547 research outputs found
Afterglow Light Curve Modulated by a Highly Magnetized Millisecond Pulsar
We investigate consequences of a continuously energy-injecting central engine
of gamma-ray burst (GRB) afterglow emission, assuming that a highly magnetized
pulsar is left beaming in the core of a GRB progenitor. Beaming and continuous
energy-injection are natural consequences of the pulsar origin of GRB
afterglows. Whereas previous studies have considered continuous
energy-injection from a new-born pulsar to interpret the deviation of afterglow
light curves of GRBs from those with the simple power law behavior, a beaming
effect, which is one of the most important aspects of pulsar emissions, is
ignored in earlier investigations. We explicitly include the beaming effect and
consider a change of the beaming with time due to a dynamical evolution of a
new-born pulsar. We show that the magnitude of the afterglow from this fireball
indeed first decreases with time, subsequently rises, and declines again. One
of the most peculiar optical afterglows light curve of GRB 970508 can be
accounted for by continuous energy injection with beaming due to a highly
magnetized new-born pulsar. We discuss implications on such observational
evidence for a pulsar.Comment: 4 pages, 1 table, submitted to Astronomy and Astrophysics (Letters
Gamma-Ray Burst Afterglows from Realistic Fireballs
A GRB afterglow has been commonly thought to be due to continuous
deceleration of a postburst fireball. Many analytical models have made
simplifications for deceleration dynamics of the fireball and its radiation
property, although they are successful at explaining the overall features of
the observed afterglows. We here propose a model for a GRB afterglow in which
the evolution of a postburst fireball is in an intermediate case between the
adiabatic and highly radiative expansion. In our model, the afterglow is both
due to the contribution of the adiabatic electrons behind the external
blastwave of the fireball and due to the contribution of the radiative
electrons. In addition, this model can describe evolution of the fireball from
the extremely relativistic phase to the non-relativistic phase. Our
calculations show that the fireball will go to the adiabatic expansion phase
after about a day if the accelerated electrons are assumed to occupy the total
internal energy. In all cases considered, the fireball will go to the mildly
relativistic phase about seconds later, and to the non-relativistic
phase after several days. These results imply that the relativistic adiabatic
model cannot describe the deceleration dynamics of the several-days-later
fireball. The comparison of the calculated light curves with the observed
results at late times may imply the presence of impulsive events or energy
injection with much longer durations.Comment: 18 pages, 10 figures, plain latex file, submitted to Ap
Prompt TeV neutrinos from dissipative photospheres of gamma-ray bursts
Recently, it was suggested that a photospheric component that results from
the internal dissipation occurring in the optically thick inner parts of
relativistic outflows may be present in the prompt /X-ray emission of
gamma-ray bursts or X-ray flashes. We explore high-energy neutrino emission in
this dissipative photosphere model, assuming that the composition of the
outflow is baryon-dominated. We find that neutrino emission from proton-proton
collision process forms an interesting signature in the neutrino spectra. Under
favorable conditions for the shock dissipation site, these low-energy neutrinos
could be detected by detectors, such as Icecube. Higher energies
(\ga10 TeV) neutrino emission from proton-proton collision and photo-pion
production processes could be significantly suppressed for dissipation at
relatively small radii, due to efficient Bethe-Heitler cooling of protons
and/or radiative cooling of the secondary mesons in the photosphere radiation.
As the dissipation shocks continue further out, high energy neutrinos from
photo-pion production process becomes dominant.Comment: Accepted by ApJ Letters, some changes made following the referees'
comments, conclusions unchanged. The paper was originally submitted to PRL on
June 6 (2008); resubmitted to ApJL on Oct.1 (2008); accepted by ApJL on Dec.
