Augusto 2011 疑似找到和GRB 991112成协的mu子中微子

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Title:		Search for GeV gamma ray burst signal in the field of view of muon telescopes
Authors:		Augusto, C. R. A.; Kopenkin, V.; Navia, C. E.; Shigueoka, H.; Tsui, K. H.
Publication:		eprint arXiv:1109.5156
Publication Date:		09/2011
Origin:		ARXIV
Keywords:		Astrophysics - High Energy Astrophysical Phenomena
Comment:		9 pages, 9 figures
Bibliographic Code:		2011arXiv1109.5156A

Abstract

We present a search for the muon excess associated in time (preceding or following) with the trigger of gamma ray burst (GRB) detected by the satellite borne instruments. The 1 Hz rate data was collected at the sea level in the area located at 22S and 43W, inside the South Atlantic Anomaly (SAA) region and close to its center. Using 1 sec amd 10 sec time binning, we found the muon excess with significance $>5\sigma$ associated with the gamma ray burst GRB091112 observed by Swift BAT, Fermi GBM and Suzaku-WAM satellites. The detected muon time profile shows a rough peak at $\sim T+223$ sec ($5.2\sigma$), and with a duration of 7 sec. The FLUKA Monte Carlo simulation of the photon-to-muon conversion in the atmosphere shows that the observed muon excess can be explained by progenitor photons with energies above 10 GeV. This energy value is above the limits expected from the maximum synchrotron emission combined with the shock wave evolution. It shows constrains on the external shock synchrotron model for GRB emission in the GeV energy band and favors scenarios with overall afterglow spectrum, ranging from the optical-to-GeV photons. On the basis of the observed associations with the satellite data, we report confidence analysis of the background fluctuations and fluencies in the GeV energy region, that is important for better understanding of the GRB physics.

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Greco 2011 在伽马暴的“随机”光变里发现混沌现象

主要内容:
在随机光变里找到短时标的“吸引子”，是否说明存在内在的规律性。

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Title:		Evidence of Deterministic Components in the Apparent Randomness of GRBs: Clues of a Chaotic Dynamic
Authors:		Greco, G.; Rosa, R.; Beskin, G.; Karpov, S.; Romano, L.; Guarnieri, A.; Bartolini, C.; Bedogni, R.
Publication:		eprint arXiv:1109.5891
Publication Date:		09/2011
Origin:		ARXIV
Keywords:		Astrophysics - High Energy Astrophysical Phenomena
Comment:		13 pages, 6 figures; The article is published by Scientific Report. The original version is available at the following address: http://www.nature.com/srep/2011/110914/srep00091/full/srep00091.html
Bibliographic Code:		2011arXiv1109.5891G

Abstract

Prompt {\gamma}-ray emissions from gamma-ray bursts (GRBs) exhibit a vast range of extremely complex temporal structures with a typical variability time-scale significantly short - as fast as milliseconds. This work aims to investigate the apparent randomness of the GRB time profiles making extensive use of nonlinear techniques combining the advanced spectral method of the Singular Spectrum Analysis (SSA) with the classical tools provided by the Chaos Theory. Despite their morphological complexity, we detect evidence of a non stochastic short-term variability during the overall burst duration - seemingly consistent with a chaotic behavior. The phase space portrait of such variability shows the existence of a well-defined strange attractor underlying the erratic prompt emission structures. This scenario can shed new light on the ultra-relativistic processes believed to take place in GRB explosions and usually associated with the birth of a fast-spinning magnetar or accretion of matter onto a newly formed black hole.

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Maeder 2011 旋转的大质量恒星的演化

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就个人而言，可以通过这篇综述查看WR星的星风，知道星风的轮廓（应该不是-2）.

