Low-mass X-ray Binary Serpens Research Articles

Detection of Reflection Features in the Neutron Star Low-mass X-Ray Binary Serpens X-1 with NICER

Abstract We present Neutron Star Interior Composition Explorer (NICER) observations of the neutron star (NS) low-mass X-ray binary Serpens X-1 during the early mission phase in 2017. With the high spectral sensitivity and low-energy X-ray passband of NICER, we are able to detect the Fe L line complex in addition to the signature broad, asymmetric Fe K line. We confirm the presence of these lines by comparing the NICER data to archival observations with XMM-Newton/Reflection Grating Spectrometer (RGS) and NuSTAR. Both features originate close to the innermost stable circular orbit (ISCO). When modeling the lines with the relativistic line model relline, we find that the Fe L blend requires an inner disk radius of R ISCO and Fe K is at R ISCO (errors quoted at 90%). This corresponds to a position of km and km for a canonical NS mass ( ) and dimensionless spin value of a = 0. Additionally, we employ a new version of the relxill model tailored for NSs and determine that these features arise from a dense disk and supersolar Fe abundance.

ON THE EVOLUTION OF THE INNER DISK RADIUS WITH FLUX IN THE NEUTRON STAR LOW-MASS X-RAY BINARY SERPENS X-1

ABSTRACT We analyze the latest Suzaku observation of the bright neutron star (NS) low-mass X-ray binary Serpens X-1 taken in 2013 October and 2014 April. The observation was taken using the burst mode and only suffered mild pile-up effects. A broad iron line is clearly detected in the X-ray spectrum. We test different models and find that the iron line is asymmetric and best interpreted by relativistic reflection. The relativistically broadened iron line is generally believed to originate from the innermost regions of the accretion disk, where strong gravity causes a series of special and general relativistic effects. The iron line profile indicates an inner radius of ∼8 R G, which gives an upper limit on the size of the NS. The asymmetric iron line has been observed in a number of previous observations, which gives several inner radius measurements at different flux states. We find that the inner radius of Serpens X-1 does not evolve significantly over the range of L/L Edd ∼ 0.4–0.6, and the lack of flux dependence of the inner radius implies that the accretion disk may be truncated outside of the innermost stable circular orbit by the boundary layer, rather than the stellar magnetic field.

CONSTRAINTS ON THE NEUTRON STAR AND INNER ACCRETION FLOW IN SERPENS X-1 USING NuSTAR

We report on an observation of the neutron star low-mass X-ray binary Serpens X-1, made with NuSTAR. The extraordinary sensitivity afforded by NuSTAR facilitated the detection of a clear, robust, relativistic Fe K emission line from the inner disk. A relativistic profile is required over a single Gaussian line from any charge state of Fe at the 5-sigma level of confidence, and any two Gaussians of equal width at the same confidence. The Compton back-scattering "hump" peaking in the 10-20 keV band is detected for the first time in a neutron star X-ray binary. Fits with relativistically-blurred disk reflection models suggest that the disk likely extends close to the innermost stable circular orbit (ISCO) or stellar surface. The best-fit blurred reflection models constrain the gravitational redshift from the stellar surface to be z > 0.16. The data are broadly compatible with the disk extending to the ISCO; in that case, z > 0.22 and R < 12.6 km (assuming M = 1.4 Msun and a=0, where a = cJ/GM^2). If the star is as large or larger than its ISCO, or if the effective reflecting disk leaks across the ISCO to the surface, the redshift constraints become measurements. We discuss our results in the context of efforts to measure fundamental properties of neutron stars, and models for accretion onto compact objects.

Evidence of a Broad Relativistic Iron Line from the Neutron Star Low-Mass X-Ray Binary Serpens X-1

We report on an analysis of XMM-Newton data from the neutron star low-mass X-ray binary (LMXB) Serpens X-1 (Ser X-1). Spectral analysis of EPIC PN data indicates that the previously known broad iron Kα emission line from this source has a significantly skewed structure with a moderately extended red wing. The asymmetric shape of the line is well described with the laor and diskline models in XSPEC and strongly supports an inner accretion disk origin of the line. To our knowledge, this is the first strong evidence of a relativistic line in a neutron star LMXB. This finding suggests that the broad lines seen in other neutron star LMXBs likely originate from the inner disk as well. Detailed study of such lines opens up a new way to probe neutron star parameters and their strong gravitational fields. The red wing of the iron line from Ser X-1 is not as broad as that observed from some black hole systems. This is not unreasonable for a neutron star system, as the accretion disk has to terminate at or before the hard stellar surface. Finally, the inferred source inclination angle in the approximate range 40°-60° is consistent with the lack of dips and eclipses from Ser X-1.

A four-hours long burst from Serpens X–1

During a serendipitous observation of the BeppoSAX Wide Field Cameras, a very long type I X-ray burst was observed from the low mass X-ray binary Serpens X–1. The burst lasted for approximately 4 hours and had an exponential decay time of min (2–28 keV). The bolometric peak-luminosity is ( and the fluence ( erg. The first “normal" type I burst was observed 34 days after the superburst. This is in rough agreement with recent predictions for unstable carbon burning in a heavy element ocean.