Rubbia 2007 用伽玛暴瞬时辐射的偏振定轴子模型

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文章信息:
Constraining axion by polarized prompt emission from gamma ray bursts

Authors: A. Rubbia (ETH-Zurich), A.S. Sakharov (ETH-Zurich, CERN)

(Submitted on 20 Aug 2007 (v1), last revised 21 Aug 2007 (this version, v3))

Abstract: A polarized gamma ray emission spread over a sufficiently wide energy band from a strongly magnetized astrophysical object like gamma ray bursts (GRBs) offers an opportunity to test the hypothesis of invisible axion. The axionic induced dichroism of gamma rays at different energies should cause a misalignment of the polarization plane for higher energy events relative to that one for lower energies events resulting in the loss of statistics needed to form a pattern of the polarization signal to be recognized in a detector. According to this, any evidence of polarized gamma rays coming from an object with extended magnetic field could be interpreted as a constraint on the existence of the invisible axion for a certain parameter range. Based on reports of polarized MeV emission detected in several GRBs we derive a constraint on the axion-photon coupling. This constraint $\g_{a\gamma\gamma}\le 2.2\cdot 10^{-11} {\rm GeV^{-1}}$ calculated for the axion mass $m_a=10^{-3} {\rm eV}$ is competitive with the sensitivity of CAST and becomes even stronger for lower masses.

Cite as: arXiv:0708.2646v3 [hep-ph]

Wang 2006 用Ia型超新星定暗能量和加速处的红移

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文章信息:
Constraining Dark Energy and Cosmological Transition Redshift with Type Ia Supernovae

Authors: F. Y. Wang, Z. G. Dai (NJU)

(Submitted on 30 Aug 2007)

Abstract: The property of dark energy and the physical reason for acceleration of the present universe are two of the most difficult problems in modern cosmology. The dark energy contributes about two-thirds of the critical density of the present universe from the observations of type-Ia supernova (SNe Ia) and anisotropy of cosmic microwave background (CMB).The SN Ia observations also suggest that the universe expanded from a deceleration to an acceleration phase at some redshift, implying the existence of a nearly uniform component of dark energy with negative pressure. We use the ``gold'' sample containing 157 SNe Ia and two recent well-measured additions, SNe Ia 1994ae and 1998aq to explore the properties of dark energy and the transition redshift. For a flat universe with the cosmological constant, we measure $\Omega_{M}=0.28_{-0.05}^{+0.04}$, which is consistent with Riess et al. The transition redshift is $z_{T}=0.60_{-0.08}^{+0.06}$. We also discuss several dark energy models that define the $w(z)$ of the parameterized equation of state of dark energy including one parameter and two parameters ($w(z)$ being the ratio of the pressure to energy density). Our calculations show that the accurately calculated transition redshift varies from $z_{T}=0.29_{-0.06}^{+0.07}$ to $z_{T}=0.60_{-0.08}^{+0.06}$ across these models. We also calculate the minimum redshift $z_{c}$ at which the current observations need the universe to accelerate.

Comments:	16 pages, 5 figures, 1 table
Subjects:	Astrophysics (astro-ph)
Journal reference:	Chin. J. Astron. Astrophys. Vol. 6. 2006, No. 5, 561-571
Cite as:	arXiv:0708.4062v1 [astro-ph]

Zhang 2007 伽玛暴时标再分析

主要内容:
重新统计了有红移的伽玛暴的时标，包含Swift的数据，得到和以前一样的结论：分长短暴。不过长暴的时长和中间值与之前的有别。

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文章信息:
An analysis of the durations of Swift Gamma-Ray Bursts

Authors: Zhi-Bin Zhang, Chul-Sung Choi

(Submitted on 30 Aug 2007)

Abstract: We report the systematic analysis of the durations for Swift gamma-ray bursts (GRBs) and compare the results with those of pre-Swift data. For 95 GRBs with known redshift, we show that the observed durations have two lognormal distributions that are clearly divided at $T_{90}\simeq2$ s. This is consistent with the earlier BATSE results. The intrinsic durations also show a bimodal distribution but shift systematically toward the smaller value and the distribution exhibits a narrower width compared with the observed one. We find that the intrinsic distributions of long GRBs between Swift and pre-Swift are significantly different particularly in the width and the median value. In addition, the Swift data exhibit a wider duration dynamic range. Our present study also confirms the spectra of short GRBs are in general harder than the long GRBs.

