2,054 research outputs found
On the Structure of Quasi-Universal Jets for Gamma-Ray Bursts
The idea that GRBs originate from uniform jets has been used to explain
numerous observations of breaks in the GRB afterglow lightcurves. We explore
the possibility that GRBs instead originate from a structured jet that may be
quasi-universal, where the variation in the observed properties of GRBs is due
to the variation in the observer viewing angle. We test how various models
reproduce the jet data of Bloom, Frail, & Kulkarni (2003), which show a
negative correlation between the isotropic energy output and the inferred jet
opening angle (in a uniform jet configuration). We find, consistent with
previous studies, that a power-law structure for the jet energy as a function
of angle gives a good description. However, a Gaussian jet structure can also
reproduce the data well, particularly if the parameters of the Gaussian are
allowed some scatter. We place limits on the scatter of the parameters in both
the Gaussian and power-law models needed to reproduce the data, and discuss how
future observations will better distinguish between these models for the GRB
jet structure. In particular, the Gaussian model predicts a turnover at small
opening angles and in some cases a sharp cutoff at large angles, the former of
which may already have been observed. We also discuss the predictions each
model makes for the observed luminosity function of GRBs and compare these
predictions with the existing data.Comment: 13 pages, including 10 figures; To appear in Ap
Long lived central engines in Gamma Ray Bursts
The central engine of Gamma Ray Bursts may live much longer than the duration
of the prompt emission. Some evidence of it comes from the presence of strong
precursors, post-cursors, and X-ray flares in a sizable fraction of bursts.
Additional evidence comes from the fact that often the X-ray and the optical
afterglow light curves do not track one another, suggesting that they are two
different emission components. The typical "steep-flat-steep" behavior of the
X-ray light curve can be explained if the same central engine responsible for
the main prompt emission continues to be active for a long time, but with a
decreasing power. The early X-ray "afterglow" emission is then the extension of
the prompt emission, originating at approximately the same location, and is not
due to forward shocks. If the bulk Lorentz factor Gamma is decreasing in time,
the break ending the shallow phase can be explained, since at early times Gamma
is large, and we see only a fraction of the emitting area. Later, when Gamma
decreases, we see an increasing fraction of the emitting surface up to the time
when Gamma ~ 1/theta_j. This time ends the shallow phase of the X-ray light
curve. The origin of the late prompt emission can be the accretion of the
fall-back material, with an accretion rate dot M proportional to t^(-5/3). The
combination of this late prompt emission with the flux produced by the standard
forward shock can explain the great diversity of the optical and the X-ray
light curves.Comment: 6 pages, 6 figures, To appear in: 2008 Nanjing GRB Conference, AIP,
Eds. Y.F. Huang, Z.G. Dai, B. Zhan
Possible High-Energy Neutrino and Photon Signals from Gravitational Wave Bursts due to Double Neutron Star Mergers
As the technology of gravitational-wave and neutrino detectors becomes
increasingly mature, a multi-messenger era of astronomy is ushered in. Advanced
gravitational wave detectors are close to making a ground-breaking discovery of
gravitational wave bursts (GWBs) associated with mergers of double neutron
stars (NS-NS). It is essential to study the possible electromagnetic (EM) and
neutrino emission counterparts of these GWBs. Recent observations and numerical
simulations suggest that at least a fraction of NS-NS mergers may leave behind
a massive millisecond magnetar as the merger product. Here we show that protons
accelerated in the forward shock powered by a magnetar wind pushing the ejecta
launched during the merger process would interact with photons generated in the
dissipating magnetar wind and emit high energy neutrinos and photons. We
estimate the typical energy and fluence of the neutrinos from such a scenario.
