129 research outputs found
Inhomogeneous Jets from Neutron Star Mergers: One Jet to Rule them all
Using the resultant profiles from 3D hydrodynamic simulations of relativistic
jets interacting with neutron star merger wind ejecta, we show how the
inhomogeneity of energy and velocity {across the jet surface profile} can alter
the observed afterglow lightcurve. We find that the peak afterglow flux depends
sensitively on the observer's line-of-sight, not only via the jet inclination
but also through the jet rotation: for an observer viewing the afterglow within
the GRB-bright jet core, we find a peak flux variability on the order
dex through rotational orientation and dex for the polar inclination. An
observed afterglow's peak flux can be used to infer the jet kinetic energy, and
where a top-hat jet is assumed, we find the range of inferred jet kinetic
energies for our various model afterglow lightcurves (with fixed model
parameters), covers of the observed short GRB population.
Additionally, we present an analytic jet structure function that includes
physically motivated parameter uncertainties due to variability through the
rotation of the source. % An approximation for the change in collimation due to
the merger ejecta mass is included and we show that by considering the observed
range of merger ejecta masses from short GRB kilonova candidates, a population
of merger jets with a fixed intrinsic jet energy is capable of explaining the
observed broad diversity seen in short GRB afterglows.Comment: 21 pages, 8 figure
Gamma-Ray Bursts Trace UV Metrics of Star Formation over 3 < z < 5
We present the first uniform treatment of long duration gamma-ray burst (GRB)
host galaxy detections and upper limits over the redshift range 3<z<5, a key
epoch for observational and theoretical efforts to understand the processes,
environments, and consequences of early cosmic star formation. We contribute
deep imaging observations of 13 GRB positions yielding the discovery of eight
new host galaxies. We use this dataset in tandem with previously published
observations of 31 further GRB positions to estimate or constrain the host
galaxy rest-frame ultraviolet (UV; 1600 A) absolute magnitudes M_UV. We then
use the combined set of 44 M_UV estimates and limits to construct the M_UV
luminosity function (LF) for GRB host galaxies over 3<z<5 and compare it to
expectations from Lyman break galaxy (LBG) photometric surveys with the Hubble
Space Telescope. Adopting standard prescriptions for the luminosity dependence
of galaxy dust obscuration (and hence, total star formation rate), we find that
our LF is compatible with LBG observations over a factor of 600x in host
luminosity, from M_UV = -22.5 mag to >-15.6 mag, and with extrapolations of the
assumed Schechter-type LF well beyond this range. We review proposed
astrophysical and observational biases for our sample, and find they are for
the most part minimal. We therefore conclude, as the simplest interpretation of
our results, that GRBs successfully trace UV metrics of cosmic star formation
over the range 3<z<5. Our findings suggest GRBs are providing an accurate
picture of star formation processes from z ~3 out to the highest redshifts.Comment: publ. ApJ 809 (2015) 76; 14 figures; replacement to reflect changes
to v1 (rounding effects, diff. LF from Bouwens
GRB 191016A: A Long Gamma-Ray Burst Detected by TESS
The TESS exoplanet-hunting mission detected the rising and decaying optical
afterglow of GRB 191016A, a long Gamma-Ray Burst (GRB) detected by Swift-BAT
but without prompt XRT or UVOT follow-up due to proximity to the moon. The
afterglow has a late peak at least 1000 seconds after the BAT trigger, with a
brightest-detected TESS datapoint at 2589.7 s post-trigger. The burst was not
detected by Fermi-LAT, but was detected by Fermi-GBM without triggering,
possibly due to the gradual nature of rising light curve. Using ground-based
photometry, we estimate a photometric redshift of . Combined with the high-energy emission and optical peak time
derived from TESS, estimates of the bulk Lorentz factor
range from . The burst is relatively bright, with a peak optical
magnitude in ground-based follow-up of mag. Using published
distributions of GRB afterglows and considering the TESS sensitivity and
sampling, we estimate that TESS is likely to detect GRB afterglow per
year above its magnitude limit.Comment: 10 pages, 7 figures. Accepted for publication in the Astrophysical
Journa
GRB jet structure and the jet break
