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INTEGRAL detects Aql X-1 in outburst in a hard state

ATEL # 2299; J. Rodriguez (CEA Saclay, France), J. Chenevez (DSRI, Denmark), M. Cadolle Bel (ESA/ESAC, Spain), J. Wilms (Remeis Obs. and ECAP, Germany), E. Kuulkers (ESA/ESAC, Spain), D. Hannikainen (Metsahövi observatory, Finland)
on 16 Nov 2009; 14:42 UT
Password Certification: Jerome Rodriguez (jrodriguez@cea.fr)

Subjects: X-ray, Gamma Ray, Binaries, Black Holes, Neutron Stars, Transients

The neutron star X-ray binary/microquasar Aql X-1 has entered a new outburst (ATel #2288 and #2296).
In the framework of our INTEGRAL monitoring campaign on GRS 1915+105 (see e.g. Rodriguez et al. 2008, ApJ, 675, 1436) we have recently observed the Aquila region on three occasions.
The first set was obtained during the INTEGRAL revolution 861 between 2009-10-31:18h55 UTC and 2009-11-01:00h54 UTC. Aql X-1 is not detected in any of the energy ranges, with a rough 3-sigma 20-40 keV upper limit of about 1.06 cts/s (6.7 mCrab).
The second set was obtained during the INTEGRAL revolution 863 between 2009-11-07:13h25 UTC and 2009-11-07:19h10 UTC. Aql X-1 is detected with IBIS/ISGRI only in the 20-40 keV energy range with an intensity of 2.8 cts/s (~18 mCrab, 6.7 sigma).
The third set was obtained during the INTEGRAL revolution 865, between 2009-11-13:14h04 UTC and 2009-11-13:19h45 UTC. Aql X-1 is clearly detected with IBIS/ISGRI in all energy ranges with intensities of 13.3 cts/s (83 mCrab, 35 sigma), 6.25 cts/s (78 mCrab, 19.8 sigma), 1.82 cts/s (63 mCrab, 6.9 sigma) respectively in the 20-40, 40-80 and 80-150 keV energy ranges.
This therefore confirms that Aql X-1 is undergoing an outburst and that its hard X-ray flux has increased at least up to Nov. 13. The detection up to 150 keV and the fluxes suggest that the source was in a hard state during our last INTEGRAL observation.

Looking at the last RXTE and SWIFT observations, we however remark that the source may still be brightening at soft X-rays with a RXTE/ASM 1.2-12 keV flux slightly above 80 mCrab on November 16 (http://heasarc.gsfc.nasa.gov/cgi-bin/xte_weather/xte_lc?source=AQLX1), while the SWIFT/BAT 18-50 keV flux (http://heasarc.gsfc.nasa.gov/docs/swift/results/transients/AqlX-1/) showed some decrease between MJD 55149 (Nov 14, 97 mCrab) and MJD 55151 (Nov. 16, 70 mCrab). This may indicate that a spectral transition to a softer state is ongoing.

More observations at other wavelengths are encouraged.

We acknowledge the ISDC for promptly sending us the INTEGRAL near real time data. We also thank the NASA/HEASARC and associated institutes for the availability of the RXTE and SWIFT data.


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R. E. Rutledge , Editor
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