Giblin 2002 BATSE gamma-ray afterglow

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extended emission at the end of the gamma-ray burst in gamma-ray band.

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Title:		Extended Power-Law Decays in BATSE Gamma-Ray Bursts: Signatures of External Shocks?
Authors:		Giblin, T. W.; Connaughton, V.; van Paradijs, J.; Preece, R. D.; Briggs, M. S.; Kouveliotou, C.; Wijers, R. A. M. J.; Fishman, G. J.
Publication:		The Astrophysical Journal, Volume 570, Issue 2, pp. 573-587. (ApJ Homepage)
Publication Date:		05/2002
Origin:		UCP
ApJ Keywords:		Gamma Rays: Bursts
Abstract Copyright:		(c) 2002: The American Astronomical Society
DOI:		10.1086/339622
Bibliographic Code:		2002ApJ...570..573G

Abstract

The connection between gamma-ray bursts (GRBs) and their afterglows is currently not well understood. Afterglow models of synchrotron emission generated by external shocks in the GRB fireball model predict emission detectable in the gamma-ray regime (>~25 keV). In this paper, we present a temporal and spectral analysis of a subset of BATSE GRBs with smooth extended emission tails to search for signatures of the ``early high-energy afterglow,'' i.e., afterglow emission that initially begins in the gamma-ray phase and subsequently evolves into X-ray, UV, optical, and radio emission as the blast wave is decelerated by the ambient medium. From a sample of 40 GRBs we find that the temporal decays are best described with a power law ~t<sup>β</sup> rather than an exponential with a mean index <β>~-2. Spectral analysis shows that ~20% of these events are consistent with fast-cooling synchrotron emission for an adiabatic blast wave, three of which are consistent with the blast-wave evolution of a jet with F<sub>ν</sub>~t<sup>-p</sup>. This behavior suggests that in some cases, the emission may originate from a narrow jet, possibly consisting of ``nuggets'' whose angular sizes are less than 1/Γ, where Γ is the bulk Lorentz factor.

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my understanding on the profile of the pulses

Caito 2008 fire shell model for GRB 060614

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arXiv:0810.4855 [ps, pdf, other]

Title: GRB060614: a "fake" short GRB from a merging binary system

Authors: Letizia Caito, Maria Grazia Bernardini, Carlo Luciano Bianco, Maria Giovanna Dainotti, Roberto Guida, Remo Ruffini

Comments: 5 pages, 4 figures, submitted to A&A

Subjects: Astrophysics (astro-ph)

(Shortened) CONTEXT: [...] GRB060614 is the first nearby long duration GRB clearly not associated to a bright Ib/c supernova. Moreover, its duration (T_{90} ~ 100s) makes it hardly classifiable as a short GRB. It presents strong similarities with GRB970228, the prototype of the new class of "fake" short GRBs that appear to originate from the coalescence of binary neutron stars or white dwarfs spiraled out into the galactic halo. AIMS: Within the "canonical" GRB scenario based on the "fireshell" model, we test if GRB060614 can be a "fake", or, better, "disguised" short GRB. [...] METHODS: We fit GRB060614 light curves in Swift's BAT (15-150keV) and XRT (0.2-10keV) energy bands. Within the fireshell model, light curves are formed by two well defined and different components: the Proper-GRB (P-GRB), emitted at the fireshell transparency, and the afterglow, due to the interaction between the leftover accelerated baryonic shell and the CBM. RESULTS: We determine the two free parameters describing the GRB source within the fireshell model. [...] A small average CBM density [...] is inferred, typical of galactic halos. The first spikelike emission is identified with the P-GRB and the following prolonged emission with the peak of the afterglow.[...] CONCLUSIONS: The anomalous GRB060614 finds a natural interpretation within our canonical GRB scenario: it is a "disguised" short GRB. [...] This result points to an old binary system, likely formed by a white dwarf and a neutron star, as the progenitor of GRB060614 and well justify the absence of an associated supernova Ib/c. Particularly important for further studies of the final merging process are the temporal structures in the P-GRB down to 0.1s.

