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

Abstract

We present a spectral analysis of NuSTAR and NICER observations of the luminous, persistently accreting neutron star (NS) low-mass X-ray binary Cygnus X-2. The data were divided into different branches that the source traces out on the Z-track of the X-ray color-color diagram; namely the horizontal branch, normal branch, and the vertex between the two. The X-ray continuum spectrum was modeled in two different ways that produced a comparable quality fit. The spectra showed clear evidence of a reflection component in the form of a broadened Fe K line, as well as a lower energy emission feature near 1 keV likely due to an ionized plasma located far from the innermost accretion disk. We account for the reflection spectrum with two independent models (relxillns and rdblur*rfxconv). The inferred inclination is in agreement with earlier estimates from optical observations of ellipsoidal light curve modeling (relxillns: $i=67^{\circ }\pm 4^{\circ }$, rdblur*rfxconv: $i=60^{\circ }\pm 10^{\circ }$). The inner disk radius remains close to the NS ($R_{\mathrm{in}}\le 1.15R_{\mathrm{ISCO}}$) regardless of the source position along the Z-track or how the 1 keV feature is modeled. Given the optically determined NS mass of $1.71\pm 0.21M_{\odot }$, this corresponds to a conservative upper limit of $R_{\mathrm{in}}\le 19.5$ km for $M=1.92M_{\odot }$ or $R_{\mathrm{in}}\le 15.3$ km for $M=1.5M_{\odot }$. We compare these radius constraints to those obtained from NS gravitational wave merger events and recent NICER pulsar light curve modeling measurements.

Accepted for publication in ApJ, 13 pages, 5 tables, 5 figures

References in corpus (14)

• PSR J0030+0451 Mass and Radius from NICER Data and Implications for the Properties of Neutron Star Matter
• A NICER View of PSR J0030+0451: Millisecond Pulsar Parameter Estimation
• HI4PI: A full-sky HI survey based on EBHIS and GASS
• A NICER View of the Massive Pulsar PSR J0740+6620 Informed by Radio Timing and XMM-Newton Spectroscopy
• Thermonuclear (type-I) X-ray bursts observed by the Rossi X-ray Timing Explorer
• An Empirical Background Model for the NICER X-ray Timing Instrument
• Evaluating Spectral Models and the X-ray States of Neutron-Star X-ray Transients
• Relativistic iron emission lines in neutron star low-mass X-ray binaries as probes of neutron star radii
• A Hard Look at the Neutron Stars and Accretion Disks in 4U 1636-53, GX 17+2, and 4U 1705-44 with $\text{NuSTAR}$
• Relativistic X-ray Reflection Models for Accreting Neutron Stars
• Heating in an Extended Accretion Disk Corona along the Z-Pattern in Cyg X-2
• NICER-NuSTAR Observations of the Neutron Star Low-Mass X-ray Binary 4U 1735-44
• NuSTAR view of the Z-type neutron star low-mass X-ray binary Cygnus~X--2
• Discovery of Red-Skewed K_alpha iron line in Cyg X-2 with Suzaku

Cited by in corpus (21)

• The Tidal Disruption Event AT2021ehb: Evidence of Relativistic Disk Reflection, and Rapid Evolution of the Disk-Corona System
• Accretion geometry of the neutron star low mass X-ray binary Cyg X-2 from X-ray polarization measurements
• Thermal Effects in Binary Neutron Star Mergers
• X-ray polarimetry and spectroscopy of the neutron star low-mass X-ray binary GX 9+9: an in-depth study with IXPE and NuSTAR
• Evolution of a Relativistic Outflow and X-ray Corona in the Extreme Changing-Look AGN 1ES 1927+654
• Highly Significant Detection of X-Ray Polarization from the Brightest Accreting Neutron Star Sco X-1
• Polarization properties of the weakly magnetized neutron star X-ray binary GS 1826-238 in the high soft state
• X-ray spectro-polarimetric characterization of GX 340+0 in the horizontal branch: a highly inclined source?
• Pulse Profile Modeling of Thermonuclear Burst Oscillations I: The Effect of Neglecting Variability
• Impact of nuclear matter properties on the nucleosynthesis and the kilonova from binary neutron star merger ejecta
• Constraining the geometry of the dipping atoll 4U 1624-49 with X-ray spectroscopy and polarimetry
• X-ray polarization of Z-type neutron star low-mass X-ray binaries -- I. Model-independent, time-resolved X-ray polarimetry
• NuSTAR and AstroSat observations of GX 9$+$1: Spectral and temporal studies
• Probing the accretion geometry of the atoll source 4U 1702-429 in different spectral states with NICER, NuSTAR, and AstroSat
• Broadband spectro-temporal investigation of neutron star low-mass X-ray binary GX 349+2
• Revealing the spectral state transition of the Clocked Burster, GS 1826-238 with NuSTAR StrayCats
• X-ray spectropolarimetric characterization of the Z-source GX 340+0 in the normal branch
• AstroSat View of Transient Low-mass X-ray Binary XTE J1701-462: Spectral and Temporal Evolution along the Z-track
• Investigating the consistency of the shape and flux of X-ray reflection spectra in the hard state with an accretion disk reaching close to the black hole
• Links Between Optical and X-ray Light in Cygnus X-2
• X-ray polarization of Z-type neutron star low-mass X-ray binaries -- II. Spectropolarimetric analysis