Updated Ephemeris and Evidence for a Period Change in the Eclipsing Novalike Variable 1RXS J064434.5+334451

The bright (V ∼ 13.5) novalike variable 1RXS J064434.5+334451 (hereafter J0644) was discovered to be an eclipsing binary by Sing et al. (2007). Their photometric observations over 16 nights between 2005 February 2 UT and 2006 October 13 UT revealed deep (1–1.2 mag) eclipses that recurred with an orbital period, P_orb = 6. 4649808 ± 0.0000060 hr. Five years later Boyd (2012) used 22 new eclipse timings, along with 20 unpublished eclipse timings from 2005 to 2008 (made available by D. Sing and B. Green from their original study) to refine the period and determine an eclipse ephemeris given by: HJD_ecl = 2453403.75955(12) + 0.26937447(4) E. Unfortunately, the 20 unpublished eclipse timings remained unavailable, as they were not reproduced in the Boyd (2012) paper.

The most recent determination of the eclipse ephemeris was made by Hernández Santisteban et al. (2017). These authors acquired the original data for four eclipses from the work of Sing et al. (2007) and remeasured the times of mid-eclipse. They then combined these timings with a total of 10 additional timings that were obtained from eclipses observed in 2008 January and 2010 November and December finding HJD_ecl = 2,453,403.759533 + 0.269,374,46 E, which is nearly identical to the earlier ephemeris. The similarity is remarkable given that these authors were apparently unaware of the 22 timings that were reported in the work of Boyd (2012).

In this Research Note, we have compiled all previously published times of mid-eclipse (i.e., from Boyd 2012 and Hernández Santisteban et al. 2017), including the 20 previously unpublished timings kindly provided by David Sing from his original work (Sing et al. 2007). We have used these timings, along with seven new eclipse timings that we have recently measured to update the ephemeris for J0644 and to check the stability of the orbital period. Of these seven timings, one is based on observations taken with the Transiting Exoplanet Survey Satellite (TESS).

We obtained time-resolved photometric observations of J0644 on six nights in early 2021: February 21, March 5, 26, 27, and April 8, 9 UT. The observations were arranged to span the eclipse times predicted by the ephemeris of Hernández Santisteban et al. (2017). The 2021 February 21 and March 5 UT data were obtained with the Boyce-Astro Research Initiatives and Education Foundation (BRIEF) BARO Telescope, which is a 17-in f/6.8 Corrected Dall-Kirkham Astrograph instrument located in Tierra Del Sol, California and operated remotely. Observations on the remaining nights were made with the facilities of the Las Cumbres Observatory (LCO). The 2021 March 27 UT data were obtained with the LCO telescope at McDonald Observatory, with the rest being obtained at the Haleakala facility. All observations consist of 30 s exposures through a Sloan r filter.

In addition to our ground-based eclipse observations, TESS observed a total of 86 (mostly sequential) eclipses over the interval between 2019 December 25 and 2020 January 19 UT. Given that the TESS eclipses were concentrated over a short time span, with relatively poor temporal resolution (∼2 minutes) and signal-to-noise ratio during eclipse, we have determined a single fiducial eclipse timing for the TESS observations by fitting a linear function to the individual times of mid-eclipse and taking the intercept as the best measure of T_mid-ecl for this epoch. Times of mid-eclipse for both the ground-based and TESS observations were determined by fitting a parabola to the bottom half (as defined by the inflection points on ingress and egress) of each of the observed eclipses.

With the addition of our new eclipse measurements, we now have a total of 59 times of mid-eclipse for J0644: 14 from Hernández Santisteban et al. (2017), 22 from Boyd (2012), 16 previously unpublished timings provided by D. Sing, along with the 7 from the present study. A linear least squares fit of all 59 times of mid-eclipse yields the following ephemeris: BJD_ecl = 2,453,403.7611(2) + 0.269,374,43(2) E. The individual times of mid-eclipse and their residuals from this ephemeris are given in the machine-readable table associated with this Research Note. The residuals are also plotted in the O − C diagram shown in Figure 1. The distribution of the residuals suggest that the orbital period of J0644 may be slowly decreasing. A quadratic fit to all 59 times of mid-eclipse yield the following ephemeris: BJD_ecl = 2,453,403.7598 + 0.269,374,87 E − 2.0 × 10⁻¹¹  E².

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In order to estimate reduced χ² values of the fits, which are necessary to assess whether the improved quadratic fit is warranted, uncertainties must be specified for the individual eclipse timings. To avoid introducing unknown biases into the analysis, we have assumed that all eclipse timings are uncertain by 60 s (0.0007 day). A comparison of the reduced χ² values for the two fits (χ² = 3.5 and χ² = 1.8 for the linear and quadratic fits, respectively) suggests that neither fit provides a particularly good match to the eclipse timings, which is not necessarily surprising given that the uncertainties in the eclipse timings are poorly known and may be somewhat larger than we have assumed in our analysis. Nevertheless, if the eclipse timings and their associated errors are taken at face value, it appears that the quadratic fit offers a marginally significant improvement. In this case, we find that the orbital period of J0644 may be decreasing at a rate given by: $\dot{P}\simeq -1.5\times {10}^{-10}$. Future eclipse observations of J0644 will be required to better establish the stability of the orbital period.

We thank David Sing for providing unpublished eclipse timings from his previous work (Sing et al. 2007), Pat Boyce of BRIEF for access to the BARO Telescope, Michael Fitzgerald of the Our Solar Siblings project for assistance with image processing, and the Las Cumbres Observatory for a generous allocation of observing time.

Facilities: BARO Telescope - , Las Cumbres Observatory. -