The role of kink instability in Poynting-flux dominated jets

Authors: Dimitrios Giannios, Henk C. Spruit (Max Planck Institute for Astrophysics)
Comments: 13 pages, 7 figures, accepted for publication in A&A, typos Corrected

The role of kink instability in magnetically driven jets is explored through numerical one-dimensional steady relativistic MHD calculations. The instability is shown to have enough time to grow and influence the dynamics of Poynting-flux dominated jets. In the case of AGN jets, the flow becomes kinetic flux dominated at distances larger than ~1000 r_g because of the rapid dissipation of Poynting flux. When applied to GRB outflows, the model predicts more gradual Poynting dissipation and moderately magnetized flow at distances of ~10^{16} cm where the deceleration of the ejecta due to its interaction with the external medium is expected. The energy released by the instability can power the compact ``blazar zone'' emission and the prompt emission of GRB outflows with high radiative efficiencies.

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Spectral and timing properties of a dissipative GRB photosphere

Authors: Dimitrios Giannios, Henk C. Spruit (Max Planck Institute for Astrophysics)
Comments: 10 pages, 6 figures, A&A, submitted

We explore the observational appearance of the photosphere of an ultrarelativistic flow with internal dissipation of energy as predicted by the magnetic reconnection model. Previous study of the radiative transfer in the photospheric region has shown that gradual dissipation of energy results in a hot photosphere. There, inverse Compton scattering of the thermal radiation advected with the flow leads to powerful photospheric emission with spectral properties close to those of the observed prompt GRB emission. Here, we build on that study by calculating the spectra for a large range of the characteristics of the flow. An accurate fitting formula is given that provides the photospheric spectral energy distribution in the ~10 keV to ~10 MeV energy range (in the central engine frame) as a function of the basic physical parameters of the flow. It facilitates the direct comparison of the model predictions with observations, including the variability properties of the lightcurves. We verify that the model naturally accounts for the observed clustering in peak energies of the E*f(E) spectrum. In this model, the Amati relation indicates a tendency for the most luminous bursts to have more energy per baryon. If this tendency also holds for individual GRB pulses, the model predicts the observed narrowing of the width of pulses with increasing photon energy.

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Fermion Production in Strong Magnetic Fields through Anomalous Magnetic Moment

Authors: Hyun Kyu Lee, Yongsung Yoon
Comments: 7 pages with 1 figure

We calculate the explicit form of the real and imaginary parts of the effective potential for a uniform magnetic field which interacts with fermions with anomalous magnetic moments through the Pauli interaction. It is found that the non-vanishing imaginary part develops for a magnetic field stronger than a critical field, which is the ratio of the fermion mass to its anomalous magnetic moment. This implies the instability of the uniform magnetic field to produce fermion pairs with the production rate density $w(x) = \frac{m^{4}}{24\pi} (\frac{|\mu_a |}{m}-1)^3 (\frac{|\mu_a B|}{m}+3)$.

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The Gamma Ray Burst Luminosity Function in the Light of the Swift 2-year Data

astro-ph/0612278 [abs, ps, pdf, other] :

Title: The Gamma Ray Burst Luminosity Function in the Light of the Swift 2-year Data
Authors: R. Salvaterra, C. Chincarini
Comments: 9 pages, 3 figures, submitted to ApJ Letter

We compute the luminosity function (LF) and the formation rate of long gamma ray bursts (GRBs) by fitting the observed differential peak flux distribution obtained by the BATSE satellite in three different scenarios: i) GRBs follow the cosmic star formation and their LF is constant in time; ii) GRBs follow the cosmic star formation but the LF varies with redshift; iii) GRBs form preferentially in low-metallicity environments. We find that the differential peak flux number counts obtained by BATSE and by Swift can be reproduced using the same LF and GRB formation rate, indicating that the two satellites are observing the same GRB population. We then check the resulting redshift distributions in the light of Swift 2-year data, focusing in particular on the relatively large sample of GRBs detected at z>2.5. We show that models in which GRBs trace the cosmic star formation and are described by a constant LF are ruled out by the number of high-z Swift detections. This conclusion does not depend on the redshift distribution of bursts that lack of optical identification, nor on the existence of a decline in star formation rate at z>2, nor on the adopted faint-end of the GRB LF. Swift observations can be explained by assuming that the LF varies with time and/or that GRB formation is limited to low-metallicity environments.

