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

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. 5σ 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) × 10−15 erg cm−2 s−1 and luminosity 4. 6 (2. 0–9. 5) × 1026 erg s−1 for a 0. 5–7 keV band in the 90\% confidence range, given its distance of 41 pc. We derived the X-ray efficiency (η) concerning the spin-down luminosity to be 0. 012 (0. 0022–0. 074), a value comparable to that of ordinary 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.