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INTEGRAL detects renewed activity of Aquila X-1: beginning of a new outburst

ATel #660; J. Rodriguez (CEA Saclay, France), S. E. Shaw (U. Southampton, UK; & ISDC, Geneva, Switzerland),
on 22 Nov 2005; 12:59 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jerome Rodriguez (jrodriguez@cea.fr)

Subjects: X-ray, Gamma Ray, Request for Observations, Black Hole, Neutron Star, Transient

We observed the region around GRS 1915+105 with INTEGRAL between November 20 ~6h40 UTC and November 21 ~11h00 UTC (~100 ks observation). We report a positive detection of transient Atoll source Aquila X-1 with IBIS/ISGRI in all individual pointings in the energy ranges 20-40 and 40-80 keV.

The source is detected in the 20-40 keV, 40-80 keV, 80-150 keV ranges (mosaics) at ~55 mCrab, ~49 mCrab, and ~26 mCrab respectively. Significance maps can be found at http://isdc.unige.ch/~rodrigue/Aql/Mosa.gif The source is unfortunately outside the field of view of JEM-X.

We extracted spectra and light curves from IBIS/ISGRI. The 20-40 and 40-80 keV light curves can be found at http://isdc.unige.ch/~rodrigue/Aql/LiteAqlX1.gif

The ISGRI spectrum is well fitted by a model of a powerlaw modified by a high energy cut-off. The power law photon index is 2.34 +- 0.08, the high energy cut-off 64 +-13 kev and the e-folding 71 -32+64 keV. The photon index is significantly softer than what was observed at the peak of the April 2005 outburst (Rodriguez et al. submitted to A&A). This indicates the source may be undergoing a transition to a softer state unlike in the two previous outbursts. The 20-200 keV flux is 9.7 e-10 erg/cm2/s which is about ~45 % of the ISGRI flux at the maximum of the April 2005 outburst (Atel #464 and Rodriguez et al., submitted to A&A). The preliminary ISGRI spectrum can be found at http://isdc.unige.ch/~rodrigue/Aql/Spec.gif

The RXTE/ASM shows that the source is active in the 2-12 keV range since around November 12 2005 with an increasing soft X-ray flux.

This indicates that Aql X-1 is likely entering a new phase of outburst. This new possible outburst occurs only ~6 months after the last episode of activity (Atel #452, #464, #491). We note that the two last reported outbursts were dubbed "mini"-outbursts due to their low luminosity in soft X-ray.

Further observations at other wavelengths are strongly encouraged to know whether the source is entering one of its major outbursts, to study the mechanisms driving the early phases of outbursts, and possibly unveil the parameter triggering major/mini outbursts.