Classification of IGR J20084+3221 as an Intermediate Polar using X-ray and Optical Observations

TL;DR

This paper classifies IGR J20084+3221 as an Intermediate Polar mCV using multi-wavelength X-ray and optical observations, timing analysis, and spectral modeling, providing insights into its white dwarf properties.

Contribution

It presents the first comprehensive multi-wavelength analysis confirming IGR J20084+3221 as an Intermediate Polar with detailed spectral and timing characterization.

Findings

Significant X-ray spin period of 635 seconds identified.

X-ray spectrum consistent with an absorbed Bremsstrahlung model typical for IPs.

Optical emission lines indicate an accretion disk presence.

Abstract

IGR J20084+3221 is a previously unclassified Galactic source first detected by INTEGRAL. Chandra observations led to possible classifications of either a magnetic Cataclysmic Variable (mCV) or high mass X-ray binary (HMXB) based on the hardness of its spectrum. Here, we report follow-up observations taken by XMM-Newton, NuSTAR, and the Hale Telescope at Palomar Observatory. Based on these observations, we conclude that IGR J20084+3221 is most likely an Intermediate Polar (IP) type mCV. Timing analysis of the X-ray data found a significant peak period of $P=635.0\pm0.4$ s, which we interpret to be the spin period of the white dwarf (WD). The X-ray spectrum is well fit to an absorbed Bremsstrahlung model with components accounting for partial covering, reflection, and a fluorescent Fe-line, all typical for an IP. The optical spectrum shows clear emission lines, consistent with emission dominated by an accretion disk. We find counterparts to the source across the optical and infrared (IR) bands, and, despite uncertainties in the distance and extinction, we estimate that the source is too faint in the IR to be an HMXB. Given the evidence pointing towards an IP classification, we fit the X-ray spectrum to a post-shock region model where we find a WD mass of $M=1.09^{+0.12}_{-0.11}\mathrm{M}_{\odot}$, larger than the average mass for a WD in an mCV.