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Title:		Rotating massive stars through the ages, with applications to WR stars, Pop III stars and Gamma Ray Bursts
Authors:		Maeder, Andre; Meynet, Georges
Publication:		eprint arXiv:1109.6171
Publication Date:		09/2011
Origin:		ARXIV
Keywords:		Astrophysics - Solar and Stellar Astrophysics
Comment:		55 pages, 33 figures, 2 tables, accepted for publication in Review of Modern Physics
Bibliographic Code:		2011arXiv1109.6171M

Abstract

This article first reviews the basic physics of rotating stars and their evolution. We examine in particular the changes of the mechanical and thermal equilibrium of rotating stars. An important (predicted and observed) effect is that rotating stars are hotter at the poles and cooler at the equator. We briefly discuss the mass loss by stellar winds, which are influenced by the anisotropic temperature distribution. These anisotropies in the interior are also driving circulation currents, which transports the chemical elements and the angular momentum in stars. Internal differential rotation, if present, creates instabilities and mixing, in particular the shear mixing, the horizontal turbulence and their interactions. A major check of the model predictions concerns the changes of the surface abundances, which are modified by mass loss in the very massive stars and by rotational mixing in O- and B-type stars. We show that the observations confirm the existence of rotational mixing, with much larger effects at lower metallicities. We discuss the predictions of stellar models concerning the evolution of the surface velocities, the evolutionary tracks in the HR diagram and lifetimes, the populations of blue, red supergiants and Wolf-Rayet stars, and the progenitors of type Ibc supernovae. We show, that in many aspects, rotating models provide a much better fit than non-rotating ones. Using the same physical ingredients as those which fit the best the observations of stars at near solar metallicities, we explore the consequences of rotating models for the status of Be stars, the progenitors of Gamma Ray Bursts, the evolution of Pop III stars and of very metal poor stars, the early chemical evolution of galaxies, the origin of the C-enhanced Metal Poor stars (CEMP) and of the chemical anomalies in globular clusters.

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Granot 2011 关于侧向膨胀新的解析解

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以前的解析解认为在gamma=1/theta_j的时候就开始进入指数阶段，侧向膨胀很显著；但数值模拟发现不是这样的。Granot和Piran给出了新的解析解，得到了和数值解一致的结果。

结果是再共动系中侧向膨胀的速度 beta_theta ~ 1/(Gamma^2 delta_theta) ~ 1/(Gamma^2 theta)

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Title:		On the lateral expansion of GRB jets
Authors:		Granot, Jonathan; Piran, Tsvi
Publication:		eprint arXiv:1109.6468
Publication Date:		09/2011
Origin:		ARXIV
Keywords:		Astrophysics - High Energy Astrophysical Phenomena
Comment:		33 pages, 11 figures, submitted to MNRAS
Bibliographic Code:		2011arXiv1109.6468G

Abstract

The dynamics of GRB jets during the afterglow phase have an important effect on the interpretation of their observations and for inferring key physical parameters such as their true energy and event rate. Semi-analytic models generally predict a fast lateral expansion, where the jet opening angle asymptotically grows exponentially with its radius. Numerical simulations, however, show a much more modest lateral expansion, where the jet retains memory of its initial opening angle for a very long time, and the flow remains non-spherical until it becomes sub-relativistic, and only then gradually approaches spherical symmetry. Here we suggest a new analytic model based on a new physically derived recipe for the lateral expansion. We also generalize the model by relaxing the common approximations of ultra-relativistic motion and a narrow jet opening angle. We find that the new analytic model fits much better the results of numerical simulations, mainly because it remains valid also in the mildly relativistic, quasi spherical regime. This model shows that for modest initial jet half-opening angles, \theta_0, the outflow is not sufficiently ultra-relativistic when its Lorentz factor reaches \Gamma = 1/\theta_0 and therefore the sideways expansion is rather slow, showing no rapid, exponential phase. On the other hand, we find that jets with a very narrow initial half-opening angle, \theta_0 << 1, which are still ultra-relativistic at \Gamma = 1/\theta_0, do show a phase of rapid, exponential lateral expansion. However, even such jets that expand sideways exponentially are still not spherical when they become sub-relativistic.

