Photometric and Spectroscopic Observations of the New Eclipsing Polar ASASSN-16me

ATel #9764; Colin Littlefield (Notre Dame), Lewis M. Cook (CBA Concord), David Bersier (LJMU), Shane Davitt (Notre Dame), David Kalamarides (Notre Dame)
on 16 Nov 2016; 19:32 UT
Credential Certification: Colin Littlefield (clittlef@alumni.nd.edu)

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We have obtained time-resolved photometry and low-resolution spectroscopy of the optical transient ASASSN-16me, which we classify as an eclipsing polar. In such a system, a highly magnetized white dwarf (WD) accretes from a nearby companion star, and its magnetic field channels the accretion flow onto a compact accretion region on the WD's photosphere. Despite its compactness, the accretion region accounts for a significant fraction of the system's optical luminosity. Thus, an eclipse of the accretion region by the companion star is deep and shows extremely abrupt ingresses and egresses (e.g., Harrop-Allin et al. 1999, MNRAS, 308, 807).

For our photometric observations, we observed on four consecutive nights between 2016 November 4-7 using a combination of two telescopes: the University of Notre Dame's 80-cm Sarah L. Krizmanich Telescope and the 74-cm reflector at the Center for Backyard Astrophysics, Concord. All observations were unfiltered with a Johnson V zeropoint and were corrected to the Barycentric Julian Date in barycentric dynamical time (Eastman, Siverd, & Gaudi 2010, PASP, 122, 935).

The most striking feature of the light curve is a very deep (> 2 mag) and abrupt eclipse which is characteristic of an eclipsing polar. The eclipse ingresses and egresses are sufficiently brief that our fastest cadence of 15 sec per image could not resolve either of these transitions. During the 7-minute-long eclipse, the system fell below the detection threshold of our equipment (V~19.4). By measuring the times of eclipse ingress and egress, we find that the eclipses follow an ephemeris of T_mid[BJD] = 2457696.5407(2) + 0.066458(6)E.

When not in eclipse, ASASSN-16me varies between V ~ 17.2-18.6 in our data, although there is a great deal of cycle-to-cycle variation for both the maximum and minimum magnitude. The orbital light curve consists of a prominent, ~1.4-mag depression between orbital phases 0.30-0.63, a feature which likely corresponds with the disappearance of the accretion region behind the limb of the WD. During the bright part of the orbit, there is a wide, 0.6-mag dip of unknown origin between orbital phases 0.8 and 1.04. Although eclipsing polars sometimes show pre-eclipse dips when the accretion flow obscures the accretion region, those dips are usually significantly narrower and better-defined than the dip in ASASSN-16me.

We also obtained a low-resolution optical spectrum with the 2-m Liverpool Telescope on 2016 October 31, covering a wavelength range of 402-799 nm at a resolution of ~1.8 nm. The exposure time was 500 seconds. The spectrum displays a noisy continuum punctuated by prominent, single-peaked H-alpha and H-beta emission. There is some evidence of He II 468.6-nm emission, but neither it nor any He I lines can be confidently distinguished from the noise. The apparent weakness of He II in relation to H-beta is surprising because strong He II emission is a hallmark of polars. However, because the spectrum was centered on the start of the depression at orbital phase 0.3, it is possible that the He II-emitting regions were partially hidden behind the limb of the WD when the spectrum was taken. Higher signal-to-noise-ratio spectroscopy would provide more insight into the relative strengths of H, He I, and He II.

Although the spectrum is ambiguous, it is not inconsistent with our photometric classification of ASASSN-16me as an eclipsing polar.

Phase Plot & Spectrum