The first extragalactic ultra-compact X-ray binary : a candidate black hole-white dwarf system

Abstract

M31 UCXB-1 is one of the brightest X-ray point sources in the bulge of M31, with a peak X-ray luminosity $ L_{\mathrm{0.5-10 \: keV}} = 2.9^{+0.2}_{-0.2} \times 10^{38} \: \mathrm{erg} \: \mathrm{s}^{-1} $. Both XMM-Newton and Chandra observations have detected an eclipsing signal with a period of about 465 seconds from this source, and we note that the periodic signal is detected exclusively during the source's high-luminosity states. This signal probably originates from its orbital motion, therefore it is an ultra-compact X-ray binary (UCXB) candidate with the highest X-ray luminosity. Our theoretical analyses show that M31 UCXB-1 is in good agreement with the luminosity-orbital period relation ($ L_{\mathrm{2-10 \: keV}}-P_{\mathrm{orb}} $) of the black hole/neutron star--white dwarf (BH/NS--WD) UCXB system. Moreover, our spectral analyses indicate that the primary in M31 UCXB-1 is more likely to be a BH rather than an NS. The results show that M31 UCXB-1 is a BH--WD system, with the shortest orbital period, the possibly strongest gravitational wave emission, and the most massive white dwarf among the known UCXBs.

Figures (8)

• Figure 1: The folded light curves of five observations and the normalized Lomb-Scargle periodogram (A) The five light curves are folded with the period of 465.32 sec. The modulation is stable and the folded profiles from different epochs align in phase, demonstrating the coherence of the signal. (B) The normalized Lomb-Scargle periodogram, displaying a narrow peak at the corresponding frequency with distinct harmonics, confirming the phase stability and orbital nature of the periodicity.
• Figure 2: The folded light curve from XMM-Newton by combining five observations and a schematic picture of the orbital period The folded light curve exhibits a 465.32 sec period, which originates from the periodic obscuration of the accretion disk by the white dwarf.
• Figure 3: The $L_{\mathrm{2-10 \: keV}}-P_{\mathrm{orb}}$ diagram The location of M31 UCXB-1 (red dot, $L_{\mathrm{2-10 \: keV}} = 1.2^{+0.2}_{-0.2} \times 10^{38}$ erg $\mathrm{s}^{-1}$, $P_{\mathrm{orb}} = 465^{+3.43}_{-2.99} \: \mathrm{s}$) and the known UCXBs (black dots) are shown in this figure, with the theoretical predictions for NS--WD (green bands) and BH--WD systems (orange bands). Except for M31 UCXB-1 and 47 Tuc X-9, which are BH candidates, the other known UCXBs in this diagram are all NS systems. The green and orange bands are obtained by assuming that $M_{\mathrm{BH}} = 10 \: M_{\odot}$ and $M_{\mathrm{NS}} = 1.4 \: M_{\odot}$, with the bolometric correction factor $\lambda = L_{\mathrm{2-10 \: keV}} / L_{\mathrm{bol}} \sim 0.1-0.5$.
• Figure 4: The ranges of $\dot{m}$ as a function of $M_{1} / M_{\odot}$ and $P_{\mathrm{orb}}$ The $M_{1} / M_{\odot} < 3$ region corresponds to the NS$-$WD systems, while $M_{1} / M_{\odot} > 3$ region corresponds to the BH$-$WD systems. The darker region corresponds to higher $\dot{m}$. For the condition that $P_{\mathrm{orb}} = 465 \: \mathrm{s}$, the NS$-$WD system is super-Eddington accretion, while for BH$-$WD is sub-Eddington accretion.
• Figure 5: XMM-Newton spectral fitting results for periodic (red) and low-state (black) observations Periodic-state and low state spectra are fitted with an absorbed multicolor disk + power-law model (TBabs*(diskbb+powerlaw)). The red points and curve correspond to high-luminosity state, and the black points and fit illustrate the low/hard state.