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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Black Hole Spin Evolution: Implications for Short-hard Gamma Ray Bursts and Gravitational Wave Detection

astro-ph/0703131 [abs, ps, pdf, other] :

Title: Black Hole Spin Evolution: Implications for Short-hard Gamma Ray Bursts
and Gravitational Wave Detection


Authors:
Krzysztof Belczynski,
Ronald E. Taam,
Emmanouela Rantsiou,
Marc van der Sluys


Comments: 19 pages, 3 tables, 10 figures: submitted to ApJ

The evolution of the spin and tilt of black holes in compact black hole -
neutron star and black hole - black hole binary systems is investigated via the
population synthesis method. Based on recent results on accretion at super
Eddington rates in slim disk models, estimates of natal kicks, and the results
regarding fallback in supernova models, we obtain the black hole spin and
misalignment. It is found that the spin parameter, a_spin, is less than 0.5 for
initially non rotating black holes and the tilt, i_tilt, is less than 40 deg
for 50% of the systems in black hole - neutron star binaries. Upon comparison
with the results of black hole - neutron star merger calculations we estimate
that only a small fraction (~0.02) of these systems can potentially produce a
short-hard gamma ray burst. Only for high initial black hole spin parameters
(a_spin can this fraction be significant (~0.35). For the majority of
black holes in black hole - neutron star systems the spin magnitude is
increased to a_spin and the degree of spin misalignment (i_tilt ~ 40 deg)
is sufficiently high that the predicted gravitational radiation signal
significantly differs from that for non rotating black holes. However, due to
the (i) insensitivity of signal detection techniques to the black hole spin and
the (ii) predicted overall low contribution of black hole binaries to the
signal we find that the detection of gravitational waves are not greatly
inhibited by current searches with non spinning templates. It is pointed out
that the detection of a black hole - black hole binary inspiral system with
LIGO or VIRGO may provide a direct measurement of the initial spin of a black
hole.

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Some Theoretical Implications of Short-Hard Gamma-Ray Burst observations

astro-ph/0703129 [abs, ps, pdf, other] :

Title: Some Theoretical Implications of Short-Hard Gamma-Ray Burst observations


Authors:
Ehud Nakar


Comments: 9 pages, 2 figures; Proceedings of the 36th COSPAR Scientific
Assembly, 2006, Beijing, China. Advances in Space Research in press

Short-hard and long-soft gamma-ray bursts (GRBs) are two distinct phenomena,
but their prompt and afterglow emission show many similarities. This suggests
that two different progenitor systems lead to similar physical processes and
that the prompt and afterglow observations of short-hard GRBs (SHBs) can be
examined using models of long GRBs. Here, I discuss three conclusions that can
be drawn from SHB observations. I show that the lower limit on the Lorentz
factor of SHBs is typically ``only'' 10-50, significantly lower than that of
long GRBs. SHBs with observed X-ray afterglow after 1 day are found to be
roughly as efficient as long GRBs in converting the outflow energy into prompt
gamma-rays. Finally, I examine the origin of SHBs with X-ray dark afterglows
and find that the most plausible explanation is that these SHBs exploded in
extremely low density environment ($n \lesssim 10^{-5} cm^{-3}$)

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Swift observations of GRB 070110: an extraordinary X-ray afterglow powered by the central engine

astro-ph/0702220 [abs, ps, pdf, other] :

Title: Swift observations of GRB 070110: an extraordinary X-ray afterglow powered by the central engine
Authors: E. Troja, G. Cusumano, P. O'Brien, B. Zhang, B. Sbarufatti, V. Mangano, R. Willingale, G. Chincarini, J. P. Osborne, F. E. Marshall, D. N. Burrows, S. Campana, N. Gehrels, C. Guidorzi, H. A. Krimm, V. La Parola, E. W. Liang, T. Mineo, A. Moretti, K. L. Page, P. Romano, G. Tagliaferri, B. B. Zhang
Comments: 10 pages, 5 figures, submitted to ApJ

We present a detailed analysis of Swift multi-wavelength observations of GRB 070110 and its remarkable afterglow. The early X-ray light curve, interpreted as the tail of the prompt emission, displays a spectral evolution already seen in other gamma-ray bursts. The optical afterglow shows a shallow decay up to ~2 d after the burst, which is not consistent with standard afterglow models. The most intriguing feature is a very steep decay in the X-ray flux at ~20 ks after the burst, ending an apparent plateau. The abrupt drop of the X-ray light curve rules out an external shock as the origin of the plateau in this burst and implies long-lasting activity of the central engine. The temporal and spectral properties of the plateau phase point towards a continuous central engine emission rather than the episodic emission of X-ray flares. We suggest that the observed X-ray plateau is powered by a spinning down central engine, possibly a millisecond pulsar, which dissipates energy at an internal radius before depositing energy into the external shock.