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de Ugarte Postigo 2011 ALMA之前的红外观测

主要内容:
也就是目前的红外余辉的所有观测的总结。一共77个暴。将来ALMA可以观测到98%的暴的余辉！

ALMA座落在智利南的Llano de Chajnantor，有5000m的海拔。2011年末它就可以开始用了。

ALMA的能段

Band Frequency range Wavelength range
(GHz) (mm)
Band 1 31 – 45 6.66 – 9.67
Band 2 67 – 90 3.33 – 4.47
Band 3 84 – 116 2.58 – 3.56
Band 4 125 –163 1.84 – 2.40
Band 5 162 – 211 1.42 – 1.85
Band 6 211 – 275 1.09 – 1.42
Band 7 275 – 373 0.80 – 1.09
Band 8 385 – 500 0.60 – 0.78
Band 9 602 – 720 0.42 – 0.50
Band 10 787 – 950 0.32 – 0.38


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Title:		Pre-ALMA observations of GRBs in the mm/submm range
Authors:		de Ugarte Postigo, A.; Lundgren, A.; Martín, S.; Garcia-Appadoo, D.; de Gregorio Monsalvo, I.;Michałowski, M. J.; Thöne, C. C.; Campana, S.; Gorosabel, J.; Tanvir, N. R.; Wiersema, K.; Castro-Tirado, A. J.; Schulze, S.; De Breuck, C.; Petitpas, G.; Hjorth, J.; Jakobsson, P.; Covino, S.;Fynbo, J. P. U.; Winters, J. M.; Bremer, M.; Levan, A. J.; Salvaterra, R.
Publication:		eprint arXiv:1108.1797
Publication Date:		08/2011
Origin:		ARXIV
Keywords:		Astrophysics - Cosmology and Extragalactic Astrophysics
Comment:		21 pages, 12 figures, 5 tables (one big one!), submitted to A&A. Comments welcome, including any possible overlooked datasets
Bibliographic Code:		2011arXiv1108.1797D

Abstract

It is well known that GRBs generate an afterglow emission that can be detected from radio to X-rays during days, or even weeks after the initial explosion. The peak of this emission crosses the mm/submm range during the first hours to days, making their study in this range crucial for constraining the models. Observations have been limited until now due to the low sensitivity of the observatories in this range. We present observations of 10 GRB afterglows obtained from APEX and SMA at 230 and 345GHz and put them into context with a catalogue of all the observations that have been published until now in the spectral range that will be covered by ALMA. The catalogue of mm/submm observations collected here is the largest to date and is composed of 77 GRBs, of which 65 had afterglow observations, whereas the rest are host galaxy searches. With our programmes, we contributed with data of 10 GRBs and the discovery of 2 submm counterparts. In total, the full sample, including data from the literature, has 19 afterglow detections with redshift ranging from 0.168 to 8.2. GRBs have been detected in mm/submm wavelengths with peak luminosities spanning 2.5 orders of magnitude, the most luminous reaching 10^33erg s^-1 Hz^-1. We observe a correlation between the X-ray brightness at 0.5 days and the mm/submm peak brightness. Finally we give a rough estimate of the distribution of peak flux densities of GRB afterglows, based on the current mm/submm sample. Observations in the mm/submm bands have been shown to be crucial for our understanding of the physics of GRBs, but have until now been limited by the sensitivity of the observatories. With the start of the operations at ALMA, the sensitivity will be increased by more than an order of magnitude. Our estimates predict that, once completed, ALMA will detect up to 98% of the afterglows if observed during the passage of the peak synchrotron emission.

