伽玛暴(Gamma-Ray Burst)笔记。记录有关伽玛暴的新文章,另外也包括看的老文章、自己的想法、以及跟天文相关的一些东西。 Feel free to leave me a message by comments or by email.

星期四, 一月 31, 2008

Chang 2008 Long Term Evolution of Magnetic Turbulence in Relativistic Collisionless Shocks

主要内容:


精彩摘抄:
We conclude that initially unmagnetized relativistic shocks in electron-positron plasmas are unable to form persistent downstream magnetic fields.

文章信息:

· arXiv e-print (arXiv:0801.4583)
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Title:
Long Term Evolution of Magnetic Turbulence in Relativistic Collisionless Shocks
Authors:
Chang, Philip; Spitkovsky, Anatoly; Arons, Jonathan
Publication:
eprint arXiv:0801.4583
Publication Date:
01/2008
Origin:
ARXIV
Keywords:
Astrophysics
Comment:
4 pages, 3 figures, contributed talk at the workshop: High Energy Phenomena in Relativistic Outflows (HEPRO), Dublin, 24-28 September 2007; Downsampled version for arXiv. Full resolution version available at http://astro.berkeley.edu/~pchang/proceedings.pdf
Bibliographic Code:
2008arXiv0801.4583C

Abstract

We study the long term evolution of magnetic fields generated by an initially unmagnetized collisionless relativistic $e^+e^-$ shock. Our 2D particle-in-cell numerical simulations show that downstream of such a Weibel-mediated shock, particle distributions are approximately isotropic, relativistic Maxwellians, and the magnetic turbulence is highly intermittent spatially, nonpropagating, and decaying. Using linear kinetic theory, we find a simple analytic form for these damping rates. Our theory predicts that overall magnetic energy decays like $(\omega_p t)^{-q}$ with $q \sim 1$, which compares favorably with simulations, but predicts overly rapid damping of short wavelength modes. Magnetic trapping of particles within the magnetic structures may be the origin of this discrepancy. We conclude that initially unmagnetized relativistic shocks in electron-positron plasmas are unable to form persistent downstream magnetic fields. These results put interesting constraints on synchrotron models for the prompt and afterglow emission from GRBs.

Swift难题:无喷流拐折的暴

虽然尽了很大的努力,但还是有一些暴确定没有喷流拐折。这个问题怎么办?
这应该也属于Swift难题之一。见上一篇张冰组里的文章:
http://grblogscn.blogspot.com/2008/01/tacusin-2008-swift-x-ray-afterglows-and.html

回顾一下Swift的几个主要难题:
1. 早期快下降,现在基本认为是曲率效应;
2. 早期X射线变平,解释很多,有能量注入等;
3. X射线耀发,多半认为是中心能源再活动;
4. X射线和光学的不同时拐折;
5. 无喷流拐折的光变

Tacusin 2008 Swift X-ray Afterglows and the Missing Jet Break Problem

主要内容:
Try to find the missing jet break.
And for the real missing ones, this may become another Swift problem.

精彩摘抄:


文章信息:

· arXiv e-print (arXiv:0801.4749)
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Title:
Swift X-ray Afterglows and the Missing Jet Break Problem
Authors:
Racusin, J. L.; Liang, E. -W.; Burrows, D. N.; Morris, D. C.; Zhang, B. B.; Zhang, B.
Publication:
eprint arXiv:0801.4749
Publication Date:
01/2008
Origin:
ARXIV
Keywords:
Astrophysics
Comment:
4 pages, 4 figures, contributed talk, submitted to the proceedings of Gamma Ray Bursts 2007, Santa Fe, New Mexico, November 5-9 2007
Bibliographic Code:
2008arXiv0801.4749R

Abstract

We present a systematic survey of the temporal and spectral properties of all GRB X-ray afterglows observed by Swift-XRT between January 2005 and July 2007. We have constructed a catalog of all light curves and spectra and investigate the physical origin of each afterglow segment in the framework of the forward shock models by comparing the data with the closure relations. We search for possible jet-like breaks in the lightcurves and try to explain some of the "missing" X-ray jet breaks in the lightcurves.
Bibtex entry for this abstract Preferred format for this abstract (see Preferences)

Francisco 2008 Low-Luminosity Gamma-Ray Bursts as a Distinct GRB Population:A Monte Carlo Analysis is much popular than high-lominosity ones

主要内容:
LL-GRBs is $\sim$ 100 Gpc$^{-3}$ yr$^{-1}$ at the break luminosity, much larger than that of HL-GRBs.

精彩摘抄:


文章信息:

· arXiv e-print (arXiv:0801.4751)
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Title:
Low-Luminosity Gamma-Ray Bursts as a Distinct GRB Population:A Monte Carlo Analysis
Authors:
Virgili, Francisco; Liang, Enwei; Zhang, Bing
Publication:
eprint arXiv:0801.4751
Publication Date:
01/2008
Origin:
ARXIV
Keywords:
Astrophysics
Comment:
20 pages, 9 figures, 3 tables
Bibliographic Code:
2008arXiv0801.4751V

