Results of INTEGRAL TOO observation of the transient X-ray burster XTE J1810-189

arXiv:1004.4086v1 [astro-ph.HE] 23 Apr 2010 Results of INTEGRAL TOO observation of the transient X-ray burster XTE J1810-189 I.V. Chelovekov∗ Space Research Institute, Russian Academy of Sciences, Moscow, Russia E-mail: chelovekov@iki.rssi.ru S.A. Grebenev Space Research Institute, Russian Academy of Sciences, Moscow, Russia E-mail: grebenev@iki.rssi.ru We report results of the INTEGRAL Target of Opportunity observations of the transient X-ray burster XTE J1810-189. The observations were performed on April 3–6, 2008, soon after the discovery of the source and near the peak of its outburst. That time the source had a flux of about 50 mCrab and exhibited a hard Comptonized X-ray spectrum extending well above 100 keV. Being approximated by a power law with an exponetial cut-off in the broad 3–100 keV energy band it gave the average photon index Γ ≃1.6 and kTcuto f f ≃67 keV. We found only slight indications for changes in the index during the observation (Γ first steady decreased from ∼2.0 to ∼1.3 and then increased back to ∼2.0). However the NH value measured by absorption in the low energy part of the spectrum changed drastically and very irregularly (from ∼4 × 1022 till ∼100×1022 cm−2). There were 10 type I X-ray bursts detected from the source during these TOO observations. Assuming that the Eddington luminosity was reached during the burst with the highest peak flux we get an upper estimate for a distance to the source D = 6.4 ± 0.6 kpc. From the X-ray burst parameters we conclude that this LMXB harboures an evolved star. The Extreme sky: Sampling the Universe above 10 keV - extremesky2009, October 13-17, 2009 Otranto (Lecce) Italy ∗Speaker. c ⃝Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/ TOO observation of XTE J1810-189 I.V. Chelovekov 1. Introduction The X-ray transient XTE J1810-189 was discovered in the spring of 2008 during RXTE/PCA monitoring scans of the Galactic ridge region [7]. A pointed observation started on March 10, at 21:05 UTC revealed a variable source (30% r.m.s. fluctuations). The emission spectrum was consistent with an absorbed power law (NH ≃1 × 1022 cm−2, photon index Γ ≃1.9). The PCA flux history suggested its gradual rise since March 5. The source was observed on March 12-15 with INTEGRAL that measured a slightly steeper spectrum Γ = 2.26±0.12 in the hard > 20 keV IBIS/ISGRI energy band [9]. The observation of XTE J1810-189 on March 17, 2008 with the Swift/XRT telescope revealed the similar spectrum but the higher absorption NH = (4.2± 0.7) × 1022 cm−2 indicating that there might be an internal source of absorption in the system [4]. In a pointed observation of the source on March 26, at 12:47 UTC the RXTE/PCA detected a type I X-ray burst identifying a compact object in the system as a neutron star. Assuming the Eddington peak luminosity, the upper limit for a distance to the source was obtained D<∼11.5 kpc [8]. In this paper we report the results of TOO (Target of Opportunity) observations of XTE J1810- 189 performed with INTEGRAL on April 3-6, 2008. 2. Observations and data analysis Based on the detection of the outburst from XTE J1810-189 in 2008 March and its rising X- ray brightness it was decided to perform a TOO observation of the source with INTEGRAL. All together there were 57 individual pointing observations (43 in revolution 668 and 14 in revolution 669), each lasting 1–1.5 hours. The total exposure during this TOO observation was over 203 ks. The data set used in this research was obtained by the ISGRI (an upper detector of the IBIS telescope) and by the Module 1 of the JEM-X telescope aboard INTEGRAL. For detailed descrip- tion of the IBIS/ISGRI and JEM-X telescopes see [10, 5] and [6] respectively. Data reduction was performed using the OSA 7.0 software, distributed by the Integral Science Data Center (ISDC). In order to search for X-ray bursts we analyzed the JEM-X (3–20 keV) and IBIS/ISGRI (15–25 keV) detector light curves (count rate histories), using all the detector events, irrespective to their incidence directions. See [2] for more detailed description of the technique. The spectral fitting was performed with Xspec package, version 11.3.1. 3. Results Fig. 1 (left) shows an extended ASM light curve of XTE J1810-189 observations (2–10 keV, top panel) note that 70 counts/s = 1 Crab ≃2.2× 10−8 erg s−1cm−2. Each point of the top curve represents a 50 days averaged ASM count rate measured from XTEJ1810-189. Here we omitted all the bins, containing less then 15 observing days (≤30% fill up). The middle panel shows the same light curve with better resolution (each point corresponds to a 1 day averaged ASM count rate) and includes some history of the XTE J1810-189 discovery and observations. Note that the light curve may be contaminated by X-ray bursts represented by single point excesses (e.g. 54585), while count rate excesses consisting of several points (e.g. 54560-54570 MJD)

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