9 (2008
Shallow decay phase of GRB X-ray afterglows from relativistic wind bubbles
The postburst object of a GRB is likely to be a highly magnetized, rapidly
rotating compact object (e.g., a millisecond magnetar), which could produce an
ultrarelativistic electron-positron-pair wind. The interaction of such a wind
with an outwardly expanding fireball ejected during the burst leads to a
relativistic wind bubble (RWB). We numerically calculate the dynamics and
radiative properties of RWBs and use this model to explain the shallow decay
phase of the early X-ray afterglows observed by Swift. We find that RWBs can
fall into two types: forward-shock-dominated and reverse-shock-dominated
bubbles. Their radiation during a period of seconds is
dominated by the shocked medium and the shocked wind, respectively, based on
different magnetic energy fractions of the shocked materials. For both types,
the resulting light curves always have a shallow decay phase. In addition, we
provide an example fit to the X-ray afterglows of GRB 060813 and GRB 060814 and
show that they could be produced by forward-shock-dominated and
reverse-shock-dominated bubbles, respectively. This implies that, for some
early afterglows (e.g., GRB 060814), the long-lasting reverse shock emission is
strong enough to explain their shallow decay phase.Comment: 5 pages, 4 figures, Accepted for Publication in A&
The termination shock of a magnetar wind: a possible origin of gamma-ray burst X-ray afterglow emission
Context: Swift observations suggest that the X-ray afterglow emission of some
gamma-ray bursts (GRB) may have internal origins, and the conventional external
shock (ES) cannot be the exclusive source of the afterglow emission. Aims: If
the central compact objects of some GRBs are millisecond magentars, the
magnetar winds could play an important role in the (internal) X-ray afterglow
emission, which is our focus here. Methods: The dynamics and the synchrotron
radiation of the termination shock (TS) of the magmnetar winds, as well as the
simultaneous GRB ES, are investigated by considering the magnetization of the
winds. Results: As a result of the competition between the emission of the wind
TS and the GRB ES, two basic types of X-ray afterglows are predicted, i.e., the
TS-dominated and the ES-dominated types. Moreover, our results also show that
both of the two types of afterglows have a shallow-decay phase and a
normal-decay one, as observed by the \textit{Swift} satellite. This indicates
that some observed X-ray afterglows could be (internally) produced by the
magnetar winds, but not necessarily GRB ESs.Comment: 5 pages, 3 figure
Flares in GRB afterglows from delayed magnetic dissipation
One of the most intriguing discoveries made by the Swift satellite is the
flaring activity in about half of the afterglow lightcurves. Flares have been
observed on both long and short duration GRBs and on time scales that range
from minutes to ~1 day after the prompt emission. The rapid evolution of some
flares led to the suggestion that they are caused by late central engine
activity. Here, I propose an alternative explanation that does not need
reviving of the central engine. Flares can be powered by delayed magnetic
dissipation in strongly magnetized (i.e. with initial Poynting to kinetic flux
ratio \simmore 1) ejecta during its deceleration due to interaction with the
external medium. A closer look at the length scales of the dissipation regions
shows that magnetic dissipation can give rise to fast evolving and energetic
flares. Multiple flares are also expected in the context of the model.Comment: 5 pages, accepted for publication in A&A Letter
What do -ray bursts look like?
