TIC 378898110: A bright, short-period AM CVn binary in TESS
Kepler, S. O.; Hermes, J. J.; Dhillon, V. S.; Parsons, Steven G.; Pelisoli, Ingrid; Munday, James; Marsh, T. R.; Littlefair, S. P.; Breedt, Elmé; Dyer, Martin J.; Green, Matthew J.; Kennedy, Mark R.; Kerry, Paul; Gänsicke, Boris T.; Ashley, R. P.; Baran, Andrzej S.; Grupe, Dirk; Barlow, Brad N.; King, George W.; Sahman, David I.; Brown, Alex J.; Kaiser, Ben C.; Romero, Alejandra; Amaral, Larissa Antunes; Corcoran, Kyle
Germany, Israel, United Kingdom, United States, Brazil, Chile, Ireland, Spain, Poland
Abstract
AM CVn-type systems are ultracompact, helium-accreting binary systems that are evolutionarily linked to the progenitors of thermonuclear supernovae and are expected to be strong Galactic sources of gravitational waves detectable to upcoming space-based interferometers. AM CVn binaries with orbital periods ≲20-23 min exist in a constant high state with a permanently ionized accretion disc. We present the discovery of TIC 378898110, a bright (G = 14.3 mag), nearby (309.3 ± 1.8 pc), high-state AM CVn binary discovered in TESS two-minute-cadence photometry. At optical wavelengths, this is the third-brightest AM CVn binary known. The photometry of the system shows a 23.07172(6) min periodicity, which is likely to be the 'superhump' period and implies an orbital period in the range 22-23 min. There is no detectable spectroscopic variability. The system underwent an unusual, year-long brightening event during which the dominant photometric period changed to a shorter period (constrained to 20.5 ± 2.0 min), which we suggest may be evidence for the onset of disc-edge eclipses. The estimated mass transfer rate, $\log (\dot{M} / \mathrm{M_\odot } \, \mathrm{yr}^{-1}) = -6.8 \pm 1.0$, is unusually high and may suggest a high-mass or thermally inflated donor. The binary is detected as an X-ray source, with a flux of $9.2 ^{+4.2}_{-1.8} \times 10^{-13}$ erg cm-2 s-1 in the 0.3-10 keV range. TIC 378898110 is the shortest-period binary system discovered with TESS, and its large predicted gravitational-wave amplitude makes it a compelling verification binary for future space-based gravitational wave detectors.