Bright GRB 130427A
GRB 130427A is the brightest gamma-ray burst observed by any satellite in almost
30 years and one of the most thoroughly observed. I will present a summary of the
worldwide campaign to monitor the afterglow of this event from GHz to TeV energies
and from seconds to years after the explosion. Remarkably, the entire data set can be
described to good agreement using standard synchrotron afterglow theory, providing
strong support for the validity the basic model in describing the evolution of this event
and for GRB afterglows generally. Distinct forward and reverse shock components are
resolved in both the SED and multifrequency light curves; the late-time high-energy
emission seen by LAT is produced by the forward shock. We also infer a tenuous,
wind-stratified medium surrounding this burst, suggesting a massive, low-metallicity
progenitor. While GRB 130427A was an incredibly rare and fortuitous event its
properties are probably not intrinsically unusual, and it provides lessons for what
might be routinely achieved in the future with faster and deeper multiwavelength
follow-up of gamma-ray bursts
Gamma-ray Bursts, Classified Physically
From Galactic binary sources, to extragalactic magnetized neutron stars, to
long-duration GRBs without associated supernovae, the types of sources we now
believe capable of producing bursts of gamma-rays continues to grow apace. With
this emergent diversity comes the recognition that the traditional (and newly
formulated) high-energy observables used for identifying sub-classes does not
provide an adequate one-to-one mapping to progenitors. The popular
classification of some > 100 sec duration GRBs as ``short bursts'' is not only
an unpalatable retronym and syntactically oxymoronic but highlights the
difficultly of using what was once a purely phenomenological classification to
encode our understanding of the physics that gives rise to the events. Here we
propose a physically based classification scheme designed to coexist with the
phenomenological system already in place and argue for its utility and
necessity.Comment: 6 pages, 3 figures. Slightly expanded version of solicited paper to
be published in the Proceedings of ''Gamma Ray Bursts 2007,'' Santa Fe, New
Mexico, November 5-9. Edited by E. E. Fenimore, M. Galassi, D. Palme
Burst of the Century? A Case Study of the Afterglow of Nearby Ultra-Bright GRB 130427A
GRB 130427A is the brightest gamma-ray burst observed by any satellite in almost
30 years and one of the most thoroughly observed. I will present a summary of the
worldwide campaign to monitor the afterglow of this event from GHz to TeV energies
and from seconds to years after the explosion. Remarkably, the entire data set can be
described to good agreement using standard synchrotron afterglow theory, providing
strong support for the validity the basic model in describing the evolution of this event
and for GRB afterglows generally. Distinct forward and reverse shock components are
resolved in both the SED and multifrequency light curves; the late-time high-energy
emission seen by LAT is produced by the forward shock. We also infer a tenuous,
wind-stratified medium surrounding this burst, suggesting a massive, low-metallicity
progenitor. While GRB 130427A was an incredibly rare and fortuitous event its
properties are probably not intrinsically unusual, and it provides lessons for what
might be routinely achieved in the future with faster and deeper multiwavelength
follow-up of gamma-ray bursts
AT 2023prq: A Classical Nova in the Halo of the Andromeda Galaxy
The classical nova, AT 2023prq, was discovered on 2023 August 15 and is
located at a distance of 46 kpc from the Andromeda Galaxy (M 31). Here we
report photometry and spectroscopy of the nova. The 'very fast'
( d) and low luminosity ()
nature of the transient along with the helium in its spectra would indicate
that AT 2023prq is a 'faint-and-fast' He/N nova. Additionally, at such a large
distance from the centre of M 31, AT 2023prq is a member of the halo nova
population.Comment: 3 pages, 1 figure; Published in Research Notes of the American
Astronomical Societ
SubmilliJansky Transients in Archival Radio Observations
[ABRIDGED] We report the results of a 944-epoch survey for transient sources
with archival data from the Very Large Array spanning 22 years with a typical
epoch separation of 7 days. Observations were obtained at 5 or 8.4 GHz for a
single field of view with a full-width at half-maximum of 8.6' and 5.1',
respectively, and achieved a typical point-source detection threshold at the
beam center of ~300 microJy per epoch. Ten transient sources were detected with
a significance threshold such that only one false positive would be expected.
