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INTEGRAL and RXTE observations of XTE J1946+274 in outburst

ATEL # 2692; I. Caballero (CEA-AIM Saclay), K. Pottschmidt (CRESST/GSFC/UMBC), E. Bozzo, C. Ferrigno, A. Neronov (ISDC-University of Geneva), A. Santangelo, D.Klochkov, R.Staubert (IAAT), P.Kretschmar (ISOC/ ESAC), J.Wilms, I. Kreykenbohm, F.Fuerst (Dr Karl Remeis Obs./ECAP), G.Schoenherr (AIP), R.Rothschild, S.Suchy (CASS/UCSD)
on 23 Jun 2010; 7:50 UT
Distributed as an Instant Email Notice (Transients)
Password Certification: I. Caballero (isabel.caballero@cea.fr)

Subjects: X-ray, Request for Observations, Binaries, Neutron Stars, Pulsars, Transients
Referred to by ATEL #: 3048, 3077

The Be/X-ray binary XTE J1946+274 is undergoing an X-ray outburst that started on 2010 June 04 and has been observed by Swift and Fermi GBM (ATels #2663, #2677). The source was discovered in 1998 during a giant outburst with ASM/RXTE and BATSE/CGRO (Smith & Takeshima 1998, Wilson et al. 1998), and remained active until 2001 when it started a quiescence phase, interrupted only by the current outburst. A cyclotron line in the X-ray spectrum of the source at ~35 keV was discovered with RXTE during the 1998 outburst (Heindl et al. 2001, ApJ 563, L35). 15.8 s pulsations were observed with CGRO BATSE (Wilson et al. 2003, ApJ 584, 996).

The source has been detected also by INTEGRAL during the recent Cyg X-1 observations on 2010 June 20-21 at a flux of ~135 mCrab (20-40 keV range, ISGRI), while RXTE TOO pointed observations have been performed on June 20 and a monitoring will continue.

A preliminary timing analysis of the ISGRI observation (not corrected for the orbital motion) reveals pulsations with a spin frequency of 63.426[1] mHz or spin period 15.7664[3] s. No spin-up/down is detected during the INTEGRAL observation. This pulse frequency is slightly higher than the value obtained with Fermi GBM (ATel #2677), 63.4155[2] mHz. The ISGRI pulse profile presents a main narrow peak, a broad shoulder, and a sharp minimum, consistent with the pulse profile from the peak of the 1998 outburst (Wilson et al. 2003).

We extracted the INTEGRAL spectrum of the source from NRT data, with a total exposure of ~5.7 ks for JEMX-2 and ~16.4 ks for ISGRI (MJD start 55367.5). The source is detectable up to ~50 keV, with a 20-40 keV flux of ~1.1 e-09 erg cm-2s -1. A powerlaw with a high energy cutoff describes the data accurately, resulting in a χ2red=1.01/36 dof. The cyclotron line detected in the 1998 outburst at ~35 keV (Heindl et al. 2001) is not required in the model. The best fit parameters are Γ=1.30.2, Ecutoff=222 keV, Efold=101 keV (90 % c.l.).

We performed a preliminary analysis of the RXTE observations, using the PCA background model SkyVLE derived from the predicted SAA file and modeling the HEXTE A background from the HEXTE B off-source spectrum. The 3-50 keV spectrum is well described with a powerlaw with a high energy cutoff plus absorption. In both observations residuals are detected around 35 keV in the PCA data, however we verified that the inclusion of an absorption line did not provide a statistically significant improvement in the fit. From the first pointed observation (MJD 55367.12, PCA exposure ~0.82ks) the 20-40 keV flux is ~1.15e-09 erg cm-2 s-1. We obtain NH=5.0+0.7/-0.8e22 cm-2, Γ=1.250.04, Ecutoff=15.8+/-0.5 keV, Efold=13.70.7 keV (90 % conf) and χ2red=1.42/102 dof. From the second RXTE pointed observation (MJD 55367.18, PCA exp. ~0.53ks), the 20-40 keV flux is ~1.10e-09 erg cm-2 s-1, χ2red=1.27/102 dof, NH=5.40.5, Γ=1.240.03, Ecutoff=16.1+0.4/-0.3 keV, Efold=11.80.6 keV (90% c.l.).

Further observations are encouraged.


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