Abstract

The intriguing observations of {Swift}/BAT GRB 060218 and CGRO/BATSE burst 980425, both with much lower luminosity and redshift compared to other observed bursts, lead naturally to the question whether these low-luminosity (LL) bursts constitute a separate population from high-luminosity (HL) bursts. Utilizing Monte Carlo simulations we compare various single-component luminosity function (LF)models (single power law or broken power law) with the two-component luminosity function model proposed by Liang et al. Using various criteria, we demonstrate that the single-component LF models have great difficulty in simultaneously reproducing both the high local LL-GRB rate and the oberved distributions of redshift, luminosity, and $\log N-\log P$ for HL-GRBs. We argue that the two-component LF model is necessary, and we use the observed BATSE and Swift $\log N-\log P$ distributions to add constrains to the LL and HL-LF parameters. The LL-LF can be modeled by a smoothed, broken power law with a break at around $10^{47}$ erg s$^{-1}$, dropping steeply above this luminosity. The local rate of LL-GRBs is $\sim$ 100 Gpc$^{-3}$ yr$^{-1}$ at the break luminosity, much larger than that of HL-GRBs. The recently discovered peculiar X-ray transient XRF 080109/SN 2008D strengthens this conclusion, and requires that the LL-population LF extends further down in luminosity with a probably even higher local rate at lower luminosities.
Bibtex entry for this abstract Preferred format for this abstract (see Preferences)

星期三, 一月 30, 2008

Nishikawa 2008 Simulation for Collisionless Relativistic Jets

主要内容:
Jitter is important. Comparing with synchrotron, it can account for the complex time evolution and/or spectral structure.

精彩摘抄:


文章信息:

arXiv e-print (arXiv:0801.4390)
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Title:
Particle Acceleration, Magnetic Field Generation, and Associated Emission in Collisionless Relativistic Jets
Authors:
Nishikawa, K. -I.; Mizuno, Y.; Fishiman, G. J.; Hardee, P.
Publication:
eprint arXiv:0801.4390
Publication Date:
01/2008
Origin:
ARXIV
Keywords:
Astrophysics
Comment:
4 pages, 3 figures, contributed talk at the workshop: High Energy Phenomena in Relativistic Outflows (HEPRO), Dublin, 24-28 September 2007
Bibliographic Code:
2008arXiv0801.4390N

Abstract

Nonthermal radiation observed from astrophysical systems containing relativistic jets and shocks, e.g., active galactic nuclei (AGNs), gamma-ray bursts (GRBs), and Galactic microquasar systems usually have power-law emission spectra. Recent PIC simulations using injected relativistic electron-ion (electro-positron) jets show that acceleration occurs within the downstream jet. Shock acceleration is a ubiquitous phenomenon in astrophysical plasmas. Plasma waves and their associated instabilities (e.g., the Buneman instability, other two-streaming instability, and the Weibel instability) created in the shocks are responsible for particle (electron, positron, and ion) acceleration. The simulation results show that the Weibel instability is responsible for generating and amplifying highly nonuniform, small-scale magnetic fields. These magnetic fields contribute to the electron's transverse deflection behind the jet head. The ``jitter'' radiation from deflected electrons has different properties than synchrotron radiation which assumes a uniform magnetic field. This jitter radiation may be important to understanding the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets, and supernova remnants.
Bibtex entry for this abstract Preferred format for this abstract (see Preferences)

Evans 2008 Improving Swift-XRT positions of GRBs

主要内容:


精彩摘抄:


文章信息:

· arXiv e-print (arXiv:0801.4462)
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Title:
Improving Swift-XRT positions of GRBs
Authors:
Evans, P. A.; Beardmore, A. P.; Goad, M. R.; Osborne, J. P.; Burrows, D. N.; Gehrels, N.
Publication:
eprint arXiv:0801.4462
Publication Date:
01/2008
Origin:
ARXIV
Keywords:
Astrophysics
Comment:
4 pages, to appear in the proceedings of "Gamma Ray Bursts 2007, Santa Fe"
Bibliographic Code:
2008arXiv0801.4462E

Abstract

Since GRBs fade rapidly, it is important to publish accurate, precise positions at early times. For Swift-detected bursts, the best promptly available position is most commonly the X-ray Telescope (XRT) position. We present two processes, developed by the Swift team at Leicester, which are now routinely used to improve the precision and accuracy of the XRT positions reported by the Swift team. Both methods, which are fully automated, make use of a PSF-fitting approach which accounts for the bad columns on the CCD. The first method yields positions with 90% error radii <4.4"> Bibtex entry for this abstract Preferred format for this abstract (see Preferences)

星期二, 一月 29, 2008

Jia 2008 BATSE hardness ratio with GRB light curves

主要内容:
According to the properties (the peak of HR curves and light curves didn't occur the same time mainly), they divided the burst into 4 classes.

精彩摘抄:

an example


文章信息:
On the trend of the hardness ratio evolutionary curve
of gamma-ray bursts
Lan-Wei Jia1,2 
1National Astronomical Observatories/Yunnan Observatory, Chinese Academy of
Sciences, P. O. Box 110, Kunming 650011, China
2The Graduate School of the Chinese Academy of Sciences
Received month day; accepted month day
Abstract We analyzed a sample of 66 gamma-ray bursts (GRBs) and
statistically confirmed the prediction on the hardness ratio evolutionary
curve of GRBs made by Qin et al. (2006) based on the curvature effect. In
their analysis, GRB pulses are divided into three groups according to the
profile of their raw hardness ratio (RHR) curves. They defined RHR curve
by adding the background counts to the signal counts so that the hardness
ratio of counts within small time intervals can be evaluated. Among the
three types, very hard sources exhibit a perfect pulse-like profile (type 1),
hard bursts possess a pulse-like profile with a sinkage in its decaying phase
(type 2), and soft bursts show no pulse-like profile but have only a sinkage
profile (type 3). When analyzing type 3 sources in terms of the conventional
hardness ratio, they are indeed generally softer than those of type 1 and type
2, which is in agreement with the prediction. In addition, we found that the
minimum value of RHR is sensitive in distinguishing pulses of different
types. We propose that GRB pulses can be divided by the minimum value
of RHR and sources of different types might probably connect with different
strengthes of the shock or/and the magnetic field.

