Rapid Orbital Decay in the Ultracompact Double-degenerate Binary eRASSU J060839.5–704014

Abstract

We present timing and spectral analysis of the recently identified ultracompact double-degenerate (DD) white dwarf binary eRASSU J060839.5–704014 using observations from NICER and Einstein Probe (EP), together with archival XMM-Newton data. By phase-connecting the long-term XMM-Newton, NICER, and EP observations, we obtain a coherent quadratic timing solution, yielding an orbital period of 374.15013 (2) s and an orbital decay rate of Ṗ=−4.7(1)×10−11ss−1 . This orbital decay exceeds that measured in the prototypical DD binaries HM Cnc and V407 Vul. Assuming that the observed orbital evolution is primarily driven by gravitational-wave (GW) angular momentum loss, the inferred chirp mass is ∼0.43 M⊙, placing the source among the most massive known systems of this class. The phase-averaged spectra of NICER and EP-Follow-up X-ray Telescope (FXT) are described by a soft thermal component with temperatures of ∼126 and ∼144 eV, respectively, confirming the supersoft nature of the source. Phase-resolved spectroscopy reveals a clear decrease in temperature across the bright phase in both instruments, indicating a structured emission region with significant temperature gradients. These results establish eRASSU J060839.5–704014 as one of the most rapidly evolving ultracompact DD binaries presently known, belonging to the rare class of direct-impact ultracompact binaries, and a promising verification source for future low-frequency GW studies.