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Lindner 2011 伽马暴前身星的超新星模拟

主要内容:
用吸积产生的能量

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Title:		Simulations of Accretion Powered Supernovae in the Progenitors of Gamma Ray Bursts
Authors:		Lindner, Christopher C.; Milosavljevic, Milos; Shen, Rongfeng; Kumar, Pawan
Publication:		eprint arXiv:1108.1415
Publication Date:		08/2011
Origin:		ARXIV
Keywords:		Astrophysics - High Energy Astrophysical Phenomena
Comment:		20 Pages, 15 Figures, Submitted to ApJ
Bibliographic Code:		2011arXiv1108.1415L

Abstract

Observational evidence suggests a link between long duration gamma ray bursts (LGRBs) and Type Ic supernovae. Here, we propose a potential mechanism for Type Ic supernovae in LGRB progenitors powered solely by accretion energy. We present spherically-symmetric hydrodynamic simulations of the long-term accretion of a rotating gamma-ray burst progenitor star, a "collapsar," onto the central compact object, which we take to be a black hole. The simulations were carried out with the adaptive mesh refinement code FLASH in one spatial dimension and with rotation, an explicit shear viscosity, and convection in the mixing length theory approximation. Once the accretion flow becomes rotationally supported outside of the black hole, an accretion shock forms and traverses the stellar envelope. Energy is carried from the central geometrically thick accretion disk to the stellar envelope by convection. Energy losses through neutrino emission and nuclear photodisintegration are calculated but do not seem important following the rapid early drop of the accretion rate following circularization. We find that the shock velocity, energy, and unbound mass are sensitive to convective efficiency, effective viscosity, and initial stellar angular momentum. Our simulations show that given the appropriate combinations of stellar and physical parameters, explosions with energies ~0.5 Bethes, velocities ~3000 km/s, and unbound material masses > 5 solar masses are possible in a rapidly rotating 16 solar mass main sequence progenitor star. Further work is needed to constrain the values of these parameters, to identify the likely outcomes in more plausible and massive LRGB progenitors, and to explore nucleosynthetic implications.

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Liu 2011 低光度伽马暴也可以对超高能宇宙射线有贡献

主要内容:
Fermi的观测显示local的伽马暴不能产生那么多的超高能宇宙线，文章提出可能还有更多的没有trigger望远镜的低光度伽马暴可以对这个UHECR有贡献。

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Title:		Nearby low-luminosity GRBs as the sources of ultra-high energy cosmic rays revisited
Authors:		Liu, Ruo-Yu; Wang, Xiang-Yu; Dai, Zi-Gao
Publication:		eprint arXiv:1108.1551
Publication Date:		08/2011
Origin:		ARXIV
Keywords:		Astrophysics - High Energy Astrophysical Phenomena
Comment:		10 pages, 5 figures, accepted for publication in MNRAS
Bibliographic Code:		2011arXiv1108.1551L

Abstract

Low-luminosity gamma-ray bursts (GRBs) with luminosity . 10^49erg/s probably consititute a distinct population from the classic high-luminosity GRBs. They are the most luminous objects detected so far within ~ 100 Mpc, the horizon distance of ultra-high energy cosmic rays (UHECRs), so they are considered to be candidate sources of UHECRs. It was recently argued that the energy production rate in UHECRs is much larger than that in gamma-ray photons of long GRBs measured by the Fermi satellite, which, if true, would challenge the view that GRBs can be the sources of UHECRs. We here suggest that many of the low luminosity GRBs, due to their low luminosity, can not trigger the current GRB detectors and hence their contribution to the local gamma-ray energy production rate is missing. We find that the real local energy production rate by low-luminosity GRBs, taking into account the missing part, which constitutes a dominant fraction of the total amount, could be sufficient to account for the flux of UHECRs. Due to the low-luminosity, only intermediate-mass or heavy nuclei can be accelerated to ~ 10^20 eV. We discuss the acceleration and survival of these UHE nuclei in low-luminosity GRBs, especially in those missing low-luminosity GRBs. At last, the accompanying diffuse neutrino flux from the whole low-luminosity GRB population is calculated.

