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ATEL # 1087; S. Campana (OA Brera)
on 30 May 2007; 17:27 UT
Password Certification: Sergio Campana (email@example.com)
Subjects: X-ray, Binaries, Neutron Stars, Transients
The neutron star X-ray transient Aql X-1 has been reported to start a new outburst in ATEL \#1079 and ATEL \#1080. We analyzed two Swift observations of Aql X-1 carried out on May 28 (Photon Counting mode, 1828 s) and May 29 (Window Timing mode, 164 s). We fit together the two spectra finding a slightly fading X-ray source (about 20%) and a power law photon index moving from 1.58+/-0.05 to 1.83+/-0.08 (90% confidence level). The column density was held fixed to the best fit value of (3.9+/-0.3)x10^21 cm^-2. The unabsorbed 0.5-10 keV luminosity is (2-3)x10^36 erg s^-1 (at 4 kpc). Inspection of RXTE-ASM and Swift-BAT data do not provide a clear indication if the source is still rising to the outburst peak or it is in its decaying phase. Further optical and X-ray observations are encouraged.