Outburst of GX 304–1 monitored with INTEGRAL: positive correlation between the cyclotron line energy and flux

¹ Institut für Astronomie und Astrophysik, Universität Tübingen (IAAT), Sand 1, 72076 Tübingen, Germany
e-mail: klochkov@astro.uni-tuebingen.de
² ISDC Data Center for Astrophysics, University of Geneva, chemin d’Écogia 16, 1290 Versoix, Switzerland
³ European Space Astronomy Centre (ESA/ESAC), Science Operations Department, Villanueva de la Canãda, Madrid, Spain
⁴ CEA Saclay, DSM/IRFU/SAp – UMR AIM (7158) CNRS/CEA/Université P. Diderot, Orme des Merisiers, Bât. 709, 91191 Gif-sur-Yvette, France
⁵ Dr. Karl Remeis Sternwarte & Erlangen Centre for Astroparticle Physics, Sternwartstr. 7, 96049 Bamberg, Germany
⁶ Center for Space Science and Technology, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
⁷ Center for Astrophysics and Space Sciences, University of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0424, USA
⁸ NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
⁹ CRESST & NASA Goddard Space Flight Center, Astrophysics Science Division, Code 661, Greenbelt, MD 20771, USA

Received: 11 April 2012
Accepted: 23 May 2012

Abstract

Context. X-ray spectra of many accreting pulsars exhibit significant variations as a function of flux and thus of mass accretion rate. In some of these pulsars, the centroid energy of the cyclotron line(s), which characterizes the magnetic field strength at the site of the X-ray emission, has been found to vary systematically with flux.

Aims. GX 304−1 is a recently established cyclotron line source with a line energy around 50 keV. Since 2009, the pulsar shows regular outbursts with the peak flux exceeding one Crab. We analyze the INTEGRAL observations of the source during its outburst in January − February 2012.

Methods. The observations covered almost the entire outburst, allowing us to measure the source’s broad-band X-ray spectrum at different flux levels. We report on the variations in the spectral parameters with luminosity and focus on the variations in the cyclotron line.

Results. The centroid energy of the line is found to be positively correlated with the luminosity. We interpret this result as a manifestation of the local sub-Eddington (sub-critical) accretion regime operating in the source.