SALT high-resolution optical spectroscopy of ASASSN-18abj

ATel #12311; E. Aydi (MSU), D. A. H. Buckley (SAAO), L. Chomiuk, A. Kawash (MSU), k. Mukai (GSFC), K. Sokolovsky, and J. Strader (MSU)
on 20 Dec 2018; 05:06 UT
Credential Certification: Elias Aydi (eaydi@saao.ac.za)

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We report on high-resolution optical spectroscopy of ASASSN-18abj (AT 2018jro) which was discovered on 2018-12-12.109 UT by the ASAS-SN survey and confirmed as a dwarf nova by F. Walter (ATel #12295).

We obtained two spectra of this object under the SALT Large Science Program on transients on 2018 December 12.9 and 14.9 UT, using the High Resolution Spectrograph (HRS; Crause et al. 2014, Proc. SPIE, 91476) mounted on the Southern African Large Telescope (SALT). Observations were taken in the LR mode of HRS, covering a spectral range of 3800-8900 A at a resolution of R = 14000. The data were reduced with the SALT HRS MIDAS pipeline (Kniazev et al. 2016, MNRAS 459, 3068).

The spectra show a blue continuum with relatively strong Halpha, Hbeta, and relatively weak He I and He II lines in emission (see below a list of the line identifications). The emission lines are characterised by double emission peaks (separated by ~ 135 km/s for Hbeta and ~ 100 km/s for Halpha) and a heliocentric corrected Vr ~ -15 +/- 5 km/s (for the line centre). We measure a FWZI ~ 450 +/- 20 km/s for Hbeta and ~ 420 km/s for Halpha. The emission feature of Hbeta is superimposed on a relatively broad absorption feature (FWZI ~ 2000 +/- 200 km/s). The weak emission features of He I (4471, 4922, and 5016 A) are also superimposed on a broad emission feature. In addition broad absorption features of Hgamma and Hdelta are also present. Overall, the spectrum is typical for a dwarf nova in outburst. The EW of the Na I D interstellar absorption doublet is consistent with low extinction towards the source (Av ~ 0.06), which is considerably lower than the total reddening value (Av~ 2.6) in this direction from the Galactic reddening maps by Schlafly & Finkbeiner (2011, ApJ, 737, 103). This indicates that the dwarf nova is nearby, in agreement with the high optical and UV brightness of the outburst and negligible X-ray extinction (ATel #12294).

 
Emission line identifications: 
H I (Balmer): 4861 and 6563 A 
He I: 4471, 4922, 5016, 5876, 6678, and 7065 A 
He II: 4686 A