SAX J1810.8-2609 displays increasing hard X-ray activity

Additional material for ATel 1227:
SAX J1810.8-2609 displays increasing hard X-ray activity

by R. Galis, V. Beckmann (APC), J. Chenevez, S. Brandt (DNSC), G. Belanger, E. Kuulkers, M. Cadolle Bel, C. Sanchez-Fernandez (ESA/ESAC), A. Bazzano, I. Donnarumma, M. Fiocchi, L. Natalucci (INAF/IASF-Roma), D. Götz (CEA/Saclay), W. Hermsen (SRON), J.-C. Leyder (IAG Liege), S. Piraino (IAAT), K. Pottschmidt (UMBC/GSFC), N. Shaposhnikov (NASA/GSFC), A. Paizis, L. Sidoli (INAF/IASF-Milano), J. Tomsick (UC Berkeley), R. Walter (ISDC), and K. Watanabe (FGCU).

Subjects: X-ray, Gamma Ray, Request for Observations, Binaries, Neutron Stars, Transients

The neutron-star LMXB SAX J1810.8-2609 has been frequently observed by INTEGRAL over the last weeks. After the onset of hard X-ray activity as seen by Swift on Aug. 6-9 (ATel#1175), and by INTEGRAL on Aug. 19 (ATel#1185), the source was covered by the Galactic Bulge Monitoring Programme (Kuulkers et al. 2007, A&A 466, 595) and in the INTEGRAL Key Programme of the Galactic Center. The light curve of the last 2 weeks shows a gradual brightening, which peaked on 2007-09-21T06:01 UTC with a source flux of about 83 mCrab and 60 mCrab in the 20-40 keV and 40-80 keV band, respectively. In addition, the JEM-X1 lightcurve shows a type I X-ray burst lasting about 40s on 2007-09-24T19:53:06 with a peak flux of 1.3 ± 0.2 Crab and 1.1 ± 0.3 Crab in the 3-10 keV and 10-20 keV band, respectively (see Figure 3 below).
Analysis of IBIS/ISGRI data using INTEGRAL's latest Offline Standard Analysis package OSA 7, shows that the spectrum above 18 keV is best represented by a Comptonization model following Titarchuk 1994 (compTT), rather than by a single power law, except for the short observation on September 21, where a single power law is sufficient. As the compTT model is not well constrained in some cases, we fix the electron seed temperature to T0 = 1 keV in all cases and fit the plasma temperature (kT) which we report below. In addition, we apply a single power law model with photon index Γ in order to give a hint for the spectral slope evolution.

The spectral evolution over the last weeks appears as follows (flux in 10-10 erg cm-2 s-1):
revolution UT start exp. time [ks] flux (20-40 keV) kT [keV] Γ
600 2007-09-12T01:25 185 2.2 ± 0.3 31 ± 9 2.11 ± 0.04
601 2007-09-15T01:09 173 3.5 ± 0.4 26 ± 4 2.17 ± 0.03
602 2007-09-18T05:01 11 3.4 ± 0.9 14 ± 3 2.1 ± 0.1
603 2007-09-21T04:10 12 6.0 ± 0.5 50* 2.3 ± 0.1
604 2007-09-24T00:34 203 3.6 ± 0.3 49 ± 34 2.44 ± 0.03
* not constrained

This is the longest lasting and brightest outburst of this source observed so far by INTEGRAL. INTEGRAL continues to observe the Galactic Centre region since September 30, 2007.

Find below images and spectra extracted from IBIS/ISGRI and the light curve of the type I X-ray burst observed by JEM-X. A poster summarizing the results will be presented at the INTEGRAL Science Workshop in Sardinia

ISGRI 20-40 keV

Figure 1: INTEGRAL IBIS/ISGRI 20-40 keV significance map of the area around SAX J1810.8-2609, 2007-09-15T11:37 - 2007-09-17T13:31 (revolution 601). SAX J1810.8-2609 is covered during 170 ks of ISGRI observation time. The significance of the detection in the 15-80 keV band is 79 sigma, with a flux of about 42 mCrab.


IBIS/ISGRI spectrum of SAX J1810.8-2609

Figure 2: Spectral plot, using INTEGRAL IBIS/ISGRI data of SAX J1810.8-2609 in revolution 604.


JEM-X1 lightcurve of type I X-ray burst provided by J. Chenevez and S. Brandt (DNSC)

Figure 3: Lightcurve of the type I X-ray burst observed by INTEGRAL/JEM-X1. The time of the peak corresponds to 2007-09-24T19:53:06 when SAX J1810.8-2609 reached a flux of 1.3 ± 0.2 Crab (3-10 keV; shown in black) and 1.1 ± 0.3 Crab (10-20 keV; shown in red).

Last update: October 5th, 2007

In case of questions and comments: contact me at the APC.