A systematic analysis of the broad iron Kαline in neutron-star LMXBs withXMM-Newton

Abstract

We analysed the XMM-Newton archival observations of 16 neutron star (NS), low-mass X-ray binaries (LMXBs) to study the Fe K emission in these objects. The sample includes all the observations of NS LMXBs performed in EPIC pn timing mode with XMM-Newton publicly available until September 30, 2009. We performed a detailed data analysis considering pile-up and background effects. The properties of the Fe lines differed from previous published analyses because of either incorrect pile-up corrections or different continuum parameterisation. Eighty percent of the observations for which a spectrum can be extracted showed significant Fe line emission. We found an average line centroid of 6.67 ± 0.02 keV and a finite width, σ, of 0.33 ± 0.02 keV. The equivalent width of the lines varied between 17 and 189 eV, with an average weighted value of 42 ± 3 eV. For sources where several observations were available, the Fe line parameters changed between observations whenever the continuum changed significantly. The line parameters did not show any correlation with luminosity. Most important, we could fit the Fe line with a simple Gaussian component for all the sources. The lines did not show the asymmetric profiles that were interpreted as indicating relativistic effects in previous analyses of these LMXBs.

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

“…Nonetheless, both the results of Pandel et al (2008) and Cackett et al (2010) are based on data which were not corrected for pileup effects. As demonstrated by Ng et al (2010), re-analyses of the same XMM-Newton data considering pileup and background effects, suggested a different iron line profile. Ng et al (2010) found that the Fe lines in the three spectra of 4U 1636-53 used by Pandel et al (2008) appeared to be symmetric and could be well fit with a Gaussian profile, although, due to the statistics, a relativistic origin of the line could not be excluded.…”

“…As demonstrated by Ng et al (2010), re-analyses of the same XMM-Newton data considering pileup and background effects, suggested a different iron line profile. Ng et al (2010) found that the Fe lines in the three spectra of 4U 1636-53 used by Pandel et al (2008) appeared to be symmetric and could be well fit with a Gaussian profile, although, due to the statistics, a relativistic origin of the line could not be excluded. However, Ng et al (2010) did not include the simultaneous RXTE data in their fits, which can affect the Fe emission line profile that strongly depends on the underlying continuum.…”

“…of a "blank field" observation instead (see Ng et al 2010;Hiemstra et al 2011). Based on similar sky coordinates and column density along the line of sight, we used the timing mode observation of the BH GX 339-4 (Obs 0085680601), in which the source was not significantly detected, as our "blank field" for all six XMM-Newton observations.…”

“…Broad Fe lines are also detected in NS systems although they are weaker than in BH systems. With instruments like Chandra, XMM-Newton and Suzaku, relativistically broadened Fe emission lines have been observed in many NS LMXBs (e.g., Piraino et al 2007;Cackett et al 2008;Pandel et al 2008;di Salvo et al 2009;Cackett et al 2010; see also Ng et al 2010 for an almost complete list of NS LMXBs with Fe lines detected with XMM-Newton).…”

“…For instance, Titarchuk et al (2003) proposed that asymmetric line profiles could originate from an optically thick flow launched from the disc, which expands or contracts at relativistic speeds. We will not explore this possibility in the paper, but refer the reader to the discussion in Ng et al (2010).…”