Broad iron line in the fast spinning neutron-star system 4U 1636−53

Abstract

We analysed the X-ray spectra of six observations, simultaneously taken with XMMNewton and Rossi X-ray Timing Explorer (RXTE), of the neutron star low-mass X-ray binary 4U 1636-53. The observations cover several states of the source, and therefore a large range of inferred mass accretion rate. These six observations show a broad emission line in the spectrum at around 6.5 keV, likely due to iron. We fitted this line with a set of phenomenological models of a relativistically broadened line, plus a model that accounts for relativistically smeared and ionised reflection from the accretion disc. The latter model includes the incident emission from both the neutronstar surface or boundary layer and the corona that is responsible for the high-energy emission in these systems. From the fits with the reflection model we found that in four out of the six observations the main contribution to the reflected spectrum comes from the neutron-star surface or boundary layer, whereas in the other two observations the main contribution to the reflected spectrum comes from the corona. We found that the relative contribution of these two components is not correlated to the state of the source. From the phenomenological models we found that the iron line profile is better described by a symmetric, albeit broad, profile. The width of the line cannot be explained only by Compton broadening, and we therefore explored the case of relativistic broadening. We further found that the direct emission from the disc, boundary layer, and corona generally evolved in a manner consistent with the standard accretion disc model, with the disc and boundary layer becoming hotter and the disc moving inwards as the source changed from the hard in to the soft state. The iron line, however, did not appear to follow the same trend.

Discovery of a correlation between the frequency of the mHz quasi-periodic oscillations and the neutron-star temperature in the low-mass X-ray binary 4U 1636–53

“…As previously reported by Sanna et al (2013) the soft state and the hard state of 4U 1636-53 (see Figure 2 in Sanna et al 2013). Our results are therefore consistent with the picture that frequency drifts of the mHz QPOs are only observed in the state transition.…”

“…We found that both a thermal and a Comptonised component were necessary to fit the continuum, whereas two thermal components plus a Comptonised component (e.g., Sanna et al 2013;Lyu et al 2014) was too complex a model given the quality of the data. For the thermal component either a blackbody or an accretiondisc model could fit the data well.…”

“…We added a 0.5 % systematic error to the model. After fitting the spectra with these continuum models in all cases we found significant residuals around 6 − 7 keV, where possible emission from an iron line is expected (Pandel et al 2008;Cackett et al 2010;Sanna et al 2013;Lyu et al 2014), we therefore added a line component to the model with the central energy of the line constrained to the range 6.4 − 6.97 keV. We first used a Gaussian line to estimate some general properties of the iron line.…”

Millihertz Quasi-Periodic Oscillations in 4u 1636–536: Putting Possible Constraints on the Neutron Star Size

“…The NS nature of the compact object has been confirmed through the detection of thermonuclear X-ray bursts (Hoffman et al 1977;Galloway et al 2006;Zhang et al 2011), which show asymptotic millisecond burst oscillations, indicating a spin frequency of ∼581 Hz (Zhang et al 1997;Giles et al 2002;Strohmayer & Markwardt 2002). In addition to variability studies, the X-ray energy spectra, which show broad, asymmetric Fe emission lines, have been investigated (Pandel et al 2008;Cackett et al 2010;Ng et al 2010;Sanna et al 2013). …”

Broadband observations of the X-ray burster 4U1705-44 withBeppoSAX

“…The broad line of 4U 1705-44, with others lines detected with XMM-Newton, is also discussed in Ng et al (2010) who point out the potential role of the pileup in the broadening of the line. However, recent developments of the subject can be found in Di Salvo et al (2015), Cackett & Miller (2013), Sanna et al (2013), Egron et al (2013), Piraino et al (2012), Cackett et al (2012), andMiller et al (2010).…”