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arXiv 0704.0543astro-ph
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Swift/XRT观测到周期性超巨星快速X射线瞬变IGR J11215-5952的第五次爆发

Swift/XRT observes the fifth outburst of the periodic Supergiant Fast X-ray Transient IGR J11215-5952

P. Romano, L. Sidoli, V. Mangano, S. Mereghetti, G. Cusumano

摘要:

IGR J11215-5952是一个硬X射线瞬变源,于2005年4月被INTEGRAL发现,并被确认为新的高质量X射线双星,即超巨星快X射线瞬变(SFXTs)的成员。存档的INTEGRAL数据和RXTE观测表明,爆发的周期约为330天。因此,IGR J11215-5952是第一个显示周期性爆发的SFXT,可能与轨道周期有关。我们用雨燕进行了一次机会目标观测,主要目的是监测第五次爆发时的源行为,预计在2007年2月9日。从2007年2月4日开始,每天用雨燕观测源场两次(2ks/天),直到第五次爆发,然后每天观测约5 ks,在持续23天的监测活动期间,源上总暴露量约为73 ks。这是对SFXT爆发的最完整的监测活动。最亮耀斑期间的光谱可以用吸收功率定律很好地描述,光子指数为1,N_H~1 10^22 cm^-2。在1-10 keV的能量范围内,得到的峰值亮度约为10^36 erg·s^-1(假设光学对应物的距离为6.2 kpc)。由于灵敏度和时间覆盖范围的结合,这些Swift观测是SFXT爆发的独特数据集,并且它们允许对IGR J11215-5952从爆发开始到几乎静止的研究。我们发现,吸积阶段持续时间比以前认为的低灵敏度仪器的基础上,只观察到最明亮的耀斑。所观察到的现象是一致的,在广泛的偏心轨道与许多耀斑叠加,可能是由于情节或不均匀的吸积在近星点通道触发的流量平稳增加。

英文摘要:

IGR J11215-5952 is a hard X-ray transient source discovered in April 2005 with INTEGRAL and a confirmed member of the new class of High Mass X-ray Binaries, the Supergiant Fast X-ray Transients (SFXTs). Archival INTEGRAL data and RXTE observations showed that the outbursts occur with a periodicity of ~330 days. Thus, IGR J11215-5952 is the first SFXT displaying periodic outbursts, possibly related to the orbital period. We performed a Target of Opportunity observation with Swift with the main aim of monitoring the source behaviour around the time of the fifth outburst, expected on 2007 Feb 9. The source field was observed with Swift twice a day (2ks/day) starting from 4th February, 2007, until the fifth outburst, and then for ~5 ks a day afterwards, during a monitoring campaign that lasted 23 days for a total on-source exposure of ~73 ks. This is the most complete monitoring campaign of an outburst from a SFXT. The spectrum during the brightest flares is well described by an absorbed power law with a photon index of 1 and N_H~1 10^22 cm^-2. A 1-10 keV peak luminosity of ~10^36 erg s^-1 was derived (assuming 6.2 kpc, the distance of the optical counterpart). These Swift observations are a unique data-set for an outburst of a SFXT, thanks to the combination of sensitivity and time coverage, and they allowed a study of IGR J11215-5952 from outburst onset to almost quiescence. We find that the accretion phase lasts longer than previously thought on the basis of lower sensitivity instruments observing only the brightest flares. The observed phenomenology is consistent with a smoothly increasing flux triggered at the periastron passage in a wide eccentric orbit with many flares superimposed, possibly due to episodic or inhomogeneous accretion.

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