We investigate the shape of the jet break in within-beam gamma-ray burst (GRB) optical afterglows for various lateral jet structure profiles. We consider cases with and without lateral spreading and a range of inclinations within the jet core half-opening angle, θc. We fit model and observed afterglow light curves with a smoothly-broken power-law function with a free-parameter κ that describes the sharpness of the break. We find that the jet break is sharper (κ is greater) when lateral spreading is included than in the absence of lateral spreading. For profiles with a sharp-edged core, the sharpness parameter has a broad range of 0.1 ≲ κ ≲ 4.6, whereas profiles with a smooth-edged core have a narrower range of 0.1 ≲ κ ≲ 2.2 when models both with and without lateral spreading are included. For sharp-edged jets, the jet break sharpness depends strongly on the inclination of the system within θc, whereas for smooth-edged jets, κ is more strongly dependent on the size of θc. Using a sample of 20 GRBs, we find 9 candidate smooth-edged jet structures and 8 candidate sharp-edged jet structures, while the remaining 3 are consistent with either. The shape of the jet break, as measured by the sharpness parameter κ, can be used as an initial check for the presence of lateral structure in within-beam GRBs where the afterglow is well-sampled at and around the jet-break time
The Rapidly Flaring Afterglow of the Very Bright and Energetic GRB 070125
We report on multiwavelength observations, ranging from X-ray to radio wave bands, of the IPN-localized gamma-ray burst GRB 070125. Spectroscopic observations reveal the presence of absorption lines due to O i,Si ii,and C iv, implying a likely redshift of z ¼1:547. The well-sampled light curves, in particular from 0.5 to 4 days after the burst, suggest a jet break at 3.7 days, corresponding to a jet opening angle of 8000 s.The evolution of the afterglow light curve is achromatic at all times.Late-time observationsof the afterglow do not show evidence for emission from an underlying host galaxy or supernova. Any host galaxy would be subluminous, consistent with current GRB host galaxy samples. Evidence for strong Mg ii absorption features is not found, which is perhaps surprising in view of the relatively high redshift of this burst and the high likelihood for such features along GRB-selected lines of sight
The Rapidly Flaring Afterglow of the Very Bright and Energetic GRB 070125
We report on multiwavelength observations, ranging from X-ray to radio wave bands, of the IPN-localized gamma-ray burst GRB 070125. Spectroscopic observations reveal the presence of absorption lines due to O I, Si II, and C IV, implying a likely redshift of z = 1.547. The well-sampled light curves, in particular from 0.5 to 4 days after the burst, suggest a jet break at 3.7 days, corresponding to a jet opening angle of ~7.0°, and implying an intrinsic GRB energy in the 1-10,000 keV band of around Eγ = (6.3–6.9) × 1051 ergs (based on the fluences measured by the gamma-ray detectors of the IPN). GRB 070125 is among the brightest afterglows observed to date. The SED implies a host extinction of AV \u3c 0.9 mag . Two rebrightening episodes are observed, one with excellent time coverage, showing an increase in flux of 56% in ~8000 s. The evolution of the afterglow light curve is achromatic at all times. Late-time observations of the afterglow do not show evidence for emission from an underlying host galaxy or supernova. Any host galaxy would be subluminous, consistent with current GRB host galaxy samples. Evidence for strong Mg II absorption features is not found, which is perhaps surprising in view of the relatively high redshift of this burst and the high likelihood for such features along GRB-selected lines of sight
The Rapidly Flaring Afterglow of the Very Bright and Energetic GRB 070125
We report on multi-wavelength observations, ranging from the X-ray to radio
wave bands, of the IPN-localized gamma-ray burst GRB 070125. Spectroscopic
observations reveal the presence of absorption lines due to O I, Si II, and C
IV, implying a likely redshift of z = 1.547. The well-sampled light curves, in
particular from 0.5 to 4 days after the burst, suggest a jet break at 3.7 days,
corresponding to a jet opening angle of ~7.0 degrees, and implying an intrinsic
GRB energy in the 1 - 10,000 keV band of around E = (6.3 - 6.9)x 10^(51) erg
(based on the fluences measured by the gamma-ray detectors of the IPN network).
GRB 070125 is among the brightest afterglows observed to date. The spectral
energy distribution implies a host extinction of Av < 0.9 mag. Two
rebrightening episodes are observed, one with excellent time coverage, showing
an increase in flux of 56% in ~8000 seconds. The evolution of the afterglow
light curve is achromatic at all times. Late-time observations of the afterglow
do not show evidence for emission from an underlying host galaxy or supernova.
Any host galaxy would be subluminous, consistent with current GRB host-galaxy
samples. Evidence for strong Mg II absorption features is not found, which is
perhaps surprising in view of the relatively high redshift of this burst and
the high likelihood for such features along GRB-selected lines of sight.Comment: 50 pages, 9 figures, 5 tables Accepted to the Astrophysical Journa
How Bad Or Good Are the External forward Shock Afterglow Models of Gamma-Ray Bursts?