Gorosabel 2008 SN2008D/XRF080109 polarization

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Title:		Simultaneous polarization monitoring of SN2007uy and the axisymmetric SN2008D/XRF080109: isolating geometry from dust
Authors:		Gorosabel, J.; de Ugarte Postigo, A.; Castro-Tirado, A. J.; Agudo, I.; Jelinek, M.; Leon, S.; Augusteijn, T.; Fynbo, J. P. U.; Hjorth, J.; Michalowski, M. J.; Xu, D.; Ferrero, P.; Kann, D. A.; Klose, S.; Rossi, A.; Madrid, J. P.; LLorente, A.; Bremer, M.; Winters, J. -M.
Publication:		eprint arXiv:0810.4333
Publication Date:		10/2008
Origin:		ARXIV
Keywords:		Astrophysics
Comment:		4 pages, 1 figure, submitted to ApJL. Comments welcome
Bibliographic Code:		2008arXiv0810.4333G

Abstract

We report the results of an optical linear polarization monitoring carried out simultaneously for SN2007uy and SN2008D. The observations span 74.9 days, starting 3.6 days after the X-ray outburst of SN2008D and are distributed in 11 visits. For both SNe the polarization is dominated by the host galaxy interstellar polarization (HGIP), especially for SN2007uy. This fact is supported by 1.2 millimeter (mm) observations of NGC2770, carried out at and around the SN2008D position. The 1.2 mm data are consistent with no intrinsic emission from SN2008D and can be entirely explained by the NGC2770 dust emission. Comparing spectroscopic extinction values reported in the literature with the extinction derived from the 1.2 mm data, we conclude that the NGC2770 extinction distribution is heterogeneous. We report also a 3.3 mm flux detection at the SN2008D position, possibly due to the intrinsic SN emission, and not due to the dust. Despite the dominant HGIP, a statistical analysis of the distribution of the SN2008D Stokes parameters strongly suggests an intrinsic variable polarization component. Moreover we show that the temporal evolution of the intrinsic SN2008D polarization agrees with an aspherical axisymmetric expansion with variable eccentricity, consistent with the Collapsar model. In contrast, the SN2007uy polarization can be described by the HGIP plus a constant eccentricity expansion. We conclude that the geometric evolution of the two SNe is intrinsically different.

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Qin 2008 Two types of X-ray afterglow softening

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Title:		Two types of softening detected in X-ray afterglows of Swift bursts: internal and external shock origins?
Authors:		Qin, Y. -P.; Gupta, A. C.; Fan, J. H.; Lu, R. -J.
Publication:		eprint arXiv:0810.4379
Publication Date:		10/2008
Origin:		ARXIV
Keywords:		Astrophysics
Comment:		20 pages, 5 figures, 2 tables, Accepted for Publication to Journal of Cosmology and Astroparticle Physics (JCAP)
Bibliographic Code:		2008arXiv0810.4379Q

Abstract

The softening process observed in the steep decay phase of early X-ray afterglows of Swift bursts has remained a puzzle since its discovery. The softening process can also be observed in the later phase of the bursts and its cause has also been unknown. Recently, it was suggested that, influenced by the curvature effect, emission from high latitudes would shift the Band function spectrum from higher energy band to lower band, and this would give rise to the observed softening process accompanied by a steep decay of the flux density. The curvature effect scenario predicts that the terminating time of the softening process would be correlated with the duration of the process. In this paper, based on the data from the UNLV GRB group web-site, we found an obvious correlation between the two quantities. In addition, we found that the softening process can be divided into two classes: the early type softening ($t_{s,max}\leq "4000"s$) and the late type softening ($t_{s,max} > "4000"s$). The two types of softening show different behaviors in the duration vs. terminating time plot. In the relation between the variation rates of the flux density and spectral index during the softening process, a discrepancy between the two types of softening is also observed. According to their time scales and the discrepancy between them, we propose that the two types are of different origins: the early type is of internal shock origin and the late type is of external shock origin. The early softening is referred to the steep decay just following the prompt emission, whereas the late decay typically conceives the transition from flat decay to late afterglow decay. We suspect that there might be a great difference of the Lorentz factor in two classes which is responsible for the observed discrepancy.

