Recent torque reversal of 4U 1907+09

Abstract

We present timing and spectral analysis of RXTE‐PCA (Proportional Counter Array) observations of the accretion powered pulsar 4U 1907+09 between 2007 June and 2008 August. 4U 1907+09 had been in a spin‐down episode with a spin‐down rate of −3.54 × 10−14 Hz s−1 before 1999. From RXTE observations after 2001 March, the source showed a ∼60 per cent decrease in spin‐down magnitude, and INTEGRAL observations after 2003 March showed that source started to spin‐up. We found that the source recently entered into a new spin‐down episode with a spin‐down rate of −3.59 × 10−14 Hz s−1. This spin‐down rate is pretty close to the previous long‐term spin‐down rate of the source measured before 1999. From the spectral analysis, we showed that hydrogen column density varies with the orbital phase.

Evidence for an accretion stream in 4U 1907+09

“…The column density fit in Fig. 6 implies an interstellar column density towards 4U 1907+09 of N ism H ∼4.8 × 10 22 cm −2 , which is slightly higher than literature values (Inam et al 1997; Roberts et al 2001). The error bars in the derived N asm H are considerably large (see Fig.…”

“…The column density was extracted from spectral fits of 4U 1907+09 by Inam, Sahiner & Baykal (1997), who analysed the RXTE /PCA observations from 2007 and 2008. Data from PCA observations come from ∼2‐ks time intervals (Inam et al 1997); thus, no PCA observation covered a full orbital period. For this work, we chose to study the column density in 4U 1907+09 over the same time period as used in the flux analysis.…”

MEASUREMENTS OF CYCLOTRON FEATURES AND PULSE PERIODS IN THE HIGH-MASS X-RAY BINARIES 4U 1538–522 AND 4U 1907+09 WITH THEINTERNATIONAL GAMMA-RAY ASTROPHYSICS LABORATORY

“…The exact location of 4U 1538−522 relative to L coul varies depending on which mass is used: with the very low-mass elliptical result, the measured Luminosities and values for E cyc from other works are included, along with 90% confidence intervals. Linear fits to the data are also overplotted; due to the systematic differences between Gaussian-and Lorentzian-shaped models for the cyclotron lines, there are two curves for each source: the solid includes only sources that used Gaussian-profile models for the CRSFs, while the dotted line includes Lorentzian-modeled lines (those of Mihara et al 1998;Clark et al 1990;Rodes-Roca et al 2009;Makishima et al 1999;and Inam et al 2009) shifted upward by 1.75 keV.…”

“…For ∼15 yr following the identification of the source as a pulsar by Makishima et al (1984), the pulse period increased steadily from 437.4 s in 1984 (Makishima et al 1984) to ∼441 s in 1998, when the spin-down trend began to slow, leading to a torque reversal in 2005 (Fritz et al 2006). This was followed relatively quickly by another reversal in mid-2007 (Inam et al 2009;Şahiner et al 2011) and the source began following approximately the same spin-down trend as it had between 1987 and 1998, a trend that thus far shows no sign of changing. The initial determination of the ∼8.4 day orbital period showed flaring twice per orbit, suggesting the source was in orbit around a Be-type star (Marshall & Ricketts 1980), although subsequent optical observations indicated that the companion was more likely a supergiant (van Kerkwijk et al 1989).…”

The Magnetospheres of (Accreting) Neutron Stars

“…This means that there should be a direct relation between the X-ray pulsational period and the parameters determining r mag [34][35][36][37]. Dramatic spin-up episodes are seen, for example, during giant outbursts of Be X-ray binaries [3,[38][39][40], while many sources show extended spin up/spin down phases indicating that they are close to equilibrium [18,41,42]. These observations allow, in principle, an independent check of magnetic fields obtained from cyclotron lines (see Sect.…”