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 (firstname.lastname@example.org)
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
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
" 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
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.