X‐Ray Spectra and Pulse Frequency Changes in SAX J2103.5+4545

Abstract

The 1999 November outburst of the transient pulsar SAX J2103.5+4545 was monitored with the large area detectors of the Rossi X-Ray Timing Explorer until the pulsar faded after a year. The 358 s pulsar was spun up for 150 days, at which point the flux dropped quickly by a factor of %7, the frequency saturated, and, as the flux continued to decline, a weak spin-down began. The pulses remained strong during the decay and the spin-up/flux correlation can be fitted to the Ghosh and Lamb derivations for the spin-up caused by accretion from a thin, pressure-dominated disk, for a distance of %3.2 kpc and a surface magnetic field of %1:2Â 10 13 G. During the bright spin-up part of the outburst, the flux was subject to strong orbital modulation, peaking %3 days after periastron of the eccentric 12.68 day orbit, while during the faint part, there was little orbital modulation. The X-ray spectra were typical of accreting pulsars, describable by a cutoff power law, with an emission line near the 6.4 keV of K fluorescence from cool iron. The equivalent width of this emission did not share the orbital modulation, but nearly doubled during the faint phase, despite little change in the column density. The outburst could have been caused by an episode of increased wind from a Be star, such that a small accretion disk was formed during each periastron passage. A change in the wind and disk structure apparently occurred after 5 months such that the accretion rate was no longer modulated or the diffusion time was longer. The distance estimate implies that the X-ray luminosity observed was between 1 Â 10 36 and 6 Â 10 34 ergs s À1 , with a small but definite correlation with the intrinsic power-law spectral index.

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

“…We would like to point out that the poor quality of this spectrum is probably not only related to the very low flux level of the source (∼0.074 mCrab), but also to the fact that this short ∼3 ks observation was taken in WT mode, which is more suitable for timing analysis and for source fluxes between 1-600 mCrab. The photon indexes (Γ < 1) are consistent with the values previously reported for SAX J2103.5+4545 when it was at high luminosities (İnam et al 2004;Baykal et al 2002Baykal et al , 2007Filippova et al 2004;Camero Arranz et al 2007;Reig et al 2010). At low states, our value of the photon index during quiescence is slightly higher than previous measurements, although still compatible taking into account the uncertainties.…”

“…Top: long-term spin frequency history of SAX J2103.5+4545 since its discovery in 1997. To visualize this plot more clearly we do not display the results from Baykal et al (2002Baykal et al ( , 2007, our first XRT frequency determination from Table 3, and some values from Sidoli et al (2005). Middle: long-term optical light curve of this source.…”

“…Baykal et al (2002) found a correlation between spinup rate and X-ray flux during the 1999 outburst. This suggested the formation of an accretion disc during periastron passage of the neutron star.…”

“…The optical counterpart is a B0Ve star (V = 14.2) at a distance between 3.2 and 6.5 kpc (Baykal et al 2002;Reig et al 2004). Baykal et al (2002) found a correlation between spinup rate and X-ray flux during the 1999 outburst.…”

High-mass X-ray binaries in the Milky Way

“…A distance to the source is estimated as ≃ 4.5 − 6.9 kpc (Baykal et al, 2007). The broadband spectrum of SAX J2103.5+4545, obtained with data of RXTE and INTEGRAL observatories is typical for X-ray pulsars and can be described by a powerlaw with high energy cutoff (see, e.g., Baykal et al, 2002;Lutovinov et al, 2003c;Filippova et al, 2004;Ducci et al, 2008).…”