There have been great and rapid progresses in the field of -ray
bursts (denoted as GRBs) since BeppoSAX and other telescopes discovered their
afterglows in 1997. Here, we will first give a brief review on the
observational facts of GRBs and direct understanding from these facts, which
lead to the standard fireball model. The dynamical evolution of the fireball is
discussed, especially a generic model is proposed to describe the whole
dynamical evolution of GRB remnant from highly radiative to adiabatic, and from
ultra-relativistic to non-relativistic phase. Then, Various deviations from the
standard model are discussed to give new information about GRBs and their
environment. In order to relax the energy crisis, the beaming effects and their
possible observational evidences are also discussed in GRB's radiations.Comment: 10 pages, Latex. Invited talk at the Pacific Rim Conference on
Stellar Astrophysics, Hong Kong, China, Aug. 199
Relativistic Wind Bubbles and Afterglow Signatures
Highly magnetized, rapidly rotating compact objects are widely argued as
central energy sources of -ray bursts (GRBs). After the GRB, such a
magnetar-like object may directly lose its rotational energy through some
magnetically-driven processes, which produce an ultrarelativistic wind
dominated possibly by the energy flux of electron-positron pairs. The
interaction of such a wind with an outward-expanding fireball leads to a
relativistic wind bubble, being regarded as a relativistic version of the
well-studied Crab Nebula. We here explore the dynamics of this wind bubble and
its emission signatures. We find that when the injection energy significantly
exceeds the initial energy of the fireball, the bulk Lorentz factor of the wind
bubble decays more slowly than before, and more importantly, the reverse-shock
emission could dominate the afterglow emission, which yields a bump in
afterglow light curves. In addition, high polarization of the bump emission
would be expected if a toroidal magnetic field in the shocked wind dominates
over the random component.Comment: 7 pages including 1 figure, emulateapj style, expanded version
accepted for publication in Ap
Measuring the beaming angle of GRB 030329 by fitting the rebrightenings in its multiband afterglow
Multiple rebrightenings have been observed in the multiband afterglow of GRB
030329. Especially, a marked and quick rebrightening occurred at about t ~ 1.2
* 10^5 s. Energy injection from late and slow shells seems to be the best
interpretation for these rebrightenings. Usually it is assumed that the energy
is injected into the whole external shock. However, in the case of GRB 030329,
the rebrightenings are so quick that the usual consideration fails to give a
satisfactory fit to the observed light curves. Actually, since these late/slow
shells coast freely in the wake of the external shock, they should be cold and
may not expand laterally. The energy injection then should only occur at the
central region of the external shock. Considering this effect, we numerically
re-fit the quick rebrightenings observed in GRB 030329. By doing this, we were
able to derive the beaming angle of the energy injection process. Our result,
with a relative residual of only 5% - 10% during the major rebrightening, is
better than any previous modeling. The derived energy injection angle is about
0.035. We assume that these late shells are ejected by the central engine via
the same mechanism as those early shells that produce the prompt gamma-ray
burst. The main difference is that their velocities are much slower, so that
they catch up with the external shock very lately and manifest as the observed
quick rebrightenings. If this were true, then the derived energy injection
angle can give a good measure of the beaming angle of the prompt gamma-ray
emission. Our study may hopefully provide a novel method to measure the beaming
angle of gamma-ray bursts.Comment: 8 pages, 6 figures, Has been accepted by RAA (Research in Astronomy
and Astrophysics
Coasting external shock in wind medium: an origin for the X-ray plateau decay component in Swift GRB afterglows
The plateaus observed in about one half of the early X-ray afterglows are the
most puzzling feature in gamma-ray bursts (GRBs) detected by Swift. By
analyzing the temporal and spectral indices of a large X-ray plateau sample, we
find that 55% can be explained by external, forward shock synchrotron emission
produced by a relativistic ejecta coasting in a \rho ~ r^{-2}, wind-like
medium; no energy injection into the shock is needed. After the ejecta collects
enough medium and transitions to the adiabatic, decelerating blastwave phase,
it produces the post-plateau decay. For those bursts consistent with this
model, we find an upper limit for the initial Lorentz factor of the ejecta,
\Gamma_0 \leq 46 (\epsilon_e/0.1)^{-0.24} (\epsilon_B/0.01)^{0.17}; the
isotropic equivalent total ejecta energy is E_{iso} ~ 10^{53}
(\epsilon_e/0.1)^{-1.3} (\epsilon_B/0.01)^{-0.09} (t_b/10^4 s) erg, where
\epsilon_e and \epsilon_B are the fractions of the total energy at the shock
downstream that are carried by electrons and the magnetic field, respectively,
and t_b is the end of the plateau. Our finding supports Wolf-Rayet stars as the
progenitor stars of some GRBs. It raises intriguing questions about the origin
of an intermediate-\Gamma_0 ejecta, which we speculate is connected to the GRB
jet emergence from its host star. For the remaining 45% of the sample, the
post-plateau decline is too rapid to be explained in the coasting-in-wind
model, and energy injection appears to be required.Comment: 11 pages, 5 figures, to appear in ApJ, proof-corrected version, added
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