ChJAA

Xu 2008 XRF 080109/SN 2008D -- mildly relativisitc outflow

主要内容:
The low energy low luminosity transient may come from mildly relativistic outflow with large opening angle.

精彩摘抄:

X-ray and UVOT data.

文章信息:

arXiv e-print (arXiv:0801.4325)
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Title:
Mildly relativistic X-ray transient 080109 and SN2008D: Towards a continuum from energetic GRB/XRF to ordinary Ibc SN
Authors:
Xu, Dong; Zou, Yuan-Chuan; Fan, Yi-Zhong
Publication:
eprint arXiv:0801.4325
Publication Date:
01/2008
Origin:
ARXIV
Keywords:
Astrophysics
Comment:
4 pages, 2 figures
Bibliographic Code:
2008arXiv0801.4325X

Abstract

We analyze the hitherto available space-based X-ray data as well as ground-based optical data of the X-ray transient 080109/SN2008D. From the data we suggest that (i) The initial transient (\lesssim 800 sec) is attributed to the reverse shock emission of a mildly relativistic (\Gamma \sim a few) outflow stalled by the dense stellar wind. (ii) The subsequent X-ray afterglow (\lesssim 2\times 10^4 sec) can be ascribed to the forward shock emission of the outflow, with a kinetic energy \sim 10^{46} erg, when sweeping up the stellar wind medium. (iii) The late X-ray flattening (\gtrsim 2\times 10^4$ sec) is powered by the fastest non-decelerated component of SN2008D's ejecta. (iv) The local event rate of X-ray transient has a lower limit of \sim 1.6\times 10^4 yr^{-1} Gpc^{-3}, indicating a vast majority of X-ray transients have a wide opening angle of \gtrsim 100 degree. The off-axis viewing model is less likely. (v) Transient 080109/SN2008D may lead to a continuum from GRB-SN to under-luminous GRB-/XRF-SN to X-ray transient-SN and to ordinary Ibc SN (if not every Ibc SN has a relativistic jet), as shown in Figure 2 of this Letter.
Bibtex entry for this abstract Preferred format for this abstract (see Preferences)

星期一, 一月 28, 2008

Sakamoto 2008 Swift burst data review

主要内容:
Contain many X-ray data and E_peak, E_iso, spectral index, while mainly is the temporal properties.

精彩摘抄:

Partial data

composite plot of XRF and C-GRB.

Sketch of C-GRB(classical GRB) and XRF light curves
文章信息:

arXiv e-print (arXiv:0801.4319)
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Title:
Global Properties of X-Ray Flashes and X-Ray-Rich Gamma-Ray Bursts Observed by Swift
Authors:
Sakamoto, T.; Hullinger, D.; Sato, G.; Yamazaki, R.; Barbier, L.; Barthelmy, S. D.; Cummings, J. R.; Fenimore, E. E.; Gehrels, N.; Krimm, H. A.; Lamb, D. Q.; Markwardt, C. B.; Osborne, J. P.; Palmer, D. M.; Parsons, A. M.; Stamatikos, M.; Tueller, J.
Publication:
eprint arXiv:0801.4319
Publication Date:
01/2008
Origin:
ARXIV
Keywords:
Astrophysics
Comment:
50 pages, 22 figures, Accepted for publication in ApJ
Bibliographic Code:
2008arXiv0801.4319S

Abstract

We describe and discuss the spectral and temporal characteristics of the prompt emission and X-ray afterglow emission of X-ray flashes (XRFs) and X-ray-rich gamma-ray bursts (XRRs) detected and observed by Swift between December 2004 and September 2006. We compare these characteristics to a sample of conventional classical gamma-ray bursts (C-GRBs) observed during the same period. We confirm the correlation between Epeak_obs and fluence noted by others and find further evidence that XRFs, XRRs and C-GRBs form a continuum. We also confirm that our known redshift sample is consistent with the correlation between the peak energy in the GRB rest frame (Epeak_src) and the isotropic radiated energy (Eiso), so called the Epeak_src-Eiso relation. The spectral properties of X-ray afterglows of XRFs and C-GRBs are similar, but the temporal properties of XRFs and C-GRBs are quite different. We found that the light curves of C-GRB afterglows show a break to steeper indices (shallow-to-steep break) at much earlier times than do XRF afterglows. Moreover, the overall luminosity of XRF X-ray afterglows is systematically smaller by a factor of two or more compared to that of C-GRBs. These distinct differences between the X-ray afterglows of XRFs and C-GRBs may be the key to understanding not only the mysterious shallow-to-steep break in X-ray afterglow light curves, but also the unique nature of XRFs.
Bibtex entry for this abstract Preferred format for this abstract (see Preferences

Yoon 2008 Evolution of progenitor stars of Type Ibc supernovae and long gamma-ray bursts

主要内容:


精彩摘抄:
"presenting recent evolutionary models of massive single and binary stars"

文章信息:

