A Swift/XRT observation of Aql X-1 during its recent outburst

ATel #871; R. Wijnands (University of Amsterdam), D. Maitra, C. Bailyn (Yale University), M. Linares (University of Amsterdam)
on 1 Aug 2006; 14:48 UT
Credential Certification: Rudy Wijnands (rudy@space.mit.edu)

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We report on an observation performed with the Swift/XRT of the neutron-star X-ray transient Aql X-1 which was recently found to start a new outburst as inferred from an increase in source luminosity in the R and J band data (ATEL #869, #870). The Swift/XRT observation was performed between 31 July 2006 21:47 and 22:05 UT with a duration of 996 seconds. The XRT was in the windowed timing mode. No type-I X-ray bursts were seen during the observation.

We extract the XRT spectrum from the data and found that it could satisfactorily be fitted (delta chi^2 of 1.19 for 318 degree of freedom) by a simple absorbed power law with a column density of 3.6+-0.2 E21 cm^-2 and a photon index of 2.56+-0.06. The unabsorbed 2-10 keV and 0.5-10 keV fluxes were 2.71+-0.07 and 7.9+-0.3 erg/s/cm^2, respectively.

A simple thermal spectral shape (black-body or disk black-body model) could not satisfactorily fit the data. However, the relatively large photon index obtained when using a simple power-law model might suggest that a soft, thermal component is present in addition to the power-law component (albeit not required by the data since a simple power-law model can fit the data adequately). Therefore, we also fitted the data using a combination of a black-body or disk black-body component with a power-law component.

We found for the black body plus power law combination an column density of 3.3+-0.4 E21 cm^-2, a kT of 0.25+-0.03 keV and a photon index of 2.2+-0.1. The unabsorbed 2-10 keV and 0.5-10 keV fluxes were 2.82+-0.06 and 7.4+-0.6 erg/s/cm^2, respectively. For the disk black body plus power law combination, we found an column density of 3.7+-0.4 E21 cm^-2, a kT of 0.32+-0.05 keV and a photon index of 2.2+-0.1. The unabsorbed 2-10 keV and 0.5-10 keV fluxes were 2.83+-0.05 and 7.8+-0.6 erg/s/cm^2, respectively. In both cases the soft component contributed about 20% to the 0.5-10 keV unabsorbed flux.

For a distance of approximately 5 kpc (see, Jonker & Nelemans 2004, MNRAS 354, 355), the averaged 2-10 keV and 0.5-10 keV fluxes result in associated luminosities of 0.8 and 2.3 E36 erg/s, respectively. These luminosities combined with the spectral shape strongly suggest that the source is already at a relatively high mass accretion rate (i.e., in the so-called island state of low-luminosity neutron star X-ray binaries) only several days after it first was detected in outburst in the J and R bands.