4U 1626–67 as seen by Suzaku before and after the 2008 torque reversal

¹ Universities Space Research Association (USRA), 6767 Old Madison Pike, Suite 450, Huntsville, AL 35806, USA
e-mail: camero@ice.csic.es
² NASA Goddard Space Flight Center, Astrophysics Science Division, Code 661, Greenbelt, MD 20771, USA
³ CRESST & University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
⁴ Space Science Office, ZP12, NASA/Marshall Space Flight Center, Huntsville, Alabama 35812, USA
⁵ Pulkovo Observatory, 196140 St. Petersburg, Russia

Received: 23 May 2012
Accepted: 2 September 2012

Abstract

Aims. The accretion-powered pulsar 4U 1626–67 experienced a new torque reversal at the beginning of 2008, after about 18 years of steadily spinning down. The main goal of the present work is to study this recent torque reversal that occurred in February 2008.

Methods. We present a spectral analysis of this source using two pointed observations performed by Suzaku in March 2006 and in September 2010.

Results. We confirm with Suzaku the presence of a strong emission-line complex centered on 1 keV, with the strongest line being the hydrogen-like Ne Lyα at 1.025(3) keV. We were able to resolve this complex with up to seven emission lines. A dramatic increase in the intensity of the Ne Lyα line after the 2008 torque reversal occurred, with the equivalent width of this line reaching almost the same value measured by ASCA in 1993. We also report on the detection of a cyclotron line feature centered at  ~37 keV. In spite of the increase in the X-ray luminosity (0.5−100 keV) of a factor of  ~2.8 that occurred between these two observations, no significant change in the energy of the cyclotron line feature was observed. However, the intensity of the  ~1 keV line complex increased by an overall factor of  ~8.

Conclusions. Our results favor a scenario in which the neutron star in 4U 1626–67 accretes material from a geometrically thin disk during both the spin-up and spin-down phases.