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The ongoing outbursts of Aql X-1 and Ginga 1843+009 as seen with INTEGRAL/IBIS-ISGRI

ATel #1977; Jerome Rodriguez (CEA, France), Jerome Chenevez (National Space Institute, Denmark), Diana Hannikainen (Metsahovi Radio Observatory, Finland), on behalf of a larger collaboration
on 19 Mar 2009; 12:56 UT
Credential Certification: Jerome Rodriguez (jrodriguez@cea.fr)

Subjects: X-ray, Gamma Ray, Neutron Star, Transient

Referred to by ATel #: 2000

Our INTEGRAL monitoring campaign of GRS 1915+105 allows the Aquila region of the sky to be nicely covered with the IBIS telescope. During our last observation, on March 18, 2009 (11h-16h45 UTC), both the LMXB Aql X-1 was clearly detected with the ISGRI camera at 17.5 sigma, 8.6 sigma, 6.6 sigma in the 18-40 keV, 40-80 keV, and 80-150 keV energy ranges respectively. This confirms the renewal activity of Aql X-1 mentioned in ATel #1970.

The 18-300 keV ISGRI spectrum is well fitted by a power law with an index Gamma=2.0±0.2 (Red chi=1.10 for 16 dof). The 20-200 keV flux is 1.22E-9 erg/s/cm2.
Adding a cut-off in any of its forms (highecut*po or cutoffpl), or replacing the simple model by thermal Comptonization (comptt) does not improve the fit. This clearly shows the absence of any cut-off in the energy range considered for the fit.
We remark that the spectral parameters are compatible with those obtained with INTEGRAL during the decay of the 2007 October outburst when a possible transition back to a hard state had been seen(ATel #1229). The flux we obtain here is, however about twice the value of the October 2007 one. The Swift/BAT light curve shows the source may have reached a plateau at hard X-rays.

Aql X-1 was previously detected by INTEGRAL during the Inner Galactic Disk Key Programme between March 10th and 12th. The 20-200 keV spectrum was not well represented by a simple power law (red. chi=2.26 for 6 dof). A cut-off power law provides an acceptable fit (1.7 for 5 dof). The spectral parameters are poorly constrained and we obtain Gamma=1.2 (-0.9 +0.8), Ecut=37 (-18 +117) keV. The 20-200 keV flux is 4.9E-10 erg/s/cm2.
During the latter observation at least one type I X-ray burst was observed by the ISGRI camera on March 11, 2009 at 20h35m31s UTC with a 15-25 keV peak flux >1 Crab (1.e-8 erg/cm2/s). The average persistent bolometric flux prior to the burst is ~1.e-9 erg/cm2/s, which at 5kpc distance (Jonker & Nelemans, 2004), corresponds to a luminosity of 3E36 erg/s.

Although more observations are needed to confirm this behaviour, the value of the spectral index, the flux of the source, the possible plateau at hard X-rays, and the spectral evolution between March 11 and 18 may suggest Aql X-1 is transiting to a softer state.
The source has, however, been seen to undergo several short and faint outbursts in the last years (e.g. Rodriguez, Shaw and Corbel, 2006, A&A, 451, 1045). From the current data we cannot exclude that it has reached the maximum of its outburst and will be decaying in the coming days.

In the same field, the transient HMXB Ginga 1843+009 is also undergoing an outburst (ATels #1973, #1959). The source is clearly detected in our March 18 observation by IBIS/ISGRI at significances 31.7sigma, 14.2 sigma, 4.6 sigma in the 18-40 keV, 40-80 keV, and 80-150 keV ranges respectively. The 18-200 keV ISGRI spectrum is not well fitted with a simple power law (Red. chi=3.14 for 12 dof). Adding a cut-off greatly improves the fit to red. chi=1.16 for 10 dof. We obtain Gamma=1.8 ±0.3, Ecut=39 ±7 keV and Efold=30 (-12 +11) keV. The 20-200 keV flux is 1.3E-09 ergs/cm2/s.

A Comptonized spectrum (comptt) also provides a rather good fit to the spectrum (red. chi=1.37 for 11 dof). We obtain kTe=11.2 (-1.9 +3.4) keV, Tau=3.5 (-1.1 +1.8). We note that the Swift/BAT light curve seems to show that the source is in the decreasing part of its outburst.

INTEGRAL will continue to regularly monitor this field. The short term plan can be obtained at http://www.sciops.esa.int/index.php?project=INTEGRAL&page=iframe&Fcontent=http ://integral.esac.esa.int/isocweb/schedule.html?action=intro
Simultaneous observations at all wavelengths are encouraged.

JC acknowledges support from the Instrument center for Danish Astrophysics