Spectral Evolution of GRB Tails: Central Engine and Internal Shock Afterglows?

astro-ph/0612246 [abs, ps, pdf, other] :

Title: Spectral Evolution of GRB Tails: Central Engine and Internal Shock Afterglows?
Authors: Bin-Bin Zhang, En-Wei Liang, Bing Zhang
Comments: 11 pages,2 figures

A steep decay segment tens to hundreds of seconds after the gamma-ray burst (GRB) prompt emission is commonly observed in the {\em Swift} XRT light curves, which is regarded as the tail emission of the prompt gamma-rays. The most straightforward interpretation is the curvature effect due to delay of propagation of photons from larger angles with respect to the line of sight. Prompted by the observed strong spectral evolution in the tails of GRB 060218 and GRB 060614, we present a systematic time-resolved spectral analysis of 17 bright GRB tails observed by XRT. While 7 tails in our sample have no spectral evolution and can be explained with the curvature effect, the other 10 tails all show significant hard-to-soft spectral evolution. A toy model that combines the curvature effect with an underlying putative central engine afterglow component can roughly explain the observed light curves and spectral evolutions for 7 of them. The suggested central engine afterglow is typically soft ($\beta=2.5\sim 6.4$) and decays as normal GRB afterglows (typically $\alpha=0.8\sim 1.5$), similar to the late afterglow of GRB 060218. There are 3 cases (GRB 050724, GRB 060218, and GRB 060614) that cannot be described by this model. We suggest that these tails may be interpreted as an internal shock afterglow due to cooling of the shock-heated region. More detailed physical models are called for to understand these two possibly new types of afterglows.

Simulations of the Disk-Jet Interaction in GRS 1915+105 and Other Systems

astro-ph/0612236 [abs, ps, pdf, other] :

Title: Simulations of the Disk-Jet Interaction in GRS 1915+105 and Other Systems
Authors: David M. Rothstein (Cornell University)
Comments: 10 pages, 6 figures; accepted for publication in the proceedings of VI Microquasar Workshop: Microquasars and Beyond, Sept 18-22 2006, Como, Italy, ed: T. Belloni (2006), PoS(MQW6)037

After an X-ray binary experiences a transient jet ejection, it undergoes a phase in which its X-ray light curve is dominated, for some time, by thermal emission from an accretion disk surrounding the black hole. The accretion physics in the thermal-dominant state is understood better than in any other, and it is therefore the best state for comparing observations to theoretical models. Here, I present simulations that study the way a thermally-emitting disk might be expected to behave immediately after a large-scale, steady jet has been removed from the system in the form of a sudden ejection. I simulate the ejection's effect on the disk by allowing the strength of turbulence (modeled by the alpha parameter of Shakura and Sunyaev) to increase rapidly in time, and I show how this change can lead to an outburst in an otherwise-steady disk. The motivation for treating the jet removal in this way is the fact that many models for jets involve large-scale magnetic fields that should inhibit the magnetorotational instability believed to drive turbulence; this should naturally lead to a rapid increase in turbulence when the magnetic field is ejected from the system or otherwise destroyed during the ejection event. I show how the timescale and luminosity of the outburst can be controlled by the manner in which alpha is allowed to change, and I briefly discuss ways in which these simulations can be compared to observations of X-ray binaries, in particular GRS 1915+105, which shows the most complex and variable behavior of any black hole system in outburst.

Making a Short Gamma-Ray Burst from a Long one: Implications for the Nature of GRB 060614

astro-ph/0612238 [abs, ps, pdf, other] :

Title: Making a Short Gamma-Ray Burst from a Long one: Implications for the Nature of GRB 060614
Authors: Bing Zhang, Bin-Bin Zhang, En-Wei Liang, Neil Gehrels, David N. Burrows, Peter Meszaros
Comments: 7 pages, 3 figures, to appear in ApJ Letters

The absence of a supernova accompanying the nearby long GRB 060614 poses a great puzzle about the progenitor of this event and challenges the current GRB classification scheme. This burst displays a short-hard emission episode followed by extended soft emission with strong spectral evolution. Noticing that this burst has an isotropic gamma-ray energy only ~8 times that of GRB 050724, a good candidate of merger-type short GRBs, we generate a ``pseudo'' burst that is ~8 times less energetic than GRB 060614 based on the spectral properties of GRB 060614 and the Ep ~ Eiso^{1/2} (Amati) relation. We find that this pseudo-burst would have been detected by BATSE as a marginal short-duration GRB, and would have properties in the Swift BAT and XRT bands similar to GRB 050724. This suggests that GRB 060614 is likely a more intense event in the traditional short-hard GRB category as would be detected by BATSE. Events like GRB 060614 that seem to defy the traditional short vs. long classification of GRBs may require modification of our classification terminology for GRBs. By analogy with supernova classifications, we suggest that GRBs be classified into Type I (typically short and associated with old populations) and Type II (typically long and associated with young populations). We propose that GRB 060614 belongs to Type I, and predict that similar events will be detected in elliptical galaxies.