NuSTAR view of the Z-type neutron star low-mass X-ray binary Cygnus X-2

Abstract

We report on the NuSTAR observation of the Z-type neutron star low-mass X-ray binary Cygnus X-2 performed on 7 January 2015. During this observation, the source exhibited a sudden decrease in count rate (dips) and stronger variability in 3 − 79 keV X-ray lightcurve. The hardness-intensity diagram shows that the source remained in the so-called "normal branch" of the Z-track, although an extended "flaring branch" is observed during the dips. The source was in a soft spectral state with the 3 − 45 keV luminosity of L ≃ (0.5 − 1.1) × 10 38 erg s −1 , assuming a distance of 8 kpc. Both the non-dip and dip X-ray spectra are well represented by models in which the soft band is dominated by the emission from the disc, while the hard X-ray band is dominated by the Comptonized emission from the boundary layer/corona and its reflected emission from the disc. The X-ray spectrum also revealed a broad Fe Kα emission line which is nearly symmetric at the higher flux and asymmetric when the flux is reduced by a factor of ∼ 2. The relativistic reflection model predicts the inner radius of the accretion disc as R in ≃ 2.5 − 6.0 R ISCO (≃ 30 − 73 Km) for the non-dip state and R in ≃ 2.0 − 2.6 R ISCO (≃ 24 − 32 Km) for the dip state. If the inner disc is truncated due to the pressure arising from a magnetic field, this implies an upper limit of the magnetic field strength of 7.6 × 10 9 G at the magnetic poles which is consistent with other estimates.

Interstellar oxygen along the line of sight of Cygnus X-2

“…It is a bright X-ray source with a moderate column density (∼2 × 10 21 cm −2 , Juett 2004) and high flux (2.3 × 10 −9 erg s −1 cm −2 in the 0.3-2 keV band). Cygnus X-2 has been proposed as a Z-track source (Hasinger & van der Klis 1989) and is believed to reach the Eddington luminosity limit during its high state (Bałucińska-Church et al 2010;Mondal et al 2018;King & Ritter 1999). The distance of the source has been estimated to be around 7-12 kpc, and its galactic coordinates are l, b = 87.…”

Radius Constraints from Reflection Modeling of Cygnus X-2 with NuSTAR and NICER

“…Due to the luminous and persistent nature of the source, the spectral properties have been studied considerably. The source spectrum is known to show a broad Fe line feature near 6.7 keV (Smale et al 1993;Di Salvo et al 2002;Shaposhnikov et al 2009;Cackett et al 2010;Mondal et al 2018) due to reflection from the accretion disk, as well an emission line near 1 keV (Vrtilek et al 1986;Chiappetti et al 1990;Smale et al 1993;Kuulkers et al 1997;Di Salvo et al 2002;Farinelli et al 2009;Cackett et al 2010) that likely originates from collisionally excited or photoionized material further out in the disk (Vrtilek et al 1986). The source traces out the horizontal, normal, and flaring branches of the 'Z' with periods of irregular dipping activity while flaring; suggesting the presence of an extended accretion disk corona (ADC) during high intensity states (Vrtilek et al 1988;Schulz et al 2009;Bałucińska-Church et al 2010.…”

“…The disk was inferred to be close to the NS at R in 7.6 − 8.5 R g (where R g = GM/c 2 ) when using single DISKLINE component and R in 6 − 14 R g when using the full reflection model. Mondal et al (2018) recently analyzed a NuSTAR observation of Cyg X-2 in the normal branch and flaring/dipping state. The reflection component is modeled with a different blackbody reflection model known as REFLIONXBB and the disk was inferred to be far from the NS at R in 13.5 − 32.4 R g in the non-dipping state.…”

“…Here we analyze the existing NuSTAR observations of Cyg X-2 while the source is in a non-dipping state in order to carefully obtain radius constraints from reflection modeling. Though one NuSTAR observation was reported on in Mondal et al (2018), two additional NuSTAR observations were performed simultaneously with NICER. Hence we present the results of joint NICER and NuSTAR spectral modeling.…”

“…ures 1 and 2. The source went through the normal to extended flaring branch in Obs1 while in the high intensity state as reported in Mondal et al (2018). We divide the data in a similar manner by separating the non-dip emission from the dipping by creating GTIs that divide the observation at t = 45 ks.…”