Of these transients, eight were detected in only a single epoch. Two transients
were too faint to be detected in individual epochs but were detected in
two-month averages. None of the ten transients was detected in longer-term
averages or associated with persistent emission in the deep image produced from
the combination of all epochs. The cumulative rate for the short timescale
radio transients above 370 microJy at 5 and 8.4 GHz is 0.07 < R < 40 deg^-2
yr^-1, where the uncertainty is due to the unknown duration of the transients,
20 min < t_char < 7 days. A two-epoch survey for transients will detect 1.5 +/-
0.4 transient per square degrees above a flux density of 370 microJy. Two
transients are associated with galaxies at z=0.040 and z=0.249. These may be
similar to the peculiar Type Ib/c radio supernova SN 1998bw associated with GRB
980428. Six transients have no counterparts in the optical or infrared (R=27,
Ks=18). The hosts and progenitors of these transients are unknown.Comment: Accepted for ApJ; full quality figures available at
http://astro.berkeley.edu/~gbower/ps/rt.pd
Late-time VLA reobservations rule out ULIRG-like host galaxies for most pre-Swift long-duration gamma-ray bursts
We present new Jansky Very Large Array observations of five pre-Swift gamma-ray bursts for which an ultraluminous [star formation rate (SFR) > 100 M⊙; yr-1] dusty host galaxy had previously been inferred from radio or submillimetre observations taken within a few years after the burst. In four of the five cases, we no longer detect any source at the host location to limits much fainter than the original observations, ruling out the existence of an ultraluminous galaxy hosting any of these gamma-ray bursts (GRBs). We continue to detect a source at the position of GRB 980703, but it is much fainter than it was a decade ago and the inferred radio SFR (~80M⊙) is relatively modest. The radio flattening at 200-1000 d observed in the light curve of this GRB may have been caused by a decelerating counterjet oriented 180 deg away from the viewer, although an unjetted wind model can also explain the data. Our results eliminate all well-established ultraluminous infrared galaxies (ULIRGs) among the pre-Swift host population. They also rule out all cases for which an unobscured GRB was found in a galaxy dominated by heavily obscured star formation. When GRBs do occur in ULIRGs, the afterglow is almost always observed to be heavily obscured, consistent with the large dust opacities and high dust covering fractions characteristic of these systems. © 2016 The Authors
Radio Properties of z>4 Optically-Selected Quasars
We report on two programs to address differential evolution between the
radio-loud and radio-quiet quasar populations at high (z>4) redshift. Both
programs entail studying the radio properties of optically-selected quasars.
First, we have observed 32 optically-selected, high-redshift (z>4) quasars with
the VLA at 6 cm (5 GHz). These sources comprise a statistically complete and
well-understood sample. We detect four quasars above our 3-sigma limit of ~0.15
mJy, which is sufficiently sensitive to detect all radio-loud quasars at the
probed redshift range. Second, we have correlated 134 z>4 quasars, comprising
all such sources that we are aware of as of mid-1999, with FIRST and NVSS.
These two recent 1.4 GHz VLA sky surveys reach 3-sigma limits of approximately
0.6 mJy and 1.4 mJy respectively. We identify a total of 15 z>4 quasars, of
which six were not previously known to be radio-loud. The depth of these
surveys does not reach the radio-loud/radio-quiet demarcation luminosity
density (L(1.4 GHz) = 10^32.5 h(50)^(-2) ergs/s/Hz) at the redshift range
considered; this correlation therefore only provides a lower limit to the
radio-loud fraction of quasars at high-redshift. The two programs together
identify eight new radio-loud quasars at z>4, a significant increase over the
seven currently in the published literature. We find no evidence for radio-loud
fraction depending on optical luminosity for -25 > M_B > -28 at z~2, or for
-26>M_B>-28 at z>4. Our results also show no evolution in the radio-loud
fraction between z~2 and z>4 (-26>M_B>-28).Comment: 19 pages, 7 figures; to appear in The Astronomical Journal (April
2000
Late time HST UV and optical observations of AT~2018cow: extracting a cow from its background
The bright, blue, rapidly evolving AT2018cow is a well-studied peculiar
extragalactic transient. Despite an abundance of multi-wavelength data, there
still is no consensus on the nature of the event. We present our analysis of
three epochs of Hubble Space Telescope (HST) observations spanning the period
from 713-1474 days post burst, paying particular attention to uncertainties of
the transient photometry introduced by the complex background in which
AT2018cow resides. Photometric measurements show evident fading in the UV and
more subtle but significant fading in the optical. During the last HST
observation, the transient's optical/UV colours were still bluer than those of
the substantial population of compact, young, star-forming regions in the host
of AT2018cow, suggesting some continued transient contribution to the light.
However, a compact source underlying the transient would substantially modify
the resulting spectral energy distribution, depending on its contribution in
the various bands. In particular, in the optical filters, the complex, diffuse
background poses a problem for precise photometry. An underlying cluster is
expected for a supernova occurring within a young stellar environment or a
tidal-disruption event (TDE) within a dense older one. While many recent works
have focused on the supernova interpretation, we note the substantial
similarity in UV light-curve morphology between AT2018cow and several tidal
disruption events around supermassive black holes. Assuming AT2018cow arises
from a TDE-like event, we fit the late-time emission with a disc model and find
M. Further observations are necessary to
determine the late-time evolution of the transient and its immediate
environment.Comment: 16 pages, 10 figures. Accepted for publication in MNRA
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