Multi-band light curve analysis of the 40.5-minute period eclipsing double-degenerate binary SDSS J082239.54$+$304857.19

Abstract

We present the Apache Point Observatory BG40 broadband and simultaneous Gemini $r$-band and $i$-band high-speed follow-up photometry observations and analysis of the 40.5 minute period eclipsing detached double-degenerate binary SDSS J082239.54$+$304857.19. Our APO data spans over 318 days and includes 13 primary eclipses, from which we precisely measure the system's orbital period and improve the time of mid-eclipse measurement. We fit the light curves for each filter individually and show that this system contains a low-mass DA white dwarf with radius $R_A=0.031\pm 0.006{\mathrm{R}}_{\odot }$ and a $R_B=0.013\pm 0.005{\mathrm{R}}_{\odot }$ companion at an inclination of $i=87.7\pm 0.2^{\circ }$. We use the best-fitting eclipsing light curve model to estimate the temperature of the secondary star as $T_{\mathrm{eff}}=5200\pm 100\mathrm{K}$. Finally, while we do not record significant offsets to the expected time of mid-eclipse caused by the emission of gravitational waves with our 1-year baseline, we show that a $3\sigma$ significant measurement of the orbital decay due to gravitational waves will be possible in 2023, at which point the eclipse will occur about $8$ seconds earlier than expected.

8 pages, 8 figures, 2 tables. Accepted for publication in MNRAS

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