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Astraatmadja 2011 用千米中微子望远镜观测伽马暴TeV辐射的先期研究

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还是要很近的暴才可能看到，z<0.07,或者0.15

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Title:		On the detection of TeV gamma-rays from GRB with km-cube neutrino telescopes --- I. Muon event rate from single GRBs
Authors:		Astraatmadja, Tri L.
Publication:		eprint arXiv:1108.1720
Publication Date:		08/2011
Origin:		ARXIV
Keywords:		Astrophysics - High Energy Astrophysical Phenomena
Comment:		Thirteen pages, 14 figures. Accepted for publication in MNRAS
Bibliographic Code:		2011arXiv1108.1720A

Abstract

This is a preliminary study to examine the prospect of detecting TeV photons from $\gamma$-ray bursts (GRB) using km-size neutrino telescopes, specifically for the ANTARES neutrino telescope. Although optimized to detect upgoing neutrino-induced muons, neutrino telescopes nevertheless have a potential to detect high-energy photons by detecting downgoing muons from the electromagnetic cascade induced by the interaction of TeV photons with the Earth's atmosphere. The photon energy spectrum of a GRB is modeled by a simple power law and is normalized by simple energy considerations. Taking into account the absorption of TeV photons by cosmic infrared backgrounds, an optical depth table calculated from a model by \cite{fin10} is used and the arriving number of photons on top of the Earth atmosphere is determined. Muon production in the atmosphere is determined by considering two main channels of muon production: Pion photoproduction and direct muon pair production. The muon energy loss during their traverse from the surface to the bottom of the sea is determined using the standard muon energy loss formula. Assuming different detector sizes, the number of detectable muons from single GRB events located at different redshifts and zenith distances is determined. The background is calculated assuming it consists primarily of cosmic ray-induced downgoing muons. The detection significance is calculated and it can be concluded that to obtain at least $3\sigma$ detection significance, a typical GRB has to be located at redshift $z \lesssim 0.07$ if the detector's muon effective area is $A^{\mu}_{\rm eff} \sim 10^{-2}\;{\rm km}^{2}$, or redshift $z \lesssim 0.15$, if the muon effective area is $A^{\mu}_{\rm eff} \sim 1\;{\rm km}^{2}$.

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Wu 2011 Gamma-L_iso在伽马暴和Blazar上的统一关系

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发现两种东西放在一起也大致满足相同的关系 Gamma~L^1/3. 其中D是多普勒因子。

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Title:		A Uniform Correlation between Synchrotron Luminosity and Doppler Factor in Gamma-ray Bursts and Blazars: hint of similar intrinsic luminosities?
Authors:		Wu, Qingwen; Zou, Yuan-Chuan; Cao, Xinwu; Wang, Ding-Xiong; Chen, Liang
Publication:		eprint arXiv:1108.1442
Publication Date:		08/2011
Origin:		ARXIV
Keywords:		Astrophysics - High Energy Astrophysical Phenomena
Comment:		16 pages, 4 figures. Submitted to ApJ Letter, revised version after referee's comments
Bibliographic Code:		2011arXiv1108.1442W

Abstract

We compile 23 Gamma-ray Bursts (GRBs) and 21 blazars with estimated Doppler factors, and the Doppler factors of GRBs are estimated from their Lorentz factors by assuming their jet viewing angles \theta->0 degree. Using the conventional assumption that the prompt emission of GRBs is dominated by the synchrotron radiation, we calculate the synchrotron luminosity of GRBs from their total isotropic energy and burst duration. Intriguingly, we discover a uniform correlation between the synchrotron luminosity and Doppler factor, L_syn \propto D^3.1, for GRBs and blazars, which suggests that they may share some similar jet physics. One possible reason is that GRBs and blazars have, more or less, similar intrinsic synchrotron luminosities and both of them are strongly enhanced by the beaming effect. After Doppler and redshift-correction, we find that the intrinsic peak energy of the GRBs ranges from 0.1 to 3 keV with a typical value of 1 keV. We further correct the beaming effect for the observed luminosity of GRBs and find that there exists a positive correlation between the intrinsic synchrotron luminosity and peak energy for GRBs, which is similar to that of blazars. Our results suggest that both the intrinsic positive correlation and the beaming effect may be responsible for the observed tight correlation between the isotropic energy and the peak energy in GRBs (so called "Amati" relation).