arXiv:0708.4049

Yonetoku 2007 GRB 060904A的谱演化

主要内容:
光子谱指数演化比较剧烈，最引人注目的是在prompt晚期，指数高达5，很软。

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X射线的光变
中间一项是光子谱指数

文章信息:
Spectral evolution of GRB 060904A observed with Swift and Suzaku -- Possibility of Inefficient Electron Acceleration

Authors: Daisuke Yonetoku, et al.
(Submitted on 29 Aug 2007)

Abstract: We observed an X-ray afterglow of GRB 060904A with the Swift and Suzaku satellites. We found rapid spectral softening during both the prompt tail phase and the decline phase of an X-ray flare in the BAT and XRT data. The observed spectra were fit by power-law photon indices which rapidly changed from $\Gamma = 1.51^{+0.04}_{-0.03}$ to $\Gamma = 5.30^{+0.69}_{-0.59}$ within a few hundred seconds in the prompt tail. This is one of the steepest X-ray spectra ever observed, making it quite difficult to explain by simple electron acceleration and synchrotron radiation. Then, we applied an alternative spectral fitting using a broken power-law with exponential cutoff (BPEC) model. It is valid to consider the situation that the cutoff energy is equivalent to the synchrotron frequency of the maximum energy electrons in their energy distribution. Since the spectral cutoff appears in the soft X-ray band, we conclude the electron acceleration has been inefficient in the internal shocks of GRB 060904A. These cutoff spectra suddenly disappeared at the transition time from the prompt tail phase to the shallow decay one. After that, typical afterglow spectra with the photon indices of 2.0 are continuously and preciously monitored by both XRT and Suzaku/XIS up to 1 day since the burst trigger time. We could successfully trace the temporal history of two characteristic break energies (peak energy and cutoff energy) and they show the time dependence of $\propto t^{-3} \sim t^{-4}$ while the following afterglow spectra are quite stable. This fact indicates that the emitting material of prompt tail is due to completely different dynamics from the shallow decay component. Therefore we conclude the emission sites of two distinct phenomena obviously differ from each other.
arXiv:0708.3968 [ps, pdf, other]

Konopelko 2007 地面低能伽玛射线望远镜的设计

主要内容:
已有一些地面高能伽玛射线望远镜，大于100GeV，现在考虑设计10－100GeV的。原理也还是探测次级Cherenkov辐射。

人总是能想到办法的，以前知道伽玛射线穿不过大气层，所以要用卫星观测伽玛辐射。但是穿不过就意味着要和大气层作用，这种作用就会有次级辐射，通过观测这种次级辐射也可以反推原来的伽玛辐射的嘛。

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文章信息:
Design Study of a Low Energy IACT Array for Ground-Based Gamma-Ray Astronomy

Authors: A. Konopelko, J.P. Finley, G. Urbanski (Purdue University)

(Submitted on 28 Aug 2007)

Abstract: Recently, ground-based very high-energy (VHE) gamma-ray astronomy achieved a remarkable advancement in the development of the observational technique for the registration and study of gamma-ray emission above 100 GeV. Construction of telescopes of substantially larger sizes than the currently used 12 m class telescopes can drastically improve the sensitivity of ground-based detectors to gamma rays of energy from 10 GeV to 100 GeV. Based on Monte Carlo simulations we have studied the response of an array of three large area imaging atmospheric Cherenkov telescopes (IACT) as a prototype for a future large-scale low energy ground-based experiment. The sensitivity of a three-telescope array as a function of optical reflector size was investigated here in detail.
arXiv:0708.3865 [ps, pdf, other]

Michałowski 2007 四个宿主星系的观测

主要内容:
对GRBs 980703, 000210, 000418 and 010222四个暴的宿主星系的观测，发现这几个暗弱的星系相比一般星系：更热更年轻，更小更多尘。

这是不是就是伽玛暴宿主星系的普遍特征呢？

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文章信息:
The nature of GRB-selected submillimeter galaxies: hot and young

Authors: M. J. Michałowski, J. Hjorth, J. M. Castro Cerón, D. Watson (Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Denmark)

(Submitted on 29 Aug 2007)

