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

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 2008 February. Methods. We present a spectral analysis of this source using two pointed observations performed by Suzaku in 2006 March and in 2010 September. 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 Ly-alpha at 1.025(3) keV. We were able to resolve this complex with up to seven emission lines. A dramatic increase of the intensity of the Ne Ly-alpha 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 fact that an increase of the X-ray luminosity (0.5-100 keV) of a factor of ~2.8 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.Comment: 7 pages, 5 figures and 2 tables. Accepted in A&

Earth Occultation Imaging of the Low Energy Gamma-Ray Sky with GBM

The Earth Occultation Technique (EOT) has been applied to Fermi's Gamma-ray Burst Monitor (GBM) to perform all-sky monitoring for a predetermined catalog of hard X-ray/soft gamma-ray sources. In order to search for sources not in the catalog, thus completing the catalog and reducing a source of systematic error in EOT, an imaging method has been developed -- Imaging with a Differential filter using the Earth Occultation Method (IDEOM). IDEOM is a tomographic imaging method that takes advantage of the orbital precession of the Fermi satellite. Using IDEOM, all-sky reconstructions have been generated for ~sim 4 years of GBM data in the 12-50 keV, 50-100 keV and 100-300 keV energy bands in search of sources otherwise unmodeled by the GBM occultation analysis. IDEOM analysis resulted in the detection of 57 sources in the 12-50 keV energy band, 23 sources in the 50-100 keV energy band, and 7 sources in the 100-300 keV energy band. Seventeen sources were not present in the original GBM-EOT catalog and have now been added. We also present the first joined averaged spectra for four persistent sources detected by GBM using EOT and by the Large Area Telescope (LAT) on Fermi: NGC 1275, 3C 273, Cen A, and the Crab

Long-term pulse profile study of the Be/X-ray pulsar SAX J2103.5+4545

Aims. We present the first long-term pulse profile study of the X-ray pulsar SAX J2103.5+4545. Our main goal is to study the pulse shape correlation either with luminosity, time or energy. Methods. This Be/X-ray binary system was observed from 1999 to 2004 by RXTE PCA, and by INTEGRAL from 2002 to 2005, during the Performance and Verification (PV) phase and the Galactic Plane Scan survey (GPS). X-ray pulse profiles were obtained in different energy ranges. The long-term spectral variability of this source is studied. The long-term flux, frequency and spin-up rate histories are computed. A new set of orbital parameters are also determined. Results. The pulse shape is complex and highly variable either with time or luminosity. However, an energy dependence pattern was found. Single, double, triple or even quadruple peaks pulse profile structure was obtained. It was confirmed that SAX J2103.5+4545 becomes harder when the flux is higher. The new orbital solution obtained is: P_orb= 12.66528+-0.00051 days, e = 0.401+-0.018, w = 241.36+-2.18 and a_xsin i = 80.81+-0.67 lt-s.Comment: 9 pages, 11 figures, submitted to Astronomy & Astrophysic

Orbital Decay and Evidence of Disk Formation in the X-ray Binary Pulsar OAO 1657-415

OAO 1657-415 is an eclipsing X-ray binary wind-fed pulsar that has exhibited smooth spin-up/spin-down episodes and has undergone several torque reversals throughout its long history of observation. We present a frequency history spanning nearly 19 years of observations from the Burst and Transient Source Experiment (CGRO/BATSE) and from the Gamma-Ray Burst Monitor (Fermi/GBM). The analysis suggests two modes of accretion: one resulting in steady spin-up during which we believe a stable accretion disk is present and one that results in what appears to be a random walk in spin frequency where an unstable accretion disk forms alternating in direction ("flip flop"). Orbital elements of the pulsar system are determined at several intervals throughout this history. With these ephemerides, statistically significant orbital decay ($\dot{P}/P =(-3.40 \pm0.15)\times10^{-6} \mathrm{yr^{-1}}$) is established suggesting a transition between wind-fed and disk-mediated accretion

Recent activity of the Be/X-ray binary system SAX J2103.5+4545

Aims. We present a multiwavelength study of the Be/X-ray binary system SAX J2103.5+4545 with the goal of better characterizing the transient behaviour of this source. Methods. SAX J2103.5+4545 was observed by Swift-XRT four times in 2007 from April 25 to May 5, and during quiescence in 2012 August 31. In addition, this source has been monitored from the ground-based astronomical observatories of El Teide (Tenerife, Spain), Roque de los Muchachos (La Palma, Spain) and Sierra Nevada (Granada, Spain) since 2011 August, and from the TUBITAK National Observatory (Antalya, Turkey) since 2009 June. We have performed spectral and photometric temporal analyses in order to investigate the different states exhibited by SAX J2103.5+4545. Results. In X-rays, an absorbed power law model provided the best fit for all the XRT spectra. An iron-line feature at ~6.42 keV was present in all the observations except for that taken during quiescence in 2012. The photon indexes are consistent with previous studies of SAX J2103.5+4545 in high/low luminosity states. Pulsations were found in all the XRT data from 2007 (2.839(2) mHz; MJD 54222.02), but not during quiescence. Both optical outbursts in 2010 and 2012 lasted for about 8/9 months (as the one in 2007 probably did and the current one in 2014 might do) and were most probably caused by mass ejection events from the Be star that eventually fed the circumstellar disc. All of these outbursts started about 3 months before the triggering of the X-ray activity, and about the same period before the maximum of the H_alpha line equivalent width (in emission) was reached at only ~ -5 \AA. In this work we found that the global correlation between the BV variability and the X-ray intensity was also observed at longer wavelengths in the IR domain.Comment: 11 pages, 7 figures, and online material (2 tables). Submitted to A&A in 2014 Januar