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INTEGRAL detects an early stage of a new outburst of Aql X-1

ATel #1079; J. Rodriguez (CEA/SAp & AIM, Saclay, France), L. Prat (CEA/SAp & AIM, Saclay, France), S. E. Shaw (U. Southampton, UK), D. C. Hannikainen (Univ. of Helsinki, Finland)
on 21 May 2007; 15:00 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jerome Rodriguez (jrodriguez@cea.fr)

Subjects: Radio, Millimeter, Infra-Red, Optical, X-ray, Gamma Ray, Binary, Neutron Star, Transient

Referred to by ATel #: 1080, 1087, 1216, 1218, 1557, 1970

We monitored the region around GRS 1915+105 with INTEGRAL since March 7, 2007 with one 20 ks observation performed every second revolution (the approximative separation between two consecutive observations is 6 days).
During our last observation performed between May 20, 16h 59 UTC and May 20, 22h45 UTC (INTEGRAL revolution 561) a source, whose position is consistent with that of the transient Atoll Aql X-1 (within 0.5 arcmin between 20 and 40 keV), is clearly detected with ISGRI in the 20-40 and 40-80 keV mosaic images.
This source, we can safely associate with Aql X-1, has respective fluxes of 7.44 cts/s (~43.5 mCrab, SNR=17.2 ?) and 3.01 cts/s (45 mCrab, SNR 8.3 ?) in the 20-40 keV and 40-80 keV energy ranges. Inspection of the last RXTE/ASM pointings of the source (up to May 20 2007) indicates a marginal detection on May 20 ( at 2.9 ?) with a 2-12 keV flux of 32 mCrab.

We extracted an ISGRI spectrum from the real time data. A single power law fits the 22-80 keV spectrum well. The power law photon index is ?=2.2 +-0.3 and the 20-100 keV flux 6.4 e-10 erg/s/cm2. Although the last spectral points are of poor statistical quality they may indicate the presence of a cut-off in the spectrum. Replacing the powerlaw with a powerlaw with a high energy cut-off leads to Gamma= 2.1+-0.3 with a cut off energy>48 keV (the e- folding energy was frozen to 30 keV). With the latter model the 20-200 keV flux is only 30% of the maximum 20-200 keV flux reported for the faint April 2005 outburst (Rodriguez et al. 2006, A&A, 451, 1045). The source is unfortunately outside the field of view of JEM-X so no information on the soft X-rays is available, however the parameters of the spectral fitting, and the marginal detection by the RXTE/ASM are compatible with the source being in a hard spectral state.

Significance maps can be found at http://isdcul3.unige.ch/~rodrigue/Aql/AqlX1_2007.jpg while the 22-80 keV ?-F? spectrum and the cut off power law fit can be found at http://isdcul3.unige.ch/~rodrigue/Aql/Spec_Aql_2007.gif
Inspection of the individual pointings shows the source is detected in 4 out of 6 science windows in the 20-40 keV energy range with a flux varying from 40 to 55 mCrab (when detected), and a 3 sigma upper limit of ~25-30 mCrab when it is not detected.

The source was not detected by ISGRI in our previous observation performed on May 12 (INTEGRAL revolution 559) with a 3-sigma upper limit of about 8.5 mCrab between 20 and 40 keV.
The clear detection during rev. 561 indicates that Aql X-1 is likely to be entering a new episode of outburst. The fact that the source was not detected in our previous observation, and the hard spectral state of the source inferred from our preliminary spectral analysis, indicate that we have caught Aql X-1 in an early stage of the outburst. Further observations at all wavelengths are strongly encouraged to monitor this still poorly known rising phase of outburst of transients source, and possibly catch a spectral transition if the source undergoes a major outburst.