A self-consistent approach to the hard and soft states of 4U 1705-44

Abstract

Context. High-resolution spectroscopy has recently revealed in many low-mass X-ray binaries hosting a neutron star that the shape of the broad iron line observed in the 6.4-6.97 keV range is consistently well-fitted by a relativistically smeared line profile. Aims. The presence of other broad features, besides the iron line, together with a high S/N of the spectra offer the possibility of testing a self-consistent approach to the overall broadband reflection spectrum and evaluating the impact of the reflection component in the formation of the broadband X-ray spectra. Methods. We analyzed two XMM-Newton observations of the bright atoll source 4U 1705-44, which can be considered a prototype of the class of the persistent NS LMXBs showing both hard and soft states. The first observation was performed when the source was in a hard low flux state, the second during a soft, high-flux state. Both the spectra show broad iron emission lines. We fit the spectra using a two-component model, together with a reflection model specifically suited to the case of a neutron star, where the incident spectrum has a blackbody shape. Results. In the soft state, the reflection model, convolved with a relativistic smearing component, consistently describes the broad features present in the spectrum, and we find a clear relation between the temperature of the incident flux and the temperature of the harder X-ray component that we interpret as the boundary layer emission. In this state we find converging evidence that the boundary layer outer radius is ∼2 times the neutron star radius. In the low flux state, we observe a change in the continuum shape of the spectrum with respect to the soft state. Still, the broad local emission features can be associated with a disk reflecting matter, but in a lower ionization state, and possibly produced in an accretion disk truncated at greater distance. Conclusions. Our analysis provides strong evidence that the reflection component in soft states of LMXBs comes from to hard X-ray thermal irradiation, which we identify with the boundary layer emission, also present in the continuum model. In the hard state, the broad iron line if also produced by reflection, and the continuum disk emission can be self-consistently accounted if the disk is truncated at a greater distance than the soft state.

Testing reflection features in 4U 1705−44 withXMM-Newton,BeppoSAX, and RXTE in the hard and soft states

“…Moreover, all the smearing parameters are perfectly coherent with those previously obtained (e.g. Di D'Aì et al 2010;Piraino et al 2007). An additional iron edge is found at 8.7 keV with a significance of 11.6 σ.…”

“…Using the observations from BeppoSAX and Suzaku, Lin et al (2010) obtained constraints on the broad-band spectrum of 4U 1705−44 and find that the strength of the Fe line correlates well with the boundary layer emission in the soft state, while the Fe line is probably due to illumination of the accretion disk by the strong Comptonization emission in the hard state. An XMM-Newton observation performed in 2008 when the source was in a soft state confirmed the results obtained with Suzaku, showing a high-statistics Fe line profile and spectrum consistent with a disk-reflection scenario D'Aì et al 2010). The best fit value of the A5, page 2 of 18 inner disk radius was R in 14 R g and the corresponding inclination angle and index of the emissivity law profile were i ∼ 39 • and −2.3 respectively, in agreement with the Suzaku results.…”

“…The data analysis of the XMM/EPIC-pn spectrum was restricted to 2.4−11 keV to exclude the region around the detector Si K-edge (1.8 keV) and the mirror Au M-edge (2.3 keV) that could affect our analysis. This problem has already been noticed for the EPIC-pn observations performed in timing mode (e.g., D'Aì et al 2010;Papitto et al 2010;Egron et al 2011). The energy bands used for the other instruments are: 0.3−4 keV for the LECS, 1.8−10 keV for the MECS, 7−34 keV for HPGSPC, 15−200 keV for PDS onboard BeppoSAX, and 4−22 keV for PCA, and 15−100 keV for HEXTE onboard RXTE.…”

“…However, the conclusions coming from the spectral analysis differ. While Di D'Aì et al (2010) deduce that the iron line is clearly asymmetric and compatible with a relativistic line, using the diskline (Fabian et al 1989) and the reflection model refbb (Ballantyne 2004), respectively, Ng et al (2010) find that the iron line can be fit equally well by a Gaussian or using the laor model (Laor 1991), concluding that there was no statistical evidence of any asymmetry of the line profile. It is thus important to assess the effects of pile-up in the EPIC-pn spectrum of 4U 1705−44 in the soft state, which shows the highest signal-to-noise ratio (S/N) iron-line profile ever detected to date in a neutron star LMXB.…”

Nustar and XMM-Newton Observations of the Neutron Star X-Ray Binary 1rxs J180408.9-34205

“…However, other studies find lower ionization parameter values for an NS in the LHS (Cackett et al 2009;Papitto et al 2009;D'Aì et al 2010;Di Salvo et al 2015). However, it is likely that some of the emergent flux in the iron line region for 1RXS J1804 is partially from a region of lower ionization.…”