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Medvedev 2008 Can simulations and experiments probe relevant GRB physics?

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The answer is yes for late afterglow.

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Title:		Radiative cooling in relativistic collisionless shocks. Can simulations and experiments probe relevant GRB physics?
Authors:		Medvedev, Mikhail V.; Spitkovsky, Anatoly
Publication:		eprint arXiv:0810.4014
Publication Date:		10/2008
Origin:		ARXIV
Keywords:		Astrophysics
Comment:		15 pages, submitted to ApJ
Bibliographic Code:		2008arXiv0810.4014M

Abstract

We address the question of whether numerical particle-in-cell (PIC) simulations and laboratory laser-plasma experiments can (or will be able to, in the near future) model realistic gamma-ray burst (GRB) shocks. For this, we compare the radiative cooling time, t_cool, of relativistic electrons in the shock magnetic fields to the microscopic dynamical time of collisionless relativistic shocks -- the inverse plasma frequency of protons, omega_pp^{-1}. We obtain that for t_cool*omega_pp^{-1}\lesssim ~few hundred, the electrons cool efficiently at or near the shock jump and are capable of emitiing away a large fraction of the shock energy. Such shocks are well-resolved in existing PIC simulations; therefore, the microscopic structure can be studied in detail. Since most of the emission in such shocks would be coming from the vicinity of the shock, the spectral power of the emitted radiation can be directly obtained from finite-length simulations and compared with observational data. Such radiative shocks correspond to the internal baryon-dominated GRB shocks for the conventional range of ejecta parameters. Fermi acceleration of electrons in such shocks is limited by electron cooling, hence the emitted spectrum should be lacking a non-thermal tail, whereas its peak likely falls in the multi-MeV range. Incidentally, the conditions in internal shocks are almost identical to those in laser-produced plasmas; thus, such GRB-like plasmas can be created and studied in laboratory experiments using the presently available Petawatt-scale laser facilities. An analysis of the external shocks shows that only the highly relativistic shocks, corresponding to the extremely early afterglow phase, can have efficient electron cooling in the shock transition. We emphasize the importance of radiative PIC simulations for further studies.

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Tam 2008 VHE from nearby GRBs

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Title:		VHE Gamma-ray Afterglow Emission from Nearby GRBs
Authors:		Tam, P. H.; Xue, R. R.; Wagner, S. J.; Behera, B.; Fan, Y. Z.; Wei, D. M.
Publication:		eprint arXiv:0810.3894
Publication Date:		10/2008
Origin:		ARXIV
Keywords:		Astrophysics
Comment:		Proceeding of "Heidelberg International Symposium on High Energy Gamma-Ray Astronomy", held in Heidelberg, 7-11 July 2008, submitted to AIP Conference Proceedings. 4 pages, 3 figures, 1 table
Bibliographic Code:		2008arXiv0810.3894T

Abstract

Gamma-ray Bursts (GRBs) are among the potential extragalactic sources of very-high-energy (VHE) gamma-rays. We discuss the prospects of detecting VHE gamma-rays with current ground-based Cherenkov instruments during the afterglow phase. Using the fireball model, we calculate the synchrotron self-Compton (SSC) emission from forward-shock electrons. The modeled results are compared with the observational afterglow data taken with and/or the sensitivity level of ground-based VHE instruments (e.g. STACEE, H.E.S.S., MAGIC, VERITAS, and Whipple). We find that modeled SSC emission from bright and nearby bursts such as GRB 030329 are detectable by these instruments even with a delayed observation time of ~10 hours.

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Starling 2008 Swift important results

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We highlight a number of important results from the Swift era thus far.

