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Aql X-1 is undergoing transition from the hard state to the soft state

ATel #5136; Wenda Zhang and Wenfei Yu (Shanghai Astronomical Observatory)
on 16 Jun 2013; 20:56 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Wenfei Yu (wenfei@shao.ac.cn)

Subjects: X-ray, Request for Observations, Binary, Neutron Star, Transient

Referred to by ATel #: 5148, 5158

The neutron star system Aql X-1 has shown renewed activity in optical (Atel #5114, Atel #5129) and X-ray (Atel #5117) band, indicating it undergoes a new outburst. The BAT/15-50 keV intensity decreased sharply from 0.20+/-0.01 Crab on 2013 May 14 UT (MJD 56457) to 0.11+/-0.01 Crab on 2013 May 15 UT (MJD 56458), while the MAXI/2-10 keV intensity (calculated from MAXI/2-4 keV and MAXI/4-10 keV intensity) remains increasing, from 0.15+/-0.01 Crab on MJD 56457 to 0.27+/-0.01 Crab on MJD 56458. Along with the sharp decline of the hard X-ray intensity, the BAT(15-50 keV)/MAXI(2-10 keV) hardness ratio decreased from 1.38+/-0.06 on MJD 56457 to 0.39+/-0.03 on MJD 56458. The hardness on MJD 56458 outside the range corresponding to the hard state. We therefore conclude that the source has left the low hard state and is heading to the soft state.

A series of Swift monitoring observations was performed to cover the hard-to-soft state transition. The XRT count rate was 44.19+/-0.17 c/s on MJD 56456.8, increased to 73.64+/-0.21 c/s on MJD 56457.5, and then increased rapidly to 211.45+/-0.38 c/s on MJD 56458.9. However, the XRT 2-10 keV/0.3-2 keV hardness did not change much: 1.11+-/0.01 on MJD 56456.8, 1.07+/-0.01 on MJD 56457.5 and 1.10+/-0.00 on MJD 56458.9, in contrast to the rapid decrease of BAT(15-50 keV)/ MAXI(2-10 keV) hardness from MJD 56457 to MJD 56458.

From XRT timing analysis, we found the 0.3-10 keV fractional rms decreased from 0.22+/-0.01 on MJD 56456.8 to 0.17+/-0.01 on MJD 56457.5, then further decreased rapidly to 0.03+/-0.02 on MJD 56458.9. This rapid decrease is associated with the spectral state transition. The evolution of the variability during the state transition is dependent of photon energy. On MJD 56456.8 the fraction rms of the soft (0.3-2 keV) and the hard (2-10 keV) energy band was 0.24+/-0.02 and 0.23+/-0.02, respectively. However, on MJD 56457.5, the soft band rms decreased to 0.13+/-0.02 while the hard band rms remained as 0.22+/-0.01, indicating the soft band rms decreased earlier than that in the hard band. We fit all spectra with wabs*(diskbb+powerlaw) model and found the disk blackbody temperature was 0.14+/-0.01 keV on both MJD 56456.8 and MJD 56457.5, and increased to 1.52+/-0.09 keV on MJD 56458.9, along with the state transition indicated by BAT/MAXI monitoring and the decrease of the rms in soft band. Meanwhile, the photon index of the power-law component did not change much, at 1.65+/-0.04, 1.57+/-0.02 and 1.61+/-0.23 on MJD 56456.8, 56457.5 and 56458.9, respectively.

Since the source is undergoing a state transition, multi-wavelength observations are encouraged. We thank the Swift team for prompt scheduling of ToO observations which covered the hard-to-soft transition nicely, and both MAXI and Swift teams for providing regular monitoring of the source.