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Krühler 2011 消光很大的几个暴的宿主星系的观测

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Title:		The SEDs and Host Galaxies of the dustiest GRB afterglows
Authors:		Krühler, T.; Greiner, J.; Schady, P.; Savaglio, S.; Afonso, P. M. J.; Clemens, C.; Elliott, J.;Filgas, R.; Gruber, D.; Kann, D. A.; Klose, S.; Küpcü-Yoldas, A.; McBreen, S.; Olivares E., F.;Pierini, D.; Rau, A.; Rossi, A.; Nardini, M.; Nicuesa Guelbenzu, A.; Sudilovsky, V.; Updike, A. C.
Publication:		eprint arXiv:1108.0674
Publication Date:		08/2011
Origin:		ARXIV
Keywords:		Astrophysics - Cosmology and Extragalactic Astrophysics
Comment:		17 pages, 13 figures, resubmitted to A&A after referee report
Bibliographic Code:		2011arXiv1108.0674K

Abstract

(Abridged) Until recently the information inferred from gamma-ray burst follow-up observations was mostly limited to optically bright afterglows, biasing all demographic studies against sight-lines that contain large amounts of dust. Here, we present GRB afterglow and host observations for a sample of bursts that are exemplary of previously missed ones because of high visual extinction along the sight-line. This facilitates an investigation of the properties, geometry and location of the absorbing dust of these poorly-explored host galaxies, and a comparison to hosts from optically-selected samples. The hosts of the dustiest afterglows are diverse in their properties, but on average redder, more luminous and massive than the hosts of optically-bright events. We hence probe a different galaxy population, suggesting that previous host samples miss most of the massive, chemically-evolved and metal-rich members. This also indicates that the dust along the sight-line is often related to host properties, and thus probably located in the diffuse ISM or interstellar clouds and not in the immediate GRB environment. Some of the hosts in our sample, are blue, young or of small stellar mass illustrating that even apparently non-extinguished galaxies possess very dusty sight-lines due to a patchy dust distribution. The presented observations establish a population of luminous, massive and correspondingly chemically-evolved GRB hosts. This suggests that GRBs trace the global star-formation rate better than studies based on optically-selected host samples indicate, and the previously-claimed deficiency of high-mass host galaxies was at least partially a selection effect.

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Bosnjak 2011 用磁化喷流解释高能光子的延迟

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Title:		Magnetic jet model for GRBs and the delayed arrival of >100 MeV photons
Authors:		Bosnjak, Z.; Kumar, P.
Publication:		eprint arXiv:1108.0929
Publication Date:		08/2011
Origin:		ARXIV
Keywords:		Astrophysics - High Energy Astrophysical Phenomena
Comment:		5 pages; submitted to MNRAS
Bibliographic Code:		2011arXiv1108.0929B

Abstract

Photons of energy larger than 100 MeV from long-GRBs arrive a few seconds after <10 MeV photons do. We show that this delay is a natural consequence of a magnetic dominated relativistic jet. The much slower acceleration of a magnetic jet with radius (compared with a hot baryonic outflow) results in high energy gamma-ray photons to be converted to electron-positron pairs out to a larger radius whereas lower energy gamma-rays of energy less than ~10 MeV can escape when the jet crosses the Thomson-photosphere. The resulting delay for the arrival of high energy photons is found to be similar to the value observed by the Fermi satellite for a number of GRBs. A prediction of this model is that the delay should increase with photon energy (E) as E^{0.17} for E>100 MeV. The delay depends almost linearly on burst redshift, and on the distance from the central compact object where the jet is launched (R_0). Therefore, the delay in arrival of >10^2 MeV photons can be used to estimate burst redshift if the magnetic jet model for gamma-ray generation is correct and R_0 is roughly the same for long-GRBs.