We find that PeV neutrinos could be emitted from the shock front as long
as the ejecta could be accelerated to a relativistic speed. The diffuse
neutrino flux from these events, even under the most optimistic scenarios, is
too low to account for the two events announced by the IceCube Collaboration,
but it is only slightly lower than the diffuse flux of GRBs, making it an
important candidate for the diffuse background of PeV neutrinos. The
neutron-pion decay of these events make them a moderate contributor to the
sub-TeV gamma-ray diffuse background.Comment: Accepted for publication in PRD, minor revisio
Late-Time Optical Afterglow Observations with LBT and MDM
Using the 2.4m MDM and 8.4m Large Binocular Telescope, we observed nine GRB
afterglows to systematically probe the late time behaviors of afterglows
including jet breaks, flares, and supernova bumps. In particular, the LBT
observations have typical flux limits of 25-26 mag in the Sloan r' band, which
allows us to extend the temporal baseline for measuring jet breaks by another
decade in time scale. We detected four jet breaks (including a "textbook" jet
break in GRB070125) and a fifth candidate, all of which are not detectable
without deep, late time optical observations. In the other four cases, we do
not detect the jet breaks either because of contamination from the host galaxy
light, the presence of a supernova bump, or the intrinsic faintness of the
optical afterglow. This suggests that the basic picture that GRBs are
collimated is still valid and that the apparent lack of Swift jet breaks is due
to poorly sampled afterglow light curves, particularly at late times. Besides
the jet breaks, we also detected late time flares, which could attribute to
late central engine activities, and two supernova bumps.Comment: 5 pages, 5 figures, 2008 NANJING GAMMA-RAY BURST CONFERENCE. AIP
Conference Proceedings, Volume 1065, pp. 93-97 (2008), Eds. Y.F. Huang, Z.G.
Dai, B. Zhan
Can optical afterglows be used to discriminate between Type I and Type II GRBs?
The precise localization of short/hard (Type I) gamma-ray bursts (GRBs) in
recent years has answered many questions but raised even more. I present some
results of a systematic study of the optical afterglows of long/soft (Type II)
and short/hard (Type I) GRBs, focusing on the optical luminosity as another
puzzle piece in the classification of GRBs.Comment: 7 Pages, 2 figures, to be published in the "2008 Nanjing GRB
Conference" conference proceedings, figures have been downsample
The GRB-Supernova Connection
Long-duration gamma-ray bursts (GRBs) are believed to be produced by the core
collapse of massive stars and hence to be connected with supernovae (SNe).
Indeed, for four pairs of GRBs and SNe, spectroscopically confirmed connection
has been firmly established. For more than a dozen of GRBs the SN signature
(the `red bump') has been detected in the afterglow lightcurves. Based on the
four pairs of GRBs and SNe with spectroscopically confirmed connection a tight
correlation was found between the peak spectral energy of GRBs and the peak
bolometric luminosity of the underlying SNe. The recent discovery of X-ray
flash 080109 associated with a normal core-collapse SN 2008D confirmed this
relation and extended the GRB-SN connection. Progress on the GRB-SN connection
is briefly reviewed.Comment: 6 pages, 5 figures. To appear in the proceedings of "2008 Nanjing GRB
conference", Nanjing, 23-27 June 200
Bright broad-band afterglows of gravitational wave bursts from mergers of binary neutron stars
If double neutron star mergers leave behind a massive magnetar rather than a
black hole, a bright early afterglow can follow the gravitational wave burst
(GWB) even if there is no short gamma-ray burst (SGRB) - GWB association or
there is an association but the SGRB does not beam towards earth. Besides
directly dissipating the proto-magnetar wind as suggested by Zhang, we here
suggest that the magnetar wind could push the ejecta launched during the merger
process, and under certain conditions, would reach a relativistic speed. Such a
magnetar-powered ejecta, when interacting with the ambient medium, would
develop a bright broad-band afterglow due to synchrotron radiation. We study
this physical scenario in detail, and present the predicted X-ray, optical and
radio light curves for a range of magnetar and ejecta parameters. We show that
the X-ray and optical lightcurves usually peak around the magnetar spindown
time scale (10^3-10^5s), reaching brightness readily detectable by wide-field
X-ray and optical telescopes, and remain detectable for an extended period. The
radio afterglow peaks later, but is much brighter than the case without a
magnetar energy injection. Therefore, such bright broad-band afterglows, if
detected and combined with GWBs in the future, would be a probe of massive
millisecond magnetars and stiff equation-of-state for nuclear matter.Comment: ApJ, in pres
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