· arXiv e-print (arXiv:0801.4362)
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Title:
Evolution of progenitor stars of Type Ibc supernovae and long gamma-ray bursts
Authors:
Yoon, S. -C.; Langer, N.; Cantiello, M.; Woosley, S. E.; Glatzmaier, G. A.
Publication:
eprint arXiv:0801.4362
Publication Date:
01/2008
Origin:
ARXIV
Keywords:
Astrophysics
Comment:
6 pages, 4 figures, to appear in IAU Symp. 250, Massive Stars as Cosmic Engines, Kauai (HI), 12/2007, ed. F. Bresolin, P. Crowther, & J. Puls
Bibliographic Code:
2008arXiv0801.4362Y

Abstract

We discuss how rotation and binary interactions may be related to the diversity of type Ibc supernovae and long gamma-ray bursts. After presenting recent evolutionary models of massive single and binary stars including rotation, the Tayler-Spruit dynamo and binary interactions, we argue that the nature of SNe Ibc progenitors from binary systems may not significantly differ from that of single star progenitors in terms of rotation, and that most long GRB progenitors may be produced via the quasi-chemically homogeneous evolution at sub-solar metallicity. We also briefly discuss the possible role of magnetic fields generated in the convective core of a massive star for the transport of angular momentum, which is potentially important for future stellar evolution models of supernova and GRB progenitors.

星期三, 一月 23, 2008

Maund 2008 SN 2006aj在9.6天时的偏振

主要内容:
大约3%的偏振,说明观测上应该是偏轴。还有不同元素的偏振度和偏振角不一样,说明不同元素的轴不一样。

精彩摘抄:


文章信息:
Spectropolarimetry of SN 2006aj at 9.6 days

Authors: J. R. Maund (1), J. C. Wheeler (1), F. Patat (2), D. Baade (2), L. Wang (3), P. Hoflich (4) ((1) UT Austin, (2) ESO, (3) Texas A&M (4) FSU)
Abstract: The observational technique of spectropolarimetry has been used to directly measure the asymmetries of Supernovae (SNe), Gamma-Ray Bursts (GRBs) and X-Ray Flashes (XRFs). We wish to determine if non-axial asymmetries are present in SNe that are associated with GRBs and XRFs, given the particular alignment of the jet axis and axis of symmetry with the line of sight in these cases. We performed spectropolarimetry with the Very Large Telescope (VLT) FORS1 instrument of the Type Ic SN 2006aj, associated with the XRF 060218, at V-band maximum at 9.6 rest frame days after the detection of the XRF. Due to observations at only 3 retarder plate angles, the data were reduced assuming that the instrumental signature correction for the $U$ Stokes parameter was identical to the correction measured for $Q$. We find SN 2006aj to be highly polarized at wavelengths corresponding to the absorption minima of certain spectral lines, particularly strong for O I 7774\AA and Fe II, observed at 4200\AA with a polarization 3%. The value of the Interstellar Polarization is not well constrained by these observations and, considering the low polarization observed between 6000-6500\AA, the global asymmetry of the SN is $\lesssim 15%$. O I and Fe II lines share a polarization angle that differs from Ca II. SN 2006aj exhibits a higher degree of line polarization than other SNe associated with GRBs and XRFs. The polarization associated with spectral lines implies significant asymmetries of these elements with respect to each other and to the line of sight. This is contrary to the standard picture of SNe associated with GRBs/XRFs, where the axis of symmetry of the SN is aligned with the GRB jet axis and the line of sight.
Comments: 4 pages, 2 Figures, A&A Accepted
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0709.0004v1 [astro-ph]

Fynbo 2008 Metallicity Distributions of Gamma-ray Burst, Damped Lyman-alpha, and Lyman-break Galaxies at z=3

主要内容:


精彩摘抄:


文章信息:

arXiv e-print (arXiv:0801.3273)
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Title:
Reconciling the Metallicity Distributions of Gamma-ray Burst, Damped Lyman-alpha, and Lyman-break Galaxies at z=3
Authors:
Fynbo, Johan P. U.; Prochaska, J. Xavier; Sommer-Larsen, Jesper; Dessauges-Zavadsky, Miroslava; Moller, Palle
Publication:
eprint arXiv:0801.3273
Publication Date:
01/2008
Origin:
ARXIV
Keywords:
Astrophysics
Comment:
19 pages, 6 figures. Submitted to ApJ. Comments very welcome
Bibliographic Code:
2008arXiv0801.3273F

Abstract

We test the hypothesis that the host galaxies of long-duration gamma-ray bursts (GRBs) as well as quasar-selected damped Lyman-alpha (DLA) systems are drawn from the population of UV-selected star-forming, high-z galaxies (generally referred to as Lyman-break galaxies). Specifically, we compare the metallicity distributions of the GRB and DLA populations to simple models where these galaxies are drawn randomly from the distribution of star-forming galaxies according to their star-formation rate and HI cross-section respectively. We find that it is possible to match both observational distributions assuming very simple and constrained relations between luminosity, metallicity and HI sizes. The simple model can be tested by observing the luminosity distribution of GRB host galaxies and by measuring the luminosity and impact parameters of DLA selected galaxies as a function of metallicity. Our results support the expectation that GRB and DLA samples, in contrast to magnitude limited surveys, provide an almost complete census of star-forming galaxies at z=3.
Bibtex entry for this abstract Preferred format for this abstract (see Preferences)

星期二, 一月 22, 2008

Bagchi 2008 目前最长的射电喷流 在CGCG 049-033中

主要内容:
> 400 kpc

精彩摘抄:


文章信息:
A Giant Radio Jet Ejected by an Ultramassive Black Hole in a Single‐lobed Radio Galaxy

Joydeep Bagchi,1 Gopal‐Krishna,2 Marita Krause,3 and Santosh Joshi1,4

Received 2007 July 5; accepted 2007 October 8; published 2007 October 30

Online material: color figure

  • 1Inter‐University Centre for Astronomy and Astrophysics (IUCAA), Pune University Campus, Post Bag 4, Pune 411007, India; .

