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Significant increase of the X-ray flux and slight softening of the spectrum of Aql X-1

ATel #464; Jerome Rodriguez (CEA Saclay & ISDC Versoix), Ada Paizis (ISDC Versoix & IASF Milano), Marion Cadolle-Bel (CEA Saclay), Diana C. Hannikainen (Observatory of Helsinki) & Simon E. Shaw (Univ. Southampton & ISDC Versoix)
on 15 Apr 2005; 10:41 UT
Credential Certification: Jerome Rodriguez (jrodriguez@cea.fr)

Subjects: X-ray, Gamma Ray, Binary, Black Hole, Neutron Star, Transient, Pulsar

Referred to by ATel #: 491, 660

On April 12th, 2005, 20:49:01 UTC, INTEGRAL performed an observation of GRS 1915+105 as part of a monitoring campaign of this microquasar. During this observation, Aql X-1 has always been in the IBIS/ISGRI partially coded field of view. The IBIS/ISGRI telescope detected the source at a flux level of 141+-1 mCrab in the 18-45 keV band and 101+-3 mCrab in the 45-80 keV energy range (average values over a ~100 ksec observation, assuming a crab-like spectrum). This shows the source fluxes have increased significantly with respect to the values reported by Grebenev et al. ~10 days earlier (54+/-3 mCrab in 18-45 keV, and 64+/-8 mCrab in 45-80 keV, see ATEL #452).

Our observation also shows that Aql X-1 has softened significantly along this new outburst, since it evolved from a hardness ratio (45-80/18-45 keV) HR=1.18 (ATEL #452) to HR=0.71 (current data). We produced and fitted a preliminary (20 ks long) 20-100 keV spectrum of the source. A simple powerlaw gives a poor fit to the data. Adding a cut-off leads to a reduced chi2 of about 1 (9 dof). The power law photon index is 2.2 +/- 0.2, and the energy and the folding of the cut-off are 39 +/- 7 keV and 50 -13+22 keV respectively. The 20-100 keV flux is 1.9 x 10-9 erg cm -2 s-1. The "nu-Fnu" spectrum can be found at http://isdcul3.unige.ch/~rodrigue/images/Aql_Spectemp.gif

We remark that a softening is also seen in RXTE/ASM data between April 2 and 12 (5-12/1.2-3 keV = 2.4 on Apr 2, 5-12/1.2-3 keV = 1.59 on Apr 12). This could indicate that Aql X-1 is undergoing a transition to a soft(er) state, and therefore undergoing one of its major outburst as suggested in previous Atels (#452, #450)

Comparison with the previous mini outburst of the source (in 2004) shows, however, that such a softening was also visible in the RXTE/ASM data of the first stage of the outburst, since the (5-12/1.2-3 keV) ratio had reached values as low as ~1. Unfortunately, Aql X-1 always lies outside the JEM-X field of view, and no information can be obtained from the soft X-rays.

Further follow up observations of the source will help to disantangle between a true outburst or a mini-outburst. Our next observation of GRS 1915+105 with INTEGRAL is currently scheduled to occur between May 12 and May 15.