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Title:		The Swift satellite lives up to its name, revealing cosmic explosions as they happen
Authors:		Starling, Rhaana L. C.
Publication:		eprint arXiv:0810.3523
Publication Date:		10/2008
Origin:		ARXIV
Keywords:		Astrophysics
Comment:		11 pages, 5 figures, to appear in the Triennial Issue of Philosophical Transactions A of the Royal Society
Bibliographic Code:		2008arXiv0810.3523S

Abstract

Gamma-ray Bursts are the most powerful objects in the Universe. Discovered in the 1960's as brief flashes of gamma-radiation, we now know they emit across the entire electromagnetic spectrum, are located in distant galaxies and comprise two distinct populations, one of which may originate in the deaths of massive stars. The launch of the Swift satellite in 2004 has brought a flurry of new discoveries, advancing our understanding of these sources and the galaxies that host them. We highlight a number of important results from the Swift era thus far.

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Tam 2008 GRB observations with HESS

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several, hours after the onset. High energy

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Title:		GRB observations with HESS
Authors:		Tam, P. H.; Wagner, S. J.; Pühlhofer, G.; the HESS Collaboration
Publication:		eprint arXiv:0810.3639
Publication Date:		10/2008
Origin:		ARXIV
Keywords:		Astrophysics
Comment:		Proceeding of "Swift and GRBs: Unveiling the Relativistic Universe", held in Venice, 5-9 June 2006; Il Nuovo Cimento B, vol. 121 (2006), Issue 12, p.1595-1596; doi:10.1393/ncb/i2007-10322-x
Bibliographic Code:		2008arXiv0810.3639T

Abstract

H.E.S.S. (High Energy Stereoscopic System), which is designed to detect TeV gamma-rays, is a system of four Imaging Atmospheric Cherenkov Telescopes situated in Namibia. The system has been shown to be very successful in detecting and observing galactic and extra-galactic TeV sources. In order to explore the highest energy end of GRB spectra, a GRB observing program has been established in the H.E.S.S. collaboration. Here we introduce our GRB observing program and report on its current status.

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Eichler 2008 Spectral Lags by scattering

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Title:		Spectral Lags Explained as Scattering from Accelerated Scatterers
Authors:		Eichler, David; Manis, Hadar
Publication:		eprint arXiv:0810.3006
Publication Date:		10/2008
Origin:		ARXIV
Keywords:		Astrophysics
Comment:		2 figures
Bibliographic Code:		2008arXiv0810.3006E

Abstract

A quantitative theory of spectral lags for $\gamma$-ray bursts (GRBs) is given. The underlying hypothesis is that GRB subpulses are photons that are scattered into our line of sight by local concentrations of baryons that are accelerated by radiation pressure. For primary spectra that are power laws with exponential cutoffs, the width of the pulse and its fast rise, slow decay asymmetry is found to increase with decreasing photon energy, and the width near the exponential cutoff scales approximately as $E_{ph}^{-\eta}$, with $\eta\sim 0.4$, as observed. The spectral lag time is naturally inversely proportional to luminosity, all else being equal, also as observed.

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Xue 2008 prompt GeV from neutron component

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Title:		GeV emission from neutron-rich internal shocks of some long γ-ray bursts
Authors:		Xue, Rong-Rong; Fan, Yi-Zhong; Wei, Da-Ming
Affiliation:		AA(Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China; Graduate School, Chinese Academy of Sciences, Beijing 100039, China), AB(Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China; Niels Bohr Institute, Niels Bohr International Academy, Blegdamsvej 17, DK-2100 Copenhagen, Denmark), AC(Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China; Joint Center for Particle Nuclear Physics and Cosmology of Purple Mountain Observatory - Nanjing University, Nanjing 210008, China)
Publication:		Monthly Notices of the Royal Astronomical Society, Volume 389, Issue 1, pp. 321-324. (MNRAS Homepage)
Publication Date:		09/2008
Origin:		MNRAS
MNRAS Keywords:		radiation mechanisms: non-thermal , ISM: jets and outflows , gamma rays: bursts
Abstract Copyright:		(c) Journal compilation © 2008 RAS
DOI:		10.1111/j.1365-2966.2008.13578.x
Bibliographic Code:		2008MNRAS.389..321X

