Bright Tanscient (mag 10): ASASSN-18su

Bright Tanscient (mag 10): ASASSN-18su

Postby Francois Teyssier » Sat Aug 18, 2018 6:37 pm

Spectrum should be welcome for identification (nova or dwarf nova in ourburst?)

The Astronomer's Telegram

ATEL #11968 ATEL #11968

Title: ASAS-SN Discovery of a Very Bright (g~10.1) Transient ASASSN-18su
Author: K. Z. Stanek, C. S. Kochanek, J. V. Shields, T. A. Thompson
(OSU), L. Chomiuk, J. Strader (MSU), B. J. Shappee (IfA-Hawaii), T.
W.-S. Holoien (Carnegie Observatories), J. L. Prieto (Diego Portales;
MAS), Subo Dong (KIAA-PKU), M. Stritzinger (Aarhus)
Posted: 18 Aug 2018; 17:43 UT
Subjects:Optical, Transient

During the ongoing All Sky Automated Survey for SuperNovae (ASAS-SN,
<a href="">Shappee
et al. 2014</a>), using data from the quadruple 14-cm
"Payne-Gaposchkin" telescope in Sutherland, South Africa, we detect a
very bright, new transient source, likely an outburst of a nearby
cataclysmic variable

Object RA (J2000) DEC (J2000) Disc. UT Date Disc. g mag

ASASSN-18su 12:57:51.258 -28:30:16.82 2018-08-17.71 10.1

ASASSN-18su was discovered in images obtained on UT 2018-08-17.71
at g~10.1, and it is also detected at V~10.0 on UT 2018-08-17.97. We
do not detect (V>18.0) this object in subtracted images taken on UT
2018-08-14.98 and before.

Using <a
href=>ASAS-SN Sky Patrol</a> light curve
interface (<a
href=>Kochanek et
al. 2017)</A>, we have retrieved aperture photometry time series at
the location of ASASSN-18su in the last 30 days, and the resulting
V-band light curve can be seen <a

> here</a>. No previous outbursts</a> are detected at the position of
ASASSN-18su since ASAS-SN started observing this location in December

Follow-up observations, especially spectroscopy and rapid cadence
photometry, are strongly encouraged.

We would like to thank Las Cumbres Observatory and its staff for
their continued support of ASAS-SN. ASAS-SN is funded in part by the
Gordon and Betty Moore Foundation through grant GBMF5490 to the Ohio
State University, NSF grant AST-1515927, the Mt. Cuba Astronomical
Foundation, the Center for Cosmology and AstroParticle Physics (CCAPP)

at OSU, the Chinese Academy of Sciences South America Center for
Astronomy (CASSACA), and the Villum Fonden (Denmark).
Francois Teyssier
Posts: 1347
Joined: Fri Sep 23, 2011 1:01 pm
Location: Rouen

Re: Bright Tanscient (mag 10): ASASSN-18su

Postby Francois Teyssier » Mon Aug 20, 2018 5:20 am

The spectrum obtained by Paul Luckas
Cleraly a dwarf nova in outburst

_asassn-18su_20180819_440_Paul Luckas.png
_asassn-18su_20180819_440_Paul Luckas.png (5.31 KiB) Viewed 2077 times

Comments about the spectrum, by Steve Shore:

It's interesting that He I 5876,6678 are in weak emission (oddly there's no 7065), the emission is centered on, and about the same width as, the Halpha; there might be a hint of something at 4471 but not much. The Balmer decrement is very steep, there's no He II that I can see (or, rather, that we'd believe) and the Balmer spectrum is very strong. It's cute that this looks like a cataclysmic but the lack of He II is suspicious. The Halpha/Hbeta ratio is about 12, which would be normal for a heavily reddened object ut not this so, again, I suspect a disk (this isn't recombination). The FWZI for both emission lines is about 1600-1700 km/s. The same is true for He I 5876. The Na I D is weaker than I'd thought, going backover your spectrum and including the effects of resolution it wouldn't make he line so unsaturated so I think this agrees with what the XR picture is giving, that the neutral hydrogen column density is small. It might, in fact, be a Gaia source; there's a 17th mag source at the position within 5 arcsec but there's an offset, it has a parallax of 5.5 mas so this would be right for a cataclysmic).
Francois Teyssier
Posts: 1347
Joined: Fri Sep 23, 2011 1:01 pm
Location: Rouen

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