Hard X-Ray Emission Associated with White Dwarfs. IV. Signs of Accretion from Substellar Companions

You Hua Chu, Jesús A. Toalá, Martín A. Guerrero, Florian F. Bauer, Jana Bilikova, Robert A. Gruendl

Research output: Contribution to journalArticlepeer-review

Abstract

With an effective temperature of ≃200,000 K, KPD 0005+5106 is one of the hottest white dwarfs (WDs). ROSAT unexpectedly detected "hard"(∼1 keV) X-rays from this apparently single WD. We have obtained Chandra observations that confirm the spatial coincidence of this hard X-ray source with KPD 0005+5106. We have also obtained XMM-Newton observations of KPD 0005+5106, as well as PG 1159-035 and WD 0121-756, which are also apparently single and whose hard X-rays were detected by ROSAT at 3σ-4σ levels. The XMM-Newton spectra of the three WDs show remarkably similar shapes that can be fitted by models including a blackbody component for the stellar photospheric emission, a thermal plasma emission component, and a power-law component. Their X-ray luminosities in the 0.6-3.0 keV band range from 4 × 1029 to 4 × 1030 erg s-1. The XMM-Newton EPIC-pn soft-band (0.3-0.5 keV) light curve of KPD 0005+5106 is essentially constant, but the hard-band (0.6-3.0 keV) light curve shows periodic variations. An analysis of the generalized Lomb-Scargle periodograms for the XMM-Newton and Chandra hard-band light curves finds a convincing modulation (false-alarm probability of 0.41%) with a period of 4.7 ± 0.3 hr. Assuming that this period corresponds to a binary orbital period, the Roche radii of three viable types of companion have been calculated: M9V star, T brown dwarf, and Jupiter-like planet. Only the planet has a size larger than its Roche radius, although the M9V star and T brown dwarf may be heated by the WD and inflate past the Roche radius. Thus, all three types of companion may be donors to fuel accretion-powered hard X-ray emission.

Original languageEnglish (US)
Article number119
JournalAstrophysical Journal
Volume910
Issue number2
DOIs
StatePublished - Apr 1 2021

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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