Abstract

In the neutron-rich internal shocks model for γ-ray bursts (GRBs), the Lorentz factors (LFs) of ion shells are variable, and so are the LFs of accompanying neutron shells. For slow neutron shells with a typical LF of approximate tens, the typical β-decay radius is ~10¹⁴-10¹⁵ cm. As GRBs last long enough [T₉₀ > 14(1 + z) s], one earlier but slower ejected neutron shell will be swept successively by later ejected ion shells in the range ~10¹³-10¹⁵ cm, where slow neutrons have decayed significantly. Part of the thermal energy released in the interaction will be given to the electrons. These accelerated electrons will mainly be cooled by the prompt soft γ-rays and give rise to GeV emission. This kind of GeV emission is particularly important for some very long GRBs and is detectable for the upcoming satellite Gamma-Ray Large Area Space Telescope (GLAST).

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Eichler 2008 single model for long and short burst

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Title:		A Universal Central Engine Hypothesis for Short and Long GRBs
Authors:		Eichler, David; Guetta, Dafne; Manis, Hadar
Publication:		eprint arXiv:0810.3013
Publication Date:		10/2008
Origin:		ARXIV
Keywords:		Astrophysics
Comment:		3 figures, one with two panels
Bibliographic Code:		2008arXiv0810.3013E

Abstract

It is noted that X-ray tails (XRTs) of short, hard $\gamma$-ray bursts (SHBs) are similar to X-ray flashes (XRFs). We suggest a universal central engine hypothesis, as a way of accounting for this curiosity, in which SHBs differ from long $\gamma$-ray bursts (GRBs) in prompt emission because of the differences in the host star and attendant differences in the environment they present to the compact central engine (as opposed to differences in the central engine itself). Observational constraints and implications are discussed, especially for confirming putative detections of gravitational waves from merging compact objects.

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Mimica 2008 MHD of magnetized afterglow

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arXiv:0810.2961 [pdf]

Title: Deceleration of arbitrarily magnetized GRB ejecta: the complete evolution

Authors: P. Mimica (1), D. Giannios (2), M.A. Aloy (1) ((1) Departamento de Astronomia y Astrofisica, Universidad de Valencia, (2) Max-Planck-Institut fuer Astrophysik)

Comments: 13 pages, 10 figures, revised version sent to the referee (first version submitted on 6th of August)

Subjects: Astrophysics (astro-ph)

(Abridged) We aim to quantitatively understand the dynamical effect and observational signatures of magnetization of the GRB ejecta on the onset of the afterglow. We perform ultrahigh-resolution one-dimensional relativistic MHD simulations of the interaction of a radially expanding, magnetized ejecta with the interstellar medium. The need of ultrahigh numerical resolution derives from the extreme jump conditions in the region of interaction between the ejecta and the circumburst medium. We study the evolution of an ultrarelativistic shell all the way to a the self-similar asymptotic phase. Our simulations show that the complete evolution can be characterized in terms of two parameters, namely, the \xi parameter introduced by Sari & Piran (1995) and the magnetization \sigma_0. We exploit this property by producing numerical models where the shell Lorentz factor is \gamma_0 ~ tens and rescaling the results to arbitrarily large \gamma_0. We find that the reverse shock is typically very weak or absent for ejecta characterized by \sigma_0 >~ 1. The onset of the forward shock emission is strongly affected by the magnetization. On the other hand, the magnetic energy of the shell is transfered to the external medium on a short timescale (~several times the duration of the burst). The later forward shock emission does not contain information for the initial magnetization of the flow. The asymptotic evolution of strongly magnetized shells, after they have suffred a substantial deceleration, resembles that of hydrodynamic shells, i.e., they fully enter in the Blandford-McKee self-similar regime.

