Timing the X-ray pulsating companion of the hot-subdwarf HD 49798 with NICER

Abstract

HD 49798 is a hot subdwarf of O spectral type in a 1.55 day orbit with the X-ray source RX J0648.0-4418, a compact object with spin period of 13.2 s. We use recent data from the NICER instrument, joined with archival data from XMM-Newton and ROSAT, to obtain a phase-connected timing solution spanning ~30 years. Contrary to previous works, that relied on parameters determined through optical observations, the new timing solution could be derived using only X-ray data. We confirm that the compact object is steadily spinning up with Pdot = -2.28(2)x10^-15 s/s and obtain a refined measure of the projected semi-major axis of the compact object aX sini = 9.60(5) lightsec. This allows us to determine the inclination and masses of the system as i = 84.5(7) deg, MX = 1.220(8) Msun and Mopt = 1.41(2) Msun. We also study possible long term (~year) and orbital variations of the soft X-ray pulsed flux, without finding evidence for variability. In the light of the new findings, we discuss the nature of the compact object, concluding that the possibility of a neutron star in the subsonic propeller regime is unlikely, while accretion of the subdwarf wind onto a massive white dwarf can explain the observed luminosity and spin-up rate for a wind velocity of ~800 km/s.

Accepted for publication in MNRAS, 7 pages, 4 figures, 2 tables

References in corpus (10)

• $Gaia$ white dwarfs within 40 pc II: the volume-limited northern hemisphere sample
• An ultra-massive fast-spinning white dwarf in a peculiar binary system
• Model X-ray Spectra of Magnetic Neutron Stars with Hydrogen Atmospheres
• The hybrid CONe WD + He star scenario for the progenitors of type Ia supernovae
• A young contracting white dwarf in the peculiar binary HD 49798/RX J0648.0--4418?
• The Rapid Rotation of the Strongly Magnetic Ultramassive White Dwarf EGGR 156
• HD 49798: Its History of Binary Interaction and Future Evolution
• New X-ray observations of the hot subdwarf binary HD49798 / RXJ0648.0-4418
• Hot subdwarf wind models with accurate abundances I. Hydrogen dominated stars HD 49798 and BD+18${}^{\circ }$2647
• A blazar as the likely counterpart to 4FGL J0647.7-4418 instead of a gamma-ray binary