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arXiv 1012.4009astro-ph.HE
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短GRB 100117 A的光学余辉和z=0.92早型宿主星系

The Optical Afterglow and z=0.92 Early-type Host Galaxy of the Short GRB 100117A

Wen-fai Fong, Edo Berger, Ryan Chornock, Nial R. Tanvir, Andrew J. Levan, John F. Graham, Andrew S. Fruchter, Antonino Cucchiara, Derek B. Fox

摘要:

本文报道了短持续时间GRB 100117 A的光学余辉和早型宿主星系的发现。在爆发后8.3小时检测到微弱的余辉,r_AB = 25.46 +/- 0.20 mag.后续的光学和近红外观测发现了一个符合的紧凑红色星系,被确定为一个早期类型星系,光度红移为z~0.6-0.9(2-sigma),质量为3x10^10 M_Sun,年龄为~1 Gyr,光度L_B~ 0.5L_星。光谱观测显示在z~0.9处有一个显著的对应于Balmer 4000埃的突变,而恒星星族光谱演化模板拟合显示z~0.915,我们采用它作为宿主的红移,恒星星族年龄为~1-3 Gyr。从[OII]-3727在z=0.915处的微弱辐射中,我们推断出恒星形成率的上限约为0.1 M太阳/年,从而导致比恒星形成率<0.004 Gyr。因此,GRB 100117 A是迄今为止第二个拥有安全早型宿主的短暴(另一个是z=0.257的GRB 050724),它具有最高的短GRB红移之一。主机中心和突发位置之间的偏移为470 +/- 310 pc,是迄今为止最小的。结合旧的恒星星族年龄,这表明爆发可能起源于一个没有明显反冲速度的祖先。然而,根据光学余辉的亮度,我们推断出当λ_e和λ_B = 0.1时,其密度相对较低,约为3 × 10 ^-4 cm^-3。光学微弱的余辉和宿主的结合表明,以前的这种事件可能已经错过了,从而可能使已知的短伽玛射线暴宿主种群与z>1的早型宿主发生偏差。

英文摘要:

We present the discovery of the optical afterglow and early-type host galaxy of the short-duration GRB 100117A. The faint afterglow is detected 8.3 hr after the burst with r_AB = 25.46 +/- 0.20 mag. Follow-up optical and near-IR observations uncover a coincident compact red galaxy, identified as an early-type galaxy at a photometric redshift of z~0.6-0.9 (2-sigma) with a mass of 3x10^10 M_Sun, an age of ~1 Gyr, and a luminosity of L_B~0.5L_star. Spectroscopic observations of the host reveal a notable break corresponding to the Balmer 4000-Angstrom break at z~0.9, and stellar population spectral evolution template fits indicate z~0.915, which we adopt as the redshift of the host, with stellar population ages of ~1-3 Gyr. From a possible weak detection of [OII]-3727 emission at z=0.915 we infer an upper bound on the star formation rate of ~0.1 M_Sun per yr, leading to a specific star formation rate of <0.004 per Gyr. Thus, GRB 100117A is only the second short burst to date with a secure early-type host (the other being GRB 050724 at z=0.257) and it has one of the highest short GRB redshifts. The offset between the host center and the burst position, 470 +/- 310 pc, is the smallest to date. Combined with the old stellar population age, this indicates that the burst likely originated from a progenitor with no significant kick velocity. However, from the brightness of the optical afterglow we infer a relatively low density of n~3x10^-4 cm^-3 when epsilon_e and epsilon_B = 0.1. The combination of an optically faint afterglow and host suggest that previous such events may have been missed, thereby potentially biasing the known short GRB host population against z>1 early-type hosts.

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