Li 2008 GeV emission in Residual internal shocks

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arXiv:0810.2932 [ps, pdf, other]

Title: Prompt GeV Emission from Residual Collisions in GRB Outflows

Authors: Zhuo Li (PKU)

Comments: 4 pages, 1 figure

Subjects: Astrophysics (astro-ph)

The gamma-rays from gamma-ray bursts (GRBs) are believed to be produced by internal shocks driven by small timescale, ~1 ms, variation in the GRB outflows, and a pair-production spectral cutoff, ~1(\Gamma/300)^6 GeV (\Gamma is the bulk Lorentz factor of the outflow), is generally expected in the GRB spectrum. However, the observed optical flashes accompanying GRBs suggest that the delayed residual collisions at large radii continue to accelerate electrons. We show here that the inverse-Compton scattering of the prompt gamma-rays by these residual internal shock electrons leads to a high energy emission beyond the previously thought spectral cutoff, in agreement with the previous detections of GeV photons by EGRET in several GRBs in conjunction with MeV emission. We expect a spectral steepening in the connection between the primary and residual internal shock emission at the previously thought spectral cutoff, and expect systematic time delays of high energy photons relative to MeV emission, the discovery of which would provide stringent constraint on the outflow properties, but requires large enough collection of high energy photons by, e.g., GLAST and AGILE.

Eliasdottir 2008 Dust extinction of GRB 070802 host galaxy

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arXiv:0810.2897 [ps, pdf, other]

Title: Dust Extinction in High-z Galaxies with GRB Afterglow Spectroscopy - The 2175Å Feature at z=2.45

Authors: Á. Elíasdóttir (1), J. P. U. Fynbo (1), J. Hjorth (1), C. Ledoux (2), D. Watson (1), A. C. Andersen (1), D. Malesani (1), P. M. Vreeswijk (1), J. X. Prochaska (3), J. Sollerman (1,4), A. O. Jaunsen (5) ((1) DARK, (2) ESO, (3) UCSC, (4) Stockholm, (5) U. Oslo)

Comments: 17 pages, 11 figures, submitted to ApJ

Subjects: Astrophysics (astro-ph)

We report the clear detection of the 2175A dust absorption feature in the optical afterglow spectrum of the gamma-ray burst (GRB) GRB070802 at a redshift of z=2.45. This is the highest redshift for a detected 2175A dust bump to date, and it is the first clear detection of the 2175A bump in a GRB host galaxy, while several tens of optical afterglow spectra without the bump have been recorded in the past decade. The derived extinction curve gives A_V=0.8-1.5 depending on the assumed intrinsic slope. Of the three local extinction laws, an LMC type extinction gives the best fit to the extinction curve of the host of GRB070802. Besides the 2175A bump we find that the spectrum of GRB070802 is characterized by unusually strong low-ionization metal lines and possibly a high metallicity for a GRB sightline ([Si/H]=-0.46+/-0.38, [Zn/H]=-0.50+/-0.68). In particular, the spectrum of GRB070802 is unique for a GRB spectrum in that it shows clear CI absorption features, leading us to propose a correlation between the presence of the bump and CI. The gas to dust ratio for the host galaxy is found to be significantly lower than that of other GRB hosts with N(HI)/A_V=(2.4+/-1.0)x10^21 cm^-2 mag^-1, which lies between typical MW and LMC values. Our results are in agreement with the tentative conclusion reached by Gordon et al. 2003 that the shape of the extinction curve, in particular the presence of the bump, is affected by the UV flux density in the environment of the dust.

Gendre 2008 Possible standard candle for infrared afterglow

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arXiv:0810.2893 [ps, pdf, other]

Title: Gamma Ray burst afterglows in the infrared: a standard candle?