Abstract: We present detailed fits of the spectral energy distributions (SEDs) of four submillimeter (submm) galaxies selected by the presence of a gamma-ray burst (GRB) event (GRBs 980703, 000210, 000418 and 010222). These faint ~3 mJy submm emitters at redshift ~1 are characterized by an unusual combination of long- and short-wavelength properties, namely enhanced submm and/or radio emission combined with optical faintness and blue colors. We exclude an active galactic nucleus as the source of long-wavelength emission. From the SED fits we conclude that the four galaxies are young (ages <2>45 K) indicate that GRB host galaxies are hotter, younger, and less massive counterparts to submm-selected galaxies detected so far. Future facilities like Herschel, JCMT/SCUBA-2 and ALMA will test this hypothesis enabling measurement of dust temperatures of fainter GRB-selected galaxies.


Spectral energy distributions of submm/radio bright gamma-ray burst host galaxies

Authors: M. J. Michałowski, J. Hjorth (Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Denmark)

(Submitted on 29 Aug 2007)

Abstract: We present optical to radio spectral energy distribution fitting of the host galaxies of four long gamma-ray bursts: 980703, 000210, 000418 and 010222, which were detected at submillimetre and/or radio wavelengths. We find that only very young starburst galaxy models are consistent with the data having both blue optical colors and a pronounced submm emission. For each host we are able to construct a model consistent with the short- and long-wavelength parts of the spectra. We find galaxy ages ranging from 0.09 to 2.0 Gyrs and star formation rates ranging from 138 to 380 MSun/yr.

Cite as:

arXiv:0708.4014v1 [astro-ph]

Stratta 2007 短暴061201的暴本身和余辉的观测

主要内容:
有X射线和光学余辉，没有宿主星系的确定候选者。X射线余辉有关着，如果是喷流拐折，张角在1－2度。

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文章信息:
A study of the prompt and afterglow emission of the Short GRB 061201

Authors: G. Stratta, et. al

(Submitted on 27 Aug 2007)

Abstract: Our knowledge of the intrinsic properties of short duration Gamma-Ray Bursts has relied, so far, only upon a few cases for which the estimate of the distance and an extended, multiwavelength monitoring of the afterglow have been obtained. We carried out multiwavelength observations of the short GRB 061201 aimed at estimating its distance and studying its properties. We performed a spectral and timing analysis of the prompt and afterglow emission and discuss the results in the context of the standard fireball model. A clear temporal break was observed in the X-ray light curve about 40 minutes after the burst trigger. We find that the spectral and timing behaviour of the X-ray afterglow is consistent with a jet origin of the observed break, although the optical data can not definitively confirm this and other scenarios are possible. No underlying host galaxy down to R~26 mag was found after fading of the optical afterglow. Thus, no secure redshift could be measured for this burst. The nearest galaxy is at z=0.111 and shows evidence of star formation activity. We discuss the association of GRB 061201 with this galaxy and with the ACO S 995 galaxy cluster, from which the source is at an angular distance of 17'' and 8.5', respectively. We also test the association with a possible undetected, positionally consistent galaxy at z~1. In all these cases, in the jet interpretation, we find a jet opening angle of 1-2 degrees.

arXiv:0708.3553

Nysewander 2007 PROMPT对GRB 060607A的早期光学观测

主要内容:
PROMPT (Panchromatic Robotic Optical Monitoring and Polarimetry Telescopes)是专门为观测伽玛暴余辉准备的。观测GRB 060607A很早的光学余辉和谱。同样，光学和X射线的拐折没有什么时间上的联系。

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文章信息: PROMPT Observations of the Early-Time Optical Afterglow of GRB 060607A

Authors: M. Nysewander, D. E. Reichart, J. A. Crain, A. Foster, J. Haislip, K. Ivarsen, A. Lacluyze, A. Trotter

(Submitted on 27 Aug 2007)

