4U 1909+07: a well-hidden pearl

¹ Dr. Karl Remeis-Sternwarte & ECAP, Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
e-mail: felix.fuerst@sternwarte.uni-erlangen.de
² Center for Astrophysics & Space Sciences, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093, USA
³ CRESST and NASA Goddard Space Flight Center, Astrophysics Science Division, Code 661, Greenbelt, MD 20771, USA
⁴ Center for Space Science and Technology, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA

Received: 25 August 2010
Accepted: 23 October 2010

Abstract

We present the first detailed spectral and timing analysis of the high mass X-ray binary (HMXB) 4U 1909+07 with INTEGRAL and RXTE. 4U 1909+07 is detected in the ISGRI 20–40 keV energy band with an average countrate of 2.6 cts s^-1. The pulse period of ~604 s is not stable, but changing erratically on timescales of years. The pulse profile is strongly energy dependent: it shows a double peaked structure at low energies, the secondary pulse decreases rapidly with increasing energy and above 20 keV only the primary pulse is visible. This evolution is consistent between PCA, HEXTE, and ISGRI. The phase averaged spectrum can be well described by the sum of a photoabsorbed power law with a cutoff at high energies and a blackbody component. To investigate the pulse profile, we performed phase resolved spectral analysis. We find that the changing spectrum can be best described with a variation of the folding energy. We rule out a correlation between the black body component and the continuum variation and discuss possible accretion geometries.