  • 2National Centre for Radio Astrophysics, TIFR, Pune University Campus, Post Bag 3, Pune 411007, India.

  • 3Max‐Planck‐Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany.

  • 4Aryabhatta Research Institute of Observational Sciences (ARIES), Manora Peak, NainiTal 263129, India.

Subject headings:

galaxies: active—galaxies: ISM—galaxies: jets—radio continuum: galaxies

ABSTRACT

We report the discovery of a very unusual, highly asymmetric radio galaxy whose radio jet, the largest yet detected, emits strongly polarized synchrotron radiation and can be traced all the way from the galactic nucleus to the hot spot located 440 kpc away. This jet emanates from an extremely massive black hole (> ) and forms a strikingly compact radio lobe. No radio lobe is detected on the side of the counterjet, even though it is similar to the main jet in brightness up to a scale of tens of kiloparsecs. Thus, contrary to the nearly universal trend, the brightness asymmetry in this radio galaxy increases with distance from the nucleus. With several unusual properties, including a predominantly toroidal magnetic field, this Fanaroff‐Riley type II megajet is an exceptionally useful laboratory for testing the role of magnetic field in jet stabilization and radio lobe formation.

The Astrophysical Journal Letters, 670:L85–L88, 2007 December 1

http://www.journals.uchicago.edu/doi/abs/10.1086/524220

星期一, 一月 21, 2008

Kohta 2008 High-energy cosmic-ray nuclei from GRB

主要内容:


精彩摘抄:


文章信息:

· arXiv e-print (arXiv:0801.2861)
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Title:
High-energy cosmic-ray nuclei from high- and low-luminosity gamma-ray bursts and implications for multi-messenger astronomy
Authors:
Murase, Kohta; Ioka, Kunihito; Nagataki, Shigehiro; Nakamura, Takashi
Publication:
eprint arXiv:0801.2861
Publication Date:
01/2008
Origin:
ARXIV
Keywords:
Astrophysics
Comment:
20 pages, 9 figures
Bibliographic Code:
2008arXiv0801.2861M

Abstract

Gamma-ray burst (GRB) is one of the candidates of high-energy cosmic-ray acceleration sites. They may be also ultra-high-energy (above 3 EeV) cosmic-ray (UHECR) sources. In this paper, we discuss possibilities and implications of high-energy cosmic-ray acceleration in GRBs. (1) First, we show that not only protons but also heavier nuclei can be accelerated up to ultra-high energies in both usual high-luminosity (HL) and low-luminosity (LL) GRBs by using the Geant4. LL GRBs may also make a significant contribution to the observed UHECR flux if they form a distinct population, and we investigate cosmic-ray acceleration in LL GRBs in detail. (2) Second, we discuss implications of the GRB-UHECR hypothesis (and Hypernova-UHECR hypothesis) to cosmic-ray astronomy. HL GRBs and LL GRBs will lead to different source number densities as UHECR sources, so that the determination of the number density of UHECR sources and strength of the mean extragalactic magnetic field (EGMF) will give us useful clues to test the hypothesis. If the EGMF is sufficiently weak, only LL GRBs can be dominant as UHECR sources. If the EGMF is sufficiently strong, HL GRBs would be more suitable UHECR sources to explain the observed anisotropy. (3) The detection of high-energy neutrinos and gamma rays will also give us important clues to high-energy cosmic-ray acceleration in GRBs. We show that, when ultra-high-energy heavy nuclei such as iron can survive in GRBs, the detection of high-energy neutrinos would become more difficult. However, since escape of high-energy gamma rays from the source is easier in such cases, the detection of very high-energy gamma rays and/or secondary delayed gamma rays can be more expected.
Bibtex entry for this abstract Preferred format for this abstract (see Preferences)

星期四, 一月 17, 2008

最近伽玛暴的高能观测--没有

搜索了一下最近对伽玛暴高能光子的观测数据,有AGILE,MAGIC的,但都没有具体值,只有上限。

有观测数据的还是早期Compton卫星上的EGRET。

星期三, 一月 16, 2008

Lu 2008 Tidal disruption model for GRB 060614

主要内容:
The progenitor star of GRB 060614 may be tidally disrupted by an intermediate mass black hole.

精彩摘抄:


文章信息:

arXiv:0801.2419 [ps, pdf, other]
Title: A tidal disruption model for the gamma-ray burst of GRB 060614
Comments: 13pages, submitted to ApJ
Subjects: Astrophysics (astro-ph)

The combination of a long duration and the absence of any accompanying supernova clearly shows that GRB 060614 can not be grouped into the two conventional classes of gamma-ray bursts, i.e. the long/soft bursts deemed to be collapsars and the short/hard bursts deemed to be merging binary compact stars. A new progenitor model is required for this anomalous gamma-ray burst. We propose that GRB 060614 might be produced through the tidal disruption of a star by an intermediate mass black hole. In this scenario, the long duration and the lack of any associated supernova are naturally expected. The theoretical energy output is also consistent with observations.