Abstract: PROMPT (Panchromatic Robotic Optical Monitoring and Polarimetry Telescopes) observed the early-time optical afterglow of GRB 060607A and obtained a densely sampled multiwavelength light curve that begins only tens of seconds after the GRB. Located at Cerro Tololo Inter-American Observatory in Chile, PROMPT is designed to observe the afterglows of gamma-ray bursts using multiple automated 0.4-m telescopes that image simultaneously in many filters when the afterglow is bright and may be highly variable. The data span the interval from 44 seconds after the GRB trigger to 3.3 hours in the Bgri filters. We observe an initial peak in the light curve at approximately three minutes, followed by rebrightenings peaking around 40 minutes and again at 66 minutes. Although our data overlap with the early Swift gamma-ray and x-ray light curves, we do not see a correlation between the optical and high-energy flares. We do not find evidence for spectral evolution throughout the observations. We model the variations in the light curves and find that the most likely cause of the rebrightening episodes is a refreshment of the forward shock preceded by a rapidly fading reverse shock component, although other explanations are plausible.

arXiv:0708.3444 [ps, pdf, other]

Metzger 2007 富含中子成分的外流

主要内容:
如果中心是磁星的话，可以产生富中子的外流。NDAF

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文章信息:
On the Conditions for Neutron-Rich Gamma-Ray Burst Outflows

Authors: Brian D. Metzger, Todd A. Thompson, Eliot Quataert

(Submitted on 24 Aug 2007)

Abstract: We calculate the structure and neutron content of neutrino-heated MHD winds driven from the surface of newly-formed magnetars (``proto-magnetars'') and from the midplane of hyper-accreting disks, two of the possible central engines for gamma-ray bursts (GRBs) and hyper-energetic supernovae (SNe). Both the surface of proto-magnetars and the midplane of neutrino-cooled accretion flows (NDAFs) are electron degenerate and neutron-rich (neutron-to-proton ratio n/p >> 1). If this substantial free neutron excess is preserved to large radii in ultra-relativistic outflows, several important observational consequences may result. Weak interaction processes, however, can drive n/p to ~1 in the nondegenerate regions that obtain just above the surfaces of NDAFs and proto-magnetars. Our calculations show that mildly relativistic neutron-rich outflows from NDAFs are possible in the presence of a strong poloidal magnetic field. However, we find that neutron-rich winds possess a minimum mass-loss rate that likely precludes simultaneously neutron-rich and ultra-relativistic (Lorentz factor > 100) NDAF winds accompanying a substantial accretion power. In contrast, proto-magnetars are capable of producing neutron-rich long-duration GRB outflows ~10-30 seconds following core bounce for sub-millisecond rotation periods; such outflows would, however, accompany only extremely energetic events, in which the GRB + SN energy budget exceeds ~ 4e52 ergs. Neutron-rich highly relativistic outflows may also be produced during some short-duration GRBs by geometrically thick accretion disks formed from compact object mergers. The implications for r-process nucleosynthesis, optical transients due to non-relativistic neutron-rich winds, and Nickel production in proto-magnetar and NDAF winds are also briefly discussed.

arXiv:0708.3395

Tanaka 2007 数值模拟SN 1998bw的不对称爆发

主要内容:
数值模拟不对称爆发的动力学、核合成和辐射转移。

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文章信息:
Multi-Dimensional Simulations for Early Phase Spectra of Aspherical Hypernovae: SN 1998bw and Off-Axis Hypernovae

Authors: Masaomi Tanaka, Keiichi Maeda, Paolo A. Mazzali, Ken'ichi Nomoto

(Submitted on 23 Aug 2007)

Abstract: Early phase optical spectra of aspherical jet-like supernovae (SNe) are presented. We focus on energetic core-collapse SNe, or hypernovae. Based on hydrodynamic and nucleosynthetic models, radiative transfer in SN atmosphere is solved with a multi-dimensional Monte-Carlo radiative transfer code, SAMURAI. Since the luminosity is boosted in the jet direction, the temperature there is higher than in the equatorial plane by ~ 2,000 K. This causes anisotropic ionization in the ejecta. Emergent spectra are different depending on viewing angle, reflecting both aspherical abundance distribution and anisotropic ionization. Spectra computed with an aspherical explosion model with kinetic energy 20 x 10^{51} ergs are compatible with those of the Type Ic SN 1998bw if ~ 10-20% of the synthesized metals are mixed out to higher velocities. The simulations enable us to predict the properties of off-axis hypernovae. Even if an aspherical hypernova explosion is observed from the side, it should show hypernova-like spectra but with some differences in the line velocity, the width of the Fe absorptions and the strength of the Na I line.

arXiv:0708.3242 [ps, pdf, other]