On the X-ray efficiency of the white dwarf pulsar candidate ZTF J190132.9+145808.7

TL;DR

This study investigates the X-ray emission from the white dwarf pulsar candidate ZTF J190132.9+145808.7, providing initial evidence of X-ray signals and estimating its efficiency, suggesting it could be a source of cosmic rays like neutron star pulsars.

Contribution

First X-ray analysis of the isolated white dwarf ZTF J1901+14, estimating its X-ray efficiency and potential as a cosmic ray source, highlighting the need for further observations.

Findings

Detected marginal X-ray signals at 3.5sigma level.

Estimated X-ray efficiency comparable to neutron star pulsars.

Suggests possible curvature radiation from accelerated electrons.

Abstract

Strongly magnetized, rapidly rotating massive white dwarfs (WDs) emerge as potential outcomes of double degenerate mergers. These WDs can act as sources of non-thermal emission and cosmic rays, gethering attention as WD pulsars. In this context, we studied the X-ray emissions from ZTF J190132.9+145808.7 (hereafter ZTF J1901+14), a notable massive isolated WD in the Galaxy, using the Chandra X-ray observatory. Our results showed 3.5sigma level evidence of X-ray signals, although it is marginal. Under the assumption of a photon index of 2, we derived its intrinsic flux to be 2.3 (0.9--4.7) $\times 10^{-15}$~erg~cm$^{-2}$s$^{-1}$ and luminosity 4.6 (2.0--9.5) $\times 10^{26}$~erg~s$^{-1}$ for a 0.5--7 keV band in the 90% confidence range, given its distance of 41 pc. We derived an X-ray efficiency (eta) concerning the spin-down luminosity to be 0.012 (0.0022--0.074), a value comparable to that of ordnary neutron star pulsars. The inferred X-ray luminosity may be compatible with curvature radiation from sub-TeV electrons accelerated within open magnetic fields in the magnetosphere of ZTF J1901+14. Conducting more extensive X-ray observations is crucial to confirm whether ZTF J1901+14-like isolated WDs are also significant sources of X-rays and sub-TeV electron cosmic rays, similar to other WD pulsars in accreting systems.