Optical Spectroscopy and Swift XRT/UVOT Observations of ASASSN-14ax: Evidence of a Type Ia Supernova

ATel #6144; Ju-Jia Zhang and Jin-Ming Bai (Yunnan Observatories (YNAO); Wenda Zhang and Wenfei Yu, Shanghai Astronomical Observatory (SHAO)
on 16 May 2014; 05:08 UT
Distributed as an Instant Email Notice Supernovae
Credential Certification: Wenfei Yu (wenfei@shao.ac.cn)

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We report on an optical spectrogram (range 340-890 nm) of ASASSN-14ax (R.A. = 17h10m00s.70, Decl. = +27°06'19".6 ) that was obtained on May 15.85 UT with the 2.4-m telescope (+ YFOSC) at the LiJiang observatory of YNAO. The spectrum is consistent with a type-Ia supernova around maximum. This is an independent evidence that ASASSN-14ax is a type Ia supernova (see also ATEL #6143), which rules out other origins such as a tidal disruption event. Cross-correlation with a library of supernova spectra using the "Supernova Identification" code (SNID; Blondin and Tonry 2007, Ap.J. 666, 1024) shows that it matches with SN 2007co at +4 days. A redshift of 0.033 +/- 0.001 can be inferred for the host galaxy based on the narrow emission lines. Adopting this redshift for the host galaxy, we obtained a velocity of the Si II 635.5-nm absorption feature to be about 11660 km/s. The detailed spectrum is located at here (http://202.127.29.4/ToO/ASASSN_14ax_comp0001_snidflux.png)

Swift has taken three observations of SDSS J171000.70+270619.6, at 2014-05-13UT15:47:22, 2014-05-14UT00:12:14 and 2014-05-14UT06:27:23 with ObsIDs of 00033284001, 00033284004 and 00033284003, respectively. On the stacked Swift/XRT 0.3-10 keV image with a total exposure of 3983 s, we detected only one source positioned at 17 09 59.3,+27 06 18.9 (J2000) using Ximage/detect, which lies ~23'' away from the optical position reported in Atel #6133. For the source at the optical position we estimated an 3-sigma upper limit on count rate of 4.70E-3 cts/s using Ximage/sosta. Assuming a spectrum of power-law with photon index of 2.0 in 0.3-10 keV band and with an absorption by Galactic neutral Hydrogen, we estimated an upper limit on the source flux of 1.61E-13 erg/s/cm^2 using WebPIMMS, corresponding to 1.30E42 erg/s as the upper limit on 0.3-10 keV luminosity adopting a luminosity distance of 260 Mpc (Atel #6133).

For Swift/UVOT observations, the source was observed in UVM2 band during the first observation, all 6 bands of UVOT during the second, and UVW1 during the last. From the second to the last observation, the source was seen to decline from 17.31+/-0.05 mag to 17.44+/-0.05 mag in the UVW1 band. During the second observation, a dip of amplitude of ~0.2 mag was seen in U band. No significant variability was seen in other bands.