Swift/XRT detection of a renewed activity in the transient neutron star X-ray binary Aql X-1

ATel #8095; A. Meshcheryakov (IKI, Moscow, Russia), S. Tsygankov (Tuorla Observatory, Finland), I. Khamitov (TUG, Antalya, Turkey), I. Bikmaev (KFU/AST, Kazan, Russia), R. Burenin (IKI, Moscow, Russia), M. Eselevich (ISTP, Irkutsk, Russia), V. Vlasyuk (SAO, Nizhnij Arkhyz, Russia)
on 25 Sep 2015; 13:44 UT
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Credential Certification: Alexander Meshcheryakov (mesch@iki.rssi.ru)

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During X-ray monitoring campaign of a Neutron Star Soft X-ray Transient (NS SXT) Aql X-1 performed at Swift/XRT telescope, we discover a substantial brightening of the source in the observation carried out 2015-09-24 (started 15:39:00 UT) in comparison to previous observation 2015-09-20 (19:14:00 UT). Both observations 2015-09-24 and 2015-09-20 were taken with the same total exposure time 1.0ks. The source is clearly seen in both observations, but substantially brighter in the later. We take Swift/XRT snapshots of Aql X-1 about every 2 days since 2015-07-01 (see XRT lightcurve of AqlX-1 http://hea.iki.rssi.ru/~mesch/aqlx-1/aqlx-1_lc_quick_obs_20150925.gif ). XRT instrument was operated in the photon counting mode. The X-ray spectrum of the source for observation 2015-09-24 can be described by an absorbed power-law model with a hydrogen column density of N_H = (9.0 +/-3.0 )E21 cm-2 and a photon index of 2.7 +/-0.5. The inferred 0.3-10 keV unabsorbed flux is Fx=~2.75E-11 erg/cm2/s, which translates into an isotropic luminosity of Lx=~8.2e+34 erg/s for a distance of 5 kpc. This is about 14 times above the quiescent level of the source, as it was measured in the observation 2015-09-20. Swift/XRT discover a renewed accretion activity in Aql X-1. This activity is currently not seen by on-orbit All-Sky Monitors (MAXI, BAT). Further multiwavelength observations of Aql X-1 during ongoing accretion activity in X-ray,UV,Optical,IR and Radio bands are highly encouraged. We thank Neil Gehrels and the Swift science team for making these observations possible. Analysis of the XRT data was performed utilizing the methods described by Evans et al. (2009, MNRAS, 397, 1177) and Evans et al. (2007, A&A, 469, 379).