Is GRB 050904 a super-long burst?

Is GRB 050904 a super-long burst?

Authors:
Y. C. Zou,
Z. G. Dai,
D. Xu


Comments: 16 pages, 4 figure. Submitted on Oct. 21, 2005, accepted by ApJ


Journal-ref: Astrophys.J. 646 (2005) 1098-1103

By considering synchrotron radiative process in the internal shock model and
assuming that all internal shocks are nearly equally energetic, we analyze the
gamma-ray burst (GRB) emission at different radii corresponding to different
observed times. We apply this model to GRB 050904 and find that our analytical
results can provide a natural explanation for the multi-band observations of
GRB 050904. This suggests that the X-ray flare emission and the optical
emission of this burst could have originated from internal shocks being due to
collisions among nearly-equally-energetic shells ejected from the central
engine. Thus GRB 050904 appears to be a burst with super-long central engine
activity.

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Dust Echoes from the Ambient Medium of Gamma-Ray Bursts

astro-ph/0703158 [abs, ps, pdf, other] :

Title: Dust Echoes from the Ambient Medium of Gamma-Ray Bursts


Authors:
Kevin Heng,
Davide Lazzati,
Rosalba Perna


Comments: 24 pages, 10 figures. Accepted by ApJ

Long gamma-ray bursts (GRBs) are likely associated with the collapse of
massive stars, which produce dust and are born in dusty environments.
Absorption and scattering of ultraviolet/X-ray photons from the prompt, optical
flash and afterglow emission of the GRB produce dust echoes. We perform
time-dependent calculations of these echoes, accounting for the evolution of
the dust grain distribution due to selective grain destruction by the GRB
radiation, and for off-axis beaming. We explore cloud configurations of
differing density and size -- the echo light curve and spectrum depend on the
cloud radius, with larger clouds peaking at longer wavelengths. For a region ~
3 pc in size with hydrogen density ~ 1000 per cubic centimeter, the echo
spectrum peaks at ~ 3.6 microns and ~ 8.8 eV for thermal and scattered
components, respectively. Dust echoes should be detectable with the Very Large
Telescope up to z ~ 0.1, IRAC onboard the Spitzer Space Telescope up to z ~
0.2, and NICMOS onboard the Hubble Space Telescope up to z ~ 0.3. Furthermore,
the shape of the echo light curve allows one to infer: the jet opening angle;
the inclination of the jet axis with respect to the line of sight; the size of
the dust-emitting region. For sources with symmetric, bipolar jets, dust echoes
exhibit two bumps in the light curve, making them easily distinguishable from
the rebrightening due to an underlying supernova.

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Magnetic acceleration of relativistic AGN jets

astro-ph/0703146 [abs, ps, pdf, other] :

Title: Magnetic acceleration of relativistic AGN jets


Authors:
S.S.Komissarov,
M.V.Barkov,
N.Vlahakis,
A.Konigl


Comments: Submitted to MNRAS

We present numerical simulations of axisymmetric, magnetically driven
relativistic jets. To eliminate the dissipative effects induced by a free
boundary with an ambient medium we assume that the flow is confined by a rigid
wall of a prescribed shape, which we take to be $z\propto r^a$ (in cylindrical
coordinates, with $a$ ranging from 1 to 3). The outflows are initially cold,
sub-Alfv\'enic and Poynting flux-dominated, with a total--to--rest-mass energy
flux ratio $\mu \sim 15$. We find that in all cases they converge to a steady
state characterized by a spatially extended acceleration region. The
acceleration process is very efficient: on the outermost scale of the
simulation as much as $\sim 77%$ of the Poynting flux has been converted into
kinetic energy flux, and the terminal Lorentz factor approaches its maximum
possible value ($\Gamma_\infty \simeq \mu$). We also find a high collimation
efficiency: all our simulated jets develop a cylindrical core. We argue that
this could be the rule for current-carrying outflows that start with a low
initial Lorentz factor ($\Gamma_0 \sim 1$). Our conclusions on the high
acceleration and collimation efficiencies are not sensitive to the particular
shape of the confining boundary or to the details of the injected current
distribution, and they are qualitatively consistent with the semi-analytic
self-similar solutions derived by Vlahakis K\"onigl. We apply our results to
the interpretation of relativistic jets in AGNs: we argue that they naturally
account for the spatially extended accelerations inferred in these sources
($\Gamma_\infty \ga 10$ attained on radial scales $R\ga 10^{17} {\rm cm}$) and
are consistent with the transition to the matter-dominated regime occurring
already at $R\ga 10^{16} {\rm cm}$.

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Gamma-Ray Burst Precursors as the Remnant of the Thermal Radiation Initially Trapped in the Fireball

astro-ph/0703144 [abs, ps, pdf, other] :

Title: Gamma-Ray Burst Precursors as the Remnant of the Thermal Radiation
Initially Trapped in the Fireball


Authors:
Li-Xin Li (MPA)


Comments: 16 pages, including 13 figures

In the standard fireball model of gamma-ray bursts (GRBs), the fireball
starts with an optically thick phase. As it expands, the fireball becomes
optically thin at some stage. The thermal radiation trapped in the originally
opaque fireball then leaks out, producing a transient event. The appearance of
the event is investigated in the framework of a homogeneous, spherically
symmetric, and freely expanding fireball. We find that, generally, the event
has a time-duration shorter than that of the main burst, which is presumably
produced by the internal shock when the fireball is optically thin. In
addition, the event is separated from the main burst by a quiescent
time-interval, and is weaker than the main burst at least in a high energy
band. Hence, the event corresponds to a precursor of a GRB. The precursor event
predicted by our model has a simple and FRED (Fast Rise and Exponential Decay)
shape lightcurve, and a quasi-thermal spectrum. Typically, the characteristic
photon energy of the precursor is in the X-ray band. However, if the distortion
of the blackbody spectrum by electron scattering is considered, the inferred
photon energy could be in the gamma-ray band. Observational aspects of the GRB
precursors derived from our model are discussed.

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High Efficiency of Gamma-Ray Bursts Revisited

astro-ph/0703136 [abs, ps, pdf, other] :

Title: High Efficiency of Gamma-Ray Bursts Revisited


Authors:
Y. C. Zou,
Z. G. Dai


Comments: 12 pages, 6 figures, accepted by JASR

Using the conservation of energy and momentum during collisions of any two
shells, we consider the efficiency of gamma-ray bursts by assuming that the
ejecta from the central engine are equally massive and have the same Lorentz
factors. We calculate the efficiency and the final Lorentz factor of the merged
whole shell for different initial diversities of Lorentz factors and for
different microscopic radiative efficiency. As a result, a common high
efficiency in the range of 0.1 to 0.9 is considerable, and a very high value
near 100% is also reachable if the diversity of the Lorentz factors is large
enough.

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