The external forward shock models have been the standard paradigm to interpret the broadband afterglow data of gamma-ray bursts (GRBs). One prediction of the models is that some afterglow temporal breaks at different energy bands should be achromatic; that is, the break times should be the same in different frequencies. Multiwavelength observations in the Swift era have revealed chromatic afterglow behaviors at least in some GRBs, casting doubts on the external forward shock origin of GRB afterglows. In this paper, using a large sample of GRBs with both X-ray and optical afterglow data, we perform a systematic study to address the question: how bad or good are the external forward shock models? Our sample includes 85 GRBs up to 2014 March with well-monitored X-ray and optical light curves. Based on how well the data abide by the external forward shock models, we categorize them into five grades and three samples. The first two grades (Grade I and II) include 45 of 85 GRBs. They show evidence of, or are consistent with having, an achromatic break. The temporal and spectral behaviors in each afterglow segment are consistent with the predictions (the "closure relations") of the forward shock models. These GRBs are included in the Gold sample. The next two grades (Grade III and IV) include 37 of 85 GRBs. They are also consistent with having an achromatic break, even though one or more afterglow segments do not comply with the closure relations. These GRBs are included in the Silver sample. Finally, Grade V (3/85) shows direct evidence of chromatic behaviors, suggesting that the external shock models are inconsistent with the data. These are included in the Bad sample. We further perform statistical analyses of various observational properties (temporal index alpha, spectral index beta, break time t(b)) and model parameters (energy injection index q, electron spectral index p, jet opening angle theta(j), radiative efficiency eta(gamma), and so on) of the GRBs in the Gold sample, and derive constraints on the magnetization parameter epsilon(B) in the forward shock. Overall, we conclude that the simplest external forward shock models can account for the multiwavelength afterglow data of at least half of the GRBs. When more advanced modeling (e.g., long-lasting reverse shock, structured jets, arbitrary circumburst medium density profile) is invoked, up to >90% of the afterglows may be interpreted within the framework of the external shock models.National Basic Research Program (973 Programme) of China 2014CB845800National Natural Science Foundation of China 11303005, U1331202Guangxi Science Foundation 2013GXNSFFA019001Strategic Priority Research Program "The Emergence of Cosmological Structures" XDB09000000NASA NNX14AF85GDFG Kl 766/16-1, Kl 766/16-3Astronom
The Exotic Type Ic Broad-Lined Supernova SN 2018gep: Blurring the Line Between Supernovae and Fast Optical Transients
In the last decade a number of rapidly evolving transients have been
discovered that are not easily explained by traditional supernovae models. We
present optical and UV data on onee such object, SN 2018gep, that displayed a
fast rise with a mostly featureless blue continuum around maximum light, and
evolved to develop broad features more typical of a SN Ic-bl while retaining
significant amounts of blue flux throughout its observations. The blue excess
is most evident in its near-UV flux that is over 4 magnitudes brighter than
other stripped envelope supernovae, but also visible in optical gr colors at
early times. Its fast rise time of days
puts it squarely in the emerging class of Fast Evolving Luminous Transients, or
Fast Blue Optical Transients. With a peak absolute magnitude of M mag it is on the extreme end of both the rise time and peak magnitude
distribution for SNe Ic-bl. Only one other SN Ic-bl has similar properties,
iPTF16asu, for which less of the important early time and UV data have been
obtained. We show that the objects SNe 2018gep and iPTF16asu have similar
photometric and spectroscopic properties and that they overall share many
similarities with both SNe Ic-bl and Fast Evolving Transients. We obtain IFU
observations of the SN 2018gep host galaxy and derive a number of properties
for it. We show that the derived host galaxy properties for both SN 2018gep and
iPTF16asu are overall consistent with the SNe Ic-bl and GRB/SNe sample while
being on the extreme edge of the observed Fast Evolving Transient sample. These
photometric observations are consistent with a simple SN Ic-bl model that has
an additional form of energy injection at early times that drives the observed
rapid, blue rise, and we speculate that this additional power source may
extrapolate to the broader Fast Evolving Transient sample
GRB 221009A, The BOAT
GRB 221009A has been referred to as the Brightest Of All Time (the BOAT). We
investigate the veracity of this statement by comparing it with a half century
of prompt gamma-ray burst observations. This burst is the brightest ever
detected by the measures of peak flux and fluence. Unexpectedly, GRB 221009A
has the highest isotropic-equivalent total energy ever identified, while the
peak luminosity is at the th percentile of the known distribution. We
explore how such a burst can be powered and discuss potential implications for
ultra-long and high-redshift gamma-ray bursts. By geometric extrapolation of
the total fluence and peak flux distributions GRB 221009A appears to be a once
in 10,000 year event. Thus, while it almost certainly not the BOAT over all of
cosmic history, it may be the brightest gamma-ray burst since human
civilization began.Comment: Resubmitted to ApJ
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