Possible Detection of an Emission Cyclotron Resonance Scattering Feature from the Accretion-powered Pulsar 4U 1626-67

TL;DR

This paper reports the possible detection of an emission cyclotron resonance scattering feature in the X-ray spectrum of pulsar 4U 1626-67, based on phase-resolved analysis of Suzaku data, suggesting a new insight into the pulsar's magnetic environment.

Contribution

It presents the first evidence of an emission CRSF in a pulsar, challenging the typical absorption profile interpretation and providing new clues about the pulsar's magnetic field structure.

Findings

Detection of a potential emission CRSF at ~37 keV in the dim phase

Better fit of the dim phase spectrum with an emission profile than absorption

Statistical significance of the emission feature between 2.91e-3 and 1.53e-5

Abstract

We present analysis of 4U 1626-67, a 7.7 s pulsar in a low-mass X-ray binary system, observed with the hard X-ray detector of the Japanese X-ray satellite Suzaku in March 2006 for a net exposure of \sim88 ks. The source was detected at an average 10-60 keV flux of \sim4 x10^-10 erg cm^-2 s^-1. The phase-averaged spectrum is reproduced well by combining a negative and positive power-law times exponential cutoff (NPEX) model modified at \sim 37 keV by a cyclotron resonance scattering feature (CRSF). The phase-resolved analysis shows that the spectra at the bright phases are well fit by the NPEX with CRSF model. On the other hand, the spectrum in the dim phase lacks the NPEX high-energy cutoff component, and the CRSF can be reproduced by either an emission or an absorption profile. When fitting the dim phase spectrum with the NPEX plus Gaussian model, we find that the feature is better described in terms of an emission rather than an absorption profile. The statistical significance of this result, evaluated by means of an F-test, is between 2.91 x 10^-3 and 1.53 x 10^-5, taking into account the systematic errors in the background evaluation of HXD-PIN. We find that, the emission profile is more feasible than the absorption one for comparing the physical parameters in other phases. Therefore, we have possibly detected an emission line at the cyclotron resonance energy in the dim phase.