Follow-up of ASASSN-14ei show He lines in the spectra and Swift/XRT X-ray detection

ATel #6475; J. L. Prieto (Universidad Diego Portales; MAS), N. Morrell (LCO), D. Grupe (Morehead State), K. Z. Stanek, C. S. Kochanek, T. W.-S. Holoien, A. B. Davis, U. Basu, J. F. Beacom (Ohio State), D. Bersier (LJMU), B. J. Shappee (Carnegie Observatories), J. Brimacombe (Coral Towers Observatory), D. Szczygiel, G. Pojmanski (Warsaw University Observatory)
on 15 Sep 2014; 21:50 UT
Credential Certification: Jose L. Prieto (jose.prietok@mail.udp.cl)

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We report follow-up optical spectra and Swift X-ray/UV/optical imaging of the unusual optical transient ASASSN-14ei (ATel #6340). We obtained medium/low resolution optical spectra on UT 2014 Jul 28.4 and Aug 01.4 with the B&C spectrograph mounted on the du Pont 2.5m telescope at LCO. The spectra show a blue continuum with relatively narrow He I and He II lines in emission at redshift z=0 (see this figure). This is consistent with a He dwarf nova (AM CVn system in outburst) classification for the object, which was proposed from the periodic variations observed in the optical light curve of ~43 min and other light curve characteristics (see vsnet-alerts 17575 and CBA news). The optical light curve of ASASSN-14ei from AAVSO and ASAS-SN shows 6 short brightening episodes since 2014 July 5 (UT July 10.4, July 24.4, August 03.4, August 14.2, August 28.2, and September 10.5) with V = 11.9-13.2 mag at peak brightness and V ~ 15-17 mag between outbursts (see the optical light curve here, with the dashed vertical lines showing the times when spectra were obtained). The times between consecutive brightening peaks in the optical light curve observed so far is between 10 and 14 days.

We also obtained TOO observations of ASASSN-14ei with Swift UVOT and XRT on four epochs between UT July 24.9 and UT July 26.5, within 2 days of the peak of the second recorded outburst. The source was bright and blue in UVOT photometry, with v=12.35 +/- 0.04, b=12.15 +/- 0.03, uvw1=10.27 +/- 0.04, uvm2=10.08 +/- 0.04, uvw2=10.04 +/- 0.04 on UT July 26.1. It was also strongly detected in X-rays with the XRT in 1ks exposures. The XRT data is well-modeled with a power-law with photon index of 1.7-2.0 and flux of 3.7-4.2 x 10^-12 erg/cm²/s (0.3-10 keV).

We thank the Swift team for making these observations possible. For more information about the ASAS-SN project see the ASAS-SN Homepage and also ASAS-SN Transients.