Bellm 2008 RHESSI Spectral Fits of Swift GRBs -- E_peak

主要内容:
RHESSI is sensitive from 30 keV to 17 MeV.

精彩摘抄:

Some E_ps

文章信息:

arXiv:0801.2417 [ps, pdf, other]
Title: RHESSI Spectral Fits of Swift GRBs
Comments: 4 pages, 1 figure. To appear in the proceedings of Gamma Ray Bursts 2007, Santa Fe, New Mexico, November 5-9 2007
Subjects: Astrophysics (astro-ph)

One of the challenges of the Swift era has been accurately determining Epeak for the prompt GRB emission. RHESSI, which is sensitive from 30 keV to 17 MeV, can extend spectral coverage above the Swift-BAT bandpass. Using the public Swift data, we present results of joint spectral fits for 26 bursts co-observed by RHESSI and Swift-BAT through May 2007. We compare these fits to estimates of Epeak which rely on BAT data alone. A Bayesian Epeak estimator gives better correspondence with our measured results than an estimator relying on correlations with the Swift power law indices.

美科学家发现无任何恒星的纯暗物质星系

美科学家发现无任何恒星的纯暗物质星系

美科学家发现无任何恒星的纯暗物质星系

18
顶!
星系VIRGOHI21

新浪科技讯北京时间6月18日消息,据俄罗斯有关媒体报道,以美国康奈尔大学的科学家为首的一个国际天文研究小组发现了一个宇宙间最为神秘的星系--“暗 星系”。这个星系中没有任何普通天体(包括恒星和行星),但其质量却是太阳的1000亿倍,是实实在在的由纯暗物质构成的星系。

据从事此项研究的科学家们称,该星系名为“VIRGOHI21”。早在2000年,天文学家们就开始被这一神秘星系所吸引。但直到目前天文学家们才开始弄明白,这个星系就是所谓的“暗星系”,也就是说,与其它普通星系相比较,这个星系完全是由暗物质构成的。

在设在荷兰的“威斯特博尔克综合射电天文望远镜”(Westerbork Synthesis Radio Telescope)的帮助下,科学家们还精确测定了该星系的大小。通过“哈勃”轨道天文望远镜的进一步观测,科学家们竟然发现该星系中不存在一颗恒星, 甚至连构成普通恒星和行星的物质也找不到。

据来自康奈尔大学的天文学家们称,他们是在研究超级星系“NGC 4254”时首次意外发现了“VIRGOHI21”星系。最初,科学家们在“NGC 4254”星系周围观测到典型的气体发光现象,后来的重力测量结果显示,“NGC 4254”星系旁边肯定存在着一个巨大的星系,但是他们却无论如何也观测不到这个星系。

最近该天文小组的研究资料显示,“VIRGOHI21”暗星系的质量约为太阳的1000亿倍,它与地球的距离为5000多万光年。据康奈尔大学所 属的阿雷西伯天文观测台台长罗伯特-明琴博士表示:“如果这是一个普通的星系的话,那么性能较好的普通望远镜就可以观测得到。但遗憾的是甚至连‘哈勃’ 这样专业的轨道天文望远镜都看不到其中有任何东西,就连一颗恒星也看不到”。

我们知道,暗物质(包括暗能量)被认为是宇宙研究中最具挑战性的课题,它代表了宇宙中90%以上的物质含量,而我们可以看到的物质只占宇宙总物质 量的10%不到。暗物质无法直接观测得到,但它却能干扰星体发出的光波或引力,其存在能被明显地感受到。科学家曾对暗物质的特性提出了多种假设,但直到目 前还没有得到充分的证明。



From: http://www.qiji.cn/scinews/detailed/7080.html

星期二, 一月 15, 2008

Kaneko 2008 Broadband spectra of Bright BATSE GRBs

主要内容:
The brightest ones are different from the normal ones.

精彩摘抄:


文章信息:

arXiv e-print (arXiv:0801.1869)
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Title:
Broadband Spectral Properties of Bright High-Energy Gamma-Ray Bursts Observed with BATSE and EGRET
Authors:
Kaneko, Y.; Gonzalez, M. M.; Preece, R.; Dingus, B. L.; Briggs, M. S.
Publication:
eprint arXiv:0801.1869
Publication Date:
01/2008
Origin:
ARXIV
Keywords:
Astrophysics
Comment:
18 pages (emulateapj) including 7 figures & 2 tables. Accepted for publication in ApJ
Bibliographic Code:
2008arXiv0801.1869K

Abstract

We present the spectral analysis of duration-integrated broadband spectra (in $\sim30 $keV$-200 $MeV) of 15 bright BATSE gamma-ray bursts (GRBs). Some GRB spectra are very hard, with their spectral peak energies being above the BATSE LAD passband limit of $\sim$2 MeV. In such cases, their high-energy spectral parameters (peak energy and high-energy power-law indices) cannot be adequately constrained by BATSE LAD data alone. A few dozen bright BATSE GRBs were also observed with EGRET's calorimeter, TASC, in multi-MeV energy band, with a large effective area and fine energy resolution. Combining the BATSE and TASC data, therefore, affords spectra that span four decades of energy ($30 $keV$-200 $MeV), allowing for a broadband spectral analysis with good statistics. Studying such broadband high-energy spectra of GRB prompt emission is crucial, as they provide key clues to understanding its gamma-ray emission mechanism. Among the 15 GRB spectra, we found two cases with a significant high-energy excess, and another case with a extremely high peak energy (\epeak $\gtrsim$ 170 MeV). There have been very limited number of GRBs observed at MeV energies and above, and only a few instruments have been capable of observing GRBs in this energy band with such high sensitivity. Thus, our analysis results presented here should also help predict GRB observations with current and future high-energy instruments such as AGILE and GLAST, as well as with ground-based very-high-energy telescopes.
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Yu 2008 Nertrino from GRB-SN shock breakout

