Research Note

Possible hard X-ray shortages in bursts from KS 1731-260 and 4U 1705-44

¹ Laboratory for Particle Astrophysics, Institute of High Energy Physics, 100049 Beijing, PR China
e-mail: jilong@ihep.ac.cn; szhang@ihep.ac.cn
² European Space Astronomy Centre (ESA/ESAC), Science Operations Department, Villanueva de la Cañada, 28691 Madrid, Spain
³ Theoretical Physics Center for Science Facilities (TPCSF), CAS, 19(B) Yuquan Road, 100049 Beijing, PR China

Received: 5 November 2013
Accepted: 16 January 2014

Abstract

Aims. A hard X-ray shortage, implying the cooling of the corona, was observed during bursts of IGR J17473-272, 4U 1636-536, Aql X-1, and GS 1826-238. Apart from these four sources, we investigate here an atoll sample, in which the number of bursts for each source is larger than 5, to explore the possible additional hard X-ray shortage during Rossi X-ray timing explorer (RXTE) era.

Methods. According to the source catalog that shows type-I bursts, we analyzed all the available pointing observations of these sources carried out by the RXTE proportional counter array. We grouped and combined the bursts according to their outburst states and searched for the possible hard X-ray shortage while bursting.

Results. We found that the island states of KS 1731-260 and 4U 1705-44 show a hard X-ray shortage at significant levels of 4.5 and 4.7σ and a systematic time lag of 0.9 ± 2.1 s and 2.5 ± 2.0 s with respect to the soft X-rays, respectively. While in their banana branches and other sources, we did not find any consistent shortage.