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D'Elia 2011 暗暴GRB 100614A and GRB 100615A的观测

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Title:		GRB 100614A and GRB 100615A: two extremely dark gamma-ray bursts
Authors:		D'Elia, V.; Stratta, G.
Affiliation:		AA(INAF - Osservatorio Astronomico di Roma, via Frascati 33, 00040, Monteporzio Catone, Italydelia@mporzio.astro.it), AB(ASI-Science Data Center, via Galileo Galilei, 00044, Frascati, Italy)
Publication:		Astronomy & Astrophysics, Volume 532, id.A48 (A&A Homepage)
Publication Date:		08/2011
Origin:		EDP Sciences
Keywords:		gamma-ray burst: general, dust, extinction, stars: Population III, gamma-ray burst: individual: GRB 100614A, gamma-ray burst: individual: GRB 100615A
DOI:		10.1051/0004-6361/201116505
Bibliographic Code:		2011A&A...532A..48D

Abstract

Context. Dark gamma-ray bursts (GRBs) are sources with no or faint optical/near infrared (NIR) afterglow with respect to the X-ray one. Three possible explanations of this optical darkness have been proposed, namely: i) the GRB might be at high redshift, such that the Lyman α absorption prevents optical identifications; ii) dust in the GRB host galaxy may absorb the optical/NIR wavelengths; and iii) the optical faintness might have an intrinsic origin.
Aims: We study two dark GRBs discovered by Swift, namely, GRB 100614A and GRB 100615A. These sources are bright in the X-rays, but no optical/NIR afterglow has been detected for either source, despite the efforts of several follow-up campaigns that have been performed since soon after the GRB explosion.
Methods: We analyze the X-ray data and collect all the optical/NIR upper limits in literature for these bursts. We then build optical-to-X-ray spectral energy distributions (SEDs) at the times at which the reddest upper limits are available, and we model our SEDs with the attenuation curves of the Milky Way (MW), Small Magellanic Cloud (SMC), and one obtained for a sample of starburst galaxies.
Results: We find that to explain the deepest NIR upper limits assuming either a MW or SMC extinction law, the visual extinction towards GRB 100614A is A_V > 47 mag, while for GRB 100615A we obtain A_V > 58 mag using data taken within one day after the burst and A_V > 22 mag even 9.2 days after the trigger.
Conclusions: If these bursts were strongly extincted by dust, these results imply that a MW or SMC-like dust obscuration is unlikely to be able to explain their optical darkness. Since both GRBs are bright in X-rays, explanation iii) also cannot explain their dark classification, unless optical radiation and X-rays are not part of the same synchrotron spectrum. In particular, the X-ray emission during the first 100 - 10 000 s after the burst, shows in ~70% of the cases a "shallow phase" unexpected by the fireball model, typically not tracked at optical wavelengths, that could mimic a stronger optical dust extinction than the real one. An alternative, or complementary explanation of the previous possibility, involves greyer extinction laws. A starburst attenuation curve gives A_V > 11 (A_V > 15) for GRB 100614A (GRB 100615A) before 1 day after the burst, which is less extreme, despite still very high. Assuming high redshift in addition to extinction, implies that A_V > 10 at z = 2 and A_V > 4-5 at z = 5, regardless of the adopted extinction recipe. These lower limits are well above the A_Vcomputed for previous GRBs at known redshift, but not unlikely. A different, exotic possibility would be an extremely high redshift origin (z > 17 given the missing K detections). Population III stars are expected to emerge at z ~ 20 and can produce GRBs with energies well above those inferred for our GRBs at these redshifts. However, high N_H values (above the Galactic ones) toward our GRBs challenge this scenario. Mid- and far-IR late afterglow (>10⁵ s after trigger) observations of these extreme class of GRBs can help us to differentiate between the proposed scenarios.

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