Double White Dwarf Mergers as Progenitors of Long-Period Transients

Abstract

There is an ongoing discussion in the literature on the nature of long-period transients (LPTs), radio-emitting sources with periods ranging from hundreds to tens of thousands of seconds. Although some of these objects have been identified as white dwarf (WD) + M-dwarf binaries, this description currently does not fit the entire class. An example is GLEAM-X J162759.5-523504.3 (hereafter GLEAM-X J1627-5235), with a period of 1091 s, for which the lack of an optical counterpart disfavors the presence of such a binary system. In this case, GLEAM-X J1627-5235 could be interpreted as an isolated, massive, fast-rotating, and highly magnetized (~ 1e+9 G) WD pulsar. Its properties are consistent with a carbon-oxygen WD of mass ~1.3 Msun and radius ~2500 km, possibly supported by small-scale multipolar magnetosphere structures that keep it above the death line for WD-pulsars. We assess a double WD merger origin, modeling the post-merger rotational evolution under accretion, propeller, and magnetic braking torques. We find rotational age of ~572 Myr for GLEAM-X J1627-5235, i.e., the post-merger time required to reach its observed period. This result is consistent with current optical upper limits for GLEAM-X J1627-5235 and support the WD pulsar interpretation for this source. We also discuss how the same model can apply to other LPTs.

Figures (5)

• Figure 1: Left: Magnetic field limits set by the death line for WD-pulsars. Black-dashed curve: $r_c/R = 0.001$. Black-dotted curve: $r_c/R=0.01$. Other parameters are $R=2500$ km ($M\approx 1.3\,\text{M}_\odot$) and $h = h_{\rm max}=r_p$. Period and magnetic field values of magnetic WDs (red circles) are from Table \ref{['tab:LPT_WDs']}, while the upper limits of magnetic field for LPTs (blue triangles) are from Table \ref{['tab:LPT_WDs']}. Exceptionally, CHIME J0630+25 has a measured spindown rate, and thus the magnetic field is not an upper limit but an estimate based on dipole braking. Right: Electric potential drop limits set by the death line for WD-pulsars. Black-dashed curve: $r_c/R = 0.001$. Black-dotted curve: $r_c/R=0.01$. Other parameters are as in the left panel.
• Figure 2: Curve of the minimum age from the optical upper limits for each mass between 1.10 and 1.382 M$_{\odot}$ (blue) for GLEAM-X J1627--5235. The shaded area reflects uncertainties arising from distance and extinction.
• Figure 3: Evolution of the rotation period of GPM J1839--10 for an accretion rate of $\dot{M} = 2.0 \times 10^{-7} M_{\odot}$ yr$^{-1}$ and for different values of initial rotational periods, $P_{0} = (900, \, 1100, \, 1200, \, 1400, \, 1500, \, 1700)$ s. The dotted lines divide the evolution into three stages according to the value of $\omega$. The red dash-dotted line indicates the equilibrium period $P_{\rm eq} = 1284.8$ s. The green dashed line indicates the current rotation period of the WD.
• Figure 4: Evolution of the rotation period of GLEAM-X J1627--5235 for an accretion rate of $\dot{M} = 3.35 \times 10^{-7} M_{\odot}$ yr$^{-1}$ and for different values of initial rotational periods, $P_{0} = (800, \, 900, \, 1000, \, 1200, \, 1300, \, 1400)$ s. The dotted lines divide the evolution into three stages according to the value of $\omega$. The red dash-dotted line indicates the equilibrium period $P_{\rm eq} = 1030.0$ s. The green dashed line indicates the current rotation period of the WD.
• Figure 5: CDF of Gaia$M_G$ absolute magnitudes for WDs (bottom x-axis). The top x-axis shows the corresponding apparent $G$-band magnitudes for GLEAM-X J1627–5235, considering its estimated distance and extinction. The red and black dashed lines mark the magnitudes below which 90% and 99% of the WD population lie, respectively.