Authors: B. Gendre (1), S. Pelisson (1), M. Boer (2), S. Basa (1), A. Mazure ((1) LAM/CNRS/Universite de Provence, (2) OHP/CNRS)

Comments: 4 pages, 6 color figures. Accepted for publication as a Letter in A&A

Subjects: Astrophysics (astro-ph)

Clustering in the luminosity of the afterglows of gamma-ray burst has been reported in the optical and X-ray. We investigate the possibility that a clustering in the luminosity of the afterglows of gamma-ray burst exists in near infrared (J, H, K bands). We use observations of events occurring from 1997 to the end of 2007. We correct the gamma-ray burst afterglow light curve for distance effect and time dilation, and replace all light curves to a common distance of z=1. We used only observations of signal emitted in the near infrared (in the burst frame). We observe a clustering identical to the one observed in optical and similar to the one observed in X-ray. We thus confirm the previous works made in optical. We set a constraint on the total energy of the fireball.

Metzger 2008 Neutron-Rich Freeze-Out in compat mergers

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Title:		Neutron-Rich Freeze-Out in Viscously Spreading Accretion Disks Formed from Compact Object Mergers
Authors:		Metzger, B. D.; Piro, A. L.; Quataert, E.
Publication:		eprint arXiv:0810.2535
Publication Date:		10/2008
Origin:		ARXIV
Keywords:		Astrophysics
Comment:		12 pages, 9 figures, 1 table; submitted to MNRAS
Bibliographic Code:		2008arXiv0810.2535M

Abstract

Accretion disks with masses ~0.001-0.1 Msun form during the merger of neutron star (NS)-NS and black hole-NS binaries. Initially, such hyper-accreting disks cool efficiently by neutrino emission and their composition is driven neutron-rich by pair captures under degenerate conditions. However, as the disk viscously spreads and its temperature drops, cooling becomes inefficient and the disk becomes advective. Analytic arguments and numerical simulations suggest that once this occurs, powerful winds likely drive away most of the disk's remaining mass. We calculate the thermal evolution and nuclear composition of viscously spreading accretion disks formed from compact object mergers using one-dimensional height-integrated simulations. We show that freeze-out from weak equilibrium necessarily accompanies the disk's late-time transition to an advective state. As a result, hyper-accreting disks generically freeze out neutron-rich (with electron fraction Ye ~ 0.2-0.4), and their late-time outflows robustly synthesize rare neutron-rich isotopes. Using the measured abundances of these isotopes in our solar system, we constrain the compact object merger rate in the Milky Way to be <> 30(M_d,0/0.1 Msun)^(1/2) degrees. This is consistent with other evidence that short duration GRB outflows are less collimated than those produced in long duration GRBs.

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Mizuno 2008 MHD in relativistic jets

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Title:		Magnetohydrodynamic Effects in Propagating Relativistic Jets: Reverse Shock and Magnetic Acceleration
Authors:		Mizuno, Yosuke; Zhang, Bing; Giacomazzo, Bruno; Nishikawa, Ken-Ichi; Hardee, Philip E.; Nagataki, Shigehiro; Hartmann, Dieter H.
Publication:		eprint arXiv:0810.2779
Publication Date:		10/2008
Origin:		ARXIV
Keywords:		Astrophysics
Comment:		13 pages, 3 figures, submitted to ApJ Letters, incorporates referee's comments
Bibliographic Code:		2008arXiv0810.2779M

Abstract

We solve the Riemann problem for the deceleration of an arbitrarily magnetized relativistic flow injected into a static unmagnetized medium in one dimension. We find that for the same initial Lorentz factor, the reverse shock becomes progressively weaker with increasing magnetization \sigma (the Poynting-to kinetic energy flux ratio), and the shock becomes a rarefaction wave when \sigma exceeds a critical value, \sigma_c, defined by the balance between the magnetic pressure in the flow and the thermal pressure in the forward shock. In the rarefaction wave regime, we find that the rarefied region is accelerated to a Lorentz factor that is significantly larger than the initial value. This acceleration mechanism is due to the strong magnetic pressure in the flow. We discuss the implications of these results for models of gamma-ray bursts and active galactic nuclei.

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