主要内容:


精彩摘抄:


文章信息:

· arXiv e-print (arXiv:0801.2076)
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Title:
Neutrino emission from a GRB afterglow shock during an inner supernova shock breakout
Authors:
Yu, Yun-Wei; Dai, Zi-Gao; Zheng, Xiao-Ping
Publication:
eprint arXiv:0801.2076
Publication Date:
01/2008
Origin:
ARXIV
Keywords:
Astrophysics
Comment:
6 pages, 2 figures, accepted for publication in MNRAS
Bibliographic Code:
2008arXiv0801.2076Y

Abstract

The observations of a nearby low-luminosity gamma-ray burst (GRB) 060218 associated with supernova SN 2006aj may imply an interesting astronomical picture where a supernova shock breakout locates behind a relativistic GRB jet. Based on this picture, we study neutrino emission for early afterglows of GRB 060218-like GRBs, where neutrinos are expected to be produced from photopion interactions in a GRB blast wave that propagates into a dense wind. Relativistic protons for the interactions are accelerated by an external shock, while target photons are basically provided by the incoming thermal emission from the shock breakout and its inverse-Compton scattered component. Because of a high estimated event rate of low-luminosity GRBs, we would have more opportunities to detect afterglow neutrinos from a single nearby GRB event of this type by IceCube. Such a possible detection could provide evidence for the picture described above.

星期日, 一月 13, 2008

XRF 080109/SN 2008D


Ghisellini 2008 Central engine for X-ray shallow decay phase

主要内容:
Prolonged central engine for explaining the shallow decay phase.

精彩摘抄:
Sketch for shallow decay: Lower LF bulk collide at later time, while after the shell with gamma=1/theta catches up, the light curve will have a break. This break will not influence the Opt light curves. (But why after the break, it goes to a "normal" light curve.)

文章信息:

· arXiv e-print (arXiv:0801.1668)
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Title:
The shallow phase of X-ray afterglows
Authors:
Ghisellini, G.
Publication:
eprint arXiv:0801.1668
Publication Date:
01/2008
Origin:
ARXIV
Keywords:
Astrophysics
Comment:
4 pages, 1 figure, contributed talk for the Gamma Ray Bursts 2007 meeting. Santa Fe, New Mexico, November 5-9, 2007
Bibliographic Code:
2008arXiv0801.1668G

Abstract

We propose that the flat decay phase in the first 100-10,000 seconds of the X-ray light curve of Gamma Ray Bursts can be interpreted as prolonged activity of the central engine, producing shells of decreasing bulk Lorentz factors Gamma. The internal dissipation of these late shells produces a continuous and smooth emission, usually dominant in X-rays and sometimes in the optical. When Gamma of the late shells is larger than 1/theta_j, where theta_j is the jet opening angle, we see only a portion of the emitting surface. Eventually, Gamma becomes smaller than 1/theta_j, and the entire emitting surface is visible. When Gamma=1/theta_j there is a break in the light curve, and the plateau ends. During the plateau phase, we see the sum of the "late-prompt" emission (due to late internal dissipation), and the "real afterglow" emission (due to external shocks). A variety of different optical and X-ray light curves is possible, explaining why the X-ray and the optical light curves often do not track each other, and why they often do not have simultaneous breaks.
Bibtex entry for this abstract Preferred format for this abstract (see Preferences)

星期五, 一月 11, 2008

Weidenspointner 2008 asymmetric distribution of positrons in the Galactic disk

主要内容:
intensity: 10-3 and 10-2 photons cm-2 s-1 sr-1

精彩摘抄:
Upper: p-e annihilation line image
Lower: (b) distributions of hard LMXBs

文章信息:

Nature 451, 159-162 (10 January 2008) | doi:10.1038/nature06490; Received 30 March 2007; Accepted 16 November 2007

An asymmetric distribution of positrons in the Galactic disk revealed by gamma-rays

Georg Weidenspointner1,2,3, Gerry Skinner1,4,5, Pierre Jean1, Jürgen Knödlseder1, Peter von Ballmoos1, Giovanni Bignami1,8, Roland Diehl2, Andrew W. Strong2, Bertrand Cordier6, Stéphane Schanne6 & Christoph Winkler7

Correspondence to: Georg Weidenspointner1,2,3 Correspondence and requests for materials should be addressed to G.W. (Email: Georg.Weidenspointner@hll.mpg.de).

Top

Gamma-ray line radiation at 511 keV is the signature of electron–positron annihilation. Such radiation has been known for 30 years to come from the general direction of the Galactic Centre1, but the origin of the positrons has remained a mystery. Stellar nucleosynthesis2, 3, 4, accreting compact objects5, 6, 7, 8, and even the annihilation of exotic dark-matter particles9 have all been suggested. Here we report a distinct asymmetry in the 511-keV line emission coming from the inner Galactic disk (approx10–50° from the Galactic Centre). This asymmetry resembles an asymmetry in the distribution of low mass X-ray binaries with strong emission at photon energies >20 keV ('hard' LMXBs), indicating that they may be the dominant origin of the positrons. Although it had long been suspected that electron–positron pair plasmas may exist in X-ray binaries, it was not evident that many of the positrons could escape to lose energy and ultimately annihilate with electrons in the interstellar medium and thus lead to the emission of a narrow 511-keV line. For these models, our result implies that up to a few times 1041 positrons escape per second from a typical hard LMXB. Positron production at this level from hard LMXBs in the Galactic bulge would reduce (and possibly eliminate) the need for more exotic explanations, such as those involving dark matter.

http://www.nature.com/nature/journal/v451/n7175/full/nature06490.html#abs

Fan 2008 Polarization model in early X-ray afterglow

主要内容:
If the ejecta is Poynting-flux dominated, and geometrically asymmetric, high polarization is expected.

