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ATel 10452: SSM on AstroSat detects neutron star X-ray transient, Aql_X-1 in its outburst



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Aquila X-1
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ATel 10441: Detection of a possible new outburst in Aql X-1



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ATel 7134: Optical Observations of the February 2015 Outburst of Aql X-1: the Only Fading Stage



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ATel 6447: X-ray and Optical follow-up of the mid-2014 Outburst of Aql X-1 at peak and at low activity



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Title: Mining the Aql X-1 long term X-ray light curve
Authors: S. Campana (OA Brera), F. Coti Zelati (OA Brera, Universita' di Milano Bicocca) P. D'Avanzo (OA Brera)

Aql X-1 is the prototypical low mass X-ray binary transient. The Rossi X-ray Timing Explorer All Sky Monitor provided a ~16 yr coverage revealing 20 outbursts. This is by far the most extensive legacy of outbursts from the same source. We investigated the outbursts characteristics in terms of energetics, peak luminosities, durations, decays and recurrence times. We found that bright outbursts (peak luminosity >10^{37} erg s^-1) equal in number dimmer outbursts (<10^{36.6} erg s^-1). The peak luminosity does not correlate with outburst energetics, durations or quiescent times. We analysed the latest stages of the outbursts searching for exponential and/or linear decays. Light curve modelling led to constraints on the outer disk radius and enabled us to estimate the viscosity and the irradiation parameters. The former is larger than what has been obtained for other, shorter orbital period, transients, while the latter is somewhat smaller. This might be related to the longer orbital period of Aql X-1 with respect to other transient X-ray binaries.

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Title: Accretion Geometry of the Low-Mass X-ray Binary Aquila X-1 in the Soft and Hard States
Authors: Soki Sakurai, Shin'ya Yamada, Shunsuke Torii, Hirofumi Noda, Kazuhiro Nakazawa, Kazuo Makishima

The neutron-star Low-Mass X-ray Binary Aquila X-1 was observed seven times in total with the Suzaku X-ray observatory from September 28 to October 30 in 2007, in the decaying phase of an outburst. In order to constrain the flux-dependent accretion geometry of this source over wider energy bands than employed in most of previous works, the present study utilized two out of the seven data sets. The 0.8-31 keV spectrum on September 28, taken with the XIS and HXD-PIN for an exposure of 13.8 ks, shows an absorbed 0.8-31 keV flux of 3.6 x 10^{-9} erg s^{-1} cm^{-2}, together with typical characteristics of the soft state of this type of objects. The spectrum was successfully explained by an optically-thick disk emission plus a Comptonised blackbody component. Although these results are in general agreement with previous studies, the significance of a hard tail recently reported using the same data was inconclusive in our analysis. The spectrum acquired on October 9 for an exposure of 19.7 ks was detected over a 0.8-100 keV band with the XIS, HXD-PIN, and HXD-GSO, at an absorbed flux of 8.5 x 10^{-10} erg s^{-1} cm^{-2} (in 0.8-100 keV). It shows characteristics of the hard state, and was successfully explained by the same two continuum components but with rather different parameters including much stronger thermal Comptonisation, of which the seed photon source was identified with blackbody emission from the neutron-star surface. As a result, the accretion flow in the hard state is inferred to take a form of an optically-thick and geometrically-thin disk down to a radius of 21 4 km from the neutron star, and then turn into an optically-thin nearly-spherical hot flow.

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Title: The disc-jet coupling in the neutron star X-ray binary Aquila X-1
Authors: V. Tudose (1,2,3), R.P. Fender (4,2), M. Linares (2), D. Maitra (2), M. van der Klis (2) ((1) ASTRON, (2) Amsterdam, (3) Bucharest, (4) Southampton)

We study the accretion/ejection processes (i.e. disc/jet coupling) in the neutron star X-ray binary Aquila X-1 via a multi-wavelength approach. We use in the radio band the publicly available VLA archive containing observations of the object between 1986-2005, in the X-ray band the archival RXTE data (PCA and HEXTE) between 1997-2008, and in optical (R band) observations with the SMARTS recorded between 1998-2007. In the combined data set we find three outbursts for which quasi-simultaneous radio, optical (R band) and X-ray data exist and focus on them to some extent. We provide evidence that the disc/jet coupling in Aquila X-1 is similar to what has been observed in black hole X-ray binaries, at least from the point of view of the behaviour in the hardness-intensity diagrams (the hysteresis effect included), when the phenomenology of the jet is taken into account. Although based on a very small number of observations, a radio/X-ray correlation seems to exist for this system, with a slope of alpha=0.40 0.07 (F_{radio} \propto F_{X}^{alpha}), which is different than the slope of alpha=1.40 0.25 found for another atoll source, 4U 1728-34, but interestingly enough is relatively close to the values obtained for several black hole X-ray binaries. No significant correlation is found between the radio and optical (R band) emissions. We also report a significant drop in the radio flux from Aql X-1 above an X-ray flux of ~ 5 X 10^{-9} erg cm^{-2} s^{-1}. This behaviour, also reported in the neutron star X-ray binary 4U 1728-34, may be analogous to the suppression of radio emission in black hole X-ray binaries in bright, soft X-ray states. It suggests that from this point of view neutron star X-ray binaries can mimic the behaviour of black hole X-ray binaries in suppressing the jet in soft/disc-dominated X-ray states.

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Outburst of Aql X-1
Aql X-1 is a suspected low mass X-ray Binary.

Magnitude 18
Position(2000): RA 19 11 16.0 Dec +00 35 06

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