精彩摘抄:


文章信息:

arXiv e-print (arXiv:0801.1517)
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Title:
Polarization evolution accompanying the very early sharp decline of GRB X-ray afterglows
Authors:
Fan, Yi-Zhong; Xu, Dong; Wei, Da-Ming
Publication:
eprint arXiv:0801.1517
Publication Date:
01/2008
Origin:
ARXIV
Keywords:
Astrophysics
Comment:
9 pages including 4 figures. Accepted for publication in MNRAS
Bibliographic Code:
2008arXiv0801.1517F

Abstract

In the synchrotron radiation model, the polarization property depends on both the configuration of the magnetic field and the geometry of the visible emitting region. Some peculiar behaviors in the X-ray afterglows of {\it Swift} gamma-ray bursts (GRBs), such as energetic flares and the plateau followed by a sharp drop, might by highly linearly-polarized because the outflows powering these behaviors may be Poynting-flux dominated. Furthermore, the broken-down of the symmetry of the visible emitting region may be hiding in current X-ray data and will give rise to interesting polarization signatures. In this work we focus on the polarization accompanying the very early sharp decline of GRB X-ray afterglows. We show that strong polarization evolution is possible in both the high latitude emission model and the dying central engine model which are used to interpret this sharp X-ray decline. It is thus not easy to efficiently probe the physical origin of the very early X-ray sharp decline with future polarimetry. Strong polarization evolution is also possible in the decline phase of X-ray flares and in the shallow decline phase of X-ray light curves characterized by chromatic X-ray VS. Optical breaks. An {\it XRT}-like detector but with polarization capability on board a {\em Swift}-like satellite would be suitable to test our predictions.
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星期四, 一月 10, 2008

Petar 2008 MHD simulation for GRB and its afterglow

主要内容:
RMHD study, including synchrotron radiation simulation.

精彩摘抄:


文章信息:

· arXiv e-print (arXiv:0801.1325)
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Title:
An RMHD study of transition between prompt and afterglow GRB phases
Authors:
Mimica, Petar; Giannios, Dimitrios; Aloy, Miguel-Angel
Publication:
eprint arXiv:0801.1325
Publication Date:
01/2008
Origin:
ARXIV
Keywords:
Astrophysics
Comment:
10 pages, 3 figures, proceedings of the conference "Supernovae: lights in the darkness", October 3-5, 2007, Mao (Menorca)
Bibliographic Code:
2008arXiv0801.1325M

Abstract

We study the afterglow phases of a GRB through relativistic magnetohydrodynamic simulations. The evolution of a relativistic shell propagating into a homogeneous external medium is followed. We focus on the effect of the magnetization of the ejecta on the initial phases of the ejecta-external medium interaction. In particular we are studying the condition for the existence of a reverse shock into the ejecta, the timescale for the transfer of the energy from the shell to the shocked medium and the resulting multiwavelength light curves. To this end, we have developed a novel scheme to include non-thermal processeses which is coupled to the relativistic magnetohydrodynamic code MRGENESIS in order to compute the non-thermal synchrotron radiation.

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Curran 2008 X-ray breaks are hidden

主要内容:
Try to solve the achromatic break for X-ray and Optical afterglow, by saying that the breaks for X-ray are hidden.

精彩摘抄:


文章信息:

arXiv e-print (arXiv:0801.1449)
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Title:
The hidden X-ray breaks in afterglow light curves
Authors:
Curran, P. A.; van der Horst, A. J.; Wijers, R. A. M. J.; Starling, R. L. C.
Publication:
eprint arXiv:0801.1449
Publication Date:
01/2008
Origin:
ARXIV
Keywords:
Astrophysics
Comment:
4 pages, contributed talk, submitted to the proceedings of Gamma Ray Bursts 2007, Santa Fe, New Mexico, November 5-9 2007
Bibliographic Code:
2008arXiv0801.1449C

Abstract

Gamma-Ray Burst (GRB) afterglow observations in the Swift era have a perceived lack of achromatic jet breaks compared to the BeppoSAX, or pre-Swift era. Specifically, relatively few breaks, consistent with jet breaks, are observed in the X-ray light curves of these bursts. If these breaks are truly missing, it has serious consequences for the interpretation of GRB jet collimation and energy requirements, and the use of GRBs as standard candles. Here we address the issue of X-ray breaks which are possibly 'hidden' and hence the light curves are misinterpreted as being single power-laws. We show how a number of precedents, including GRB 990510 & GRB 060206, exist for such hidden breaks and how, even with the well sampled light curves of the Swift era, these breaks may be left misidentified. We do so by synthesising X-ray light curves and finding general trends via Monte Carlo analysis. Furthermore, in light of these simulations, we discuss how to best identify achromatic breaks in afterglow light curves via multi-wavelength analysis.
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