Friday, March 23, 2018

Bright Transient in Carina

Following the posting on the ATel #11454 about the discovery by All Sky Automated Survey for SuperNovae survey (ASAS-SN) of a new transient source, possibly a classical nova, near the Galactic plane in Carina (ASAS-SN Designation: ASASSN-18fv) I performed some follow-up of this object through a 0.43-m f/6.8 astrograph + CCD from MPC Code Q62 (iTelescope Observatory, Siding Spring).

On images taken on March 23.4, 2018 I can confirm the presence of an optical counterpart with R-filtered CCD magnitude +5.7 at coordinates:

R.A. = 10 36 15.42, Decl.= -59 35 53.7

(equinox 2000.0; UCAC-4 catalogue reference stars).

Below you can see my confirmation image (single 20-sec exposure through a 0.43-m f/6.8 astrograph + CCD), click on it for a bigger version:

While below you can see a wide-field color image (90 second exposures) I obtained on March 23.4, 2018 through a Takahashi SKY90 Apochromatic Refractor f5/6 + Color CCD. In the image ASASSN-18fv is visible at the centre of the field together with part of the Carina Nebula (click on it for a bigger version):

This transient was discovered using data from the quadruple 14-cm "Cassius" telescope in CTIO, Chile of ASAS-SN survey on images obtained on UT 2018-03-20.32 at V<10 2018-03-16.32="" also="" asas-sn="" asassn-18fv="" at="" detected="" detection="" no="" on="" possibly="" saturated="" starting="" ut="" v="">17.0) of this object in subtracted images taken on UT 2018-03-15.34 and before by the same survey. No previous outbursts or variability are detected at the position of ASASSN-18fv since ASAS-SN started observing this location in February 2016. 

According to Atel #11467 pre-discovery images have been identified on images obtained by Evryscope-South, an array of 6-cm telescopes continuously monitoring 8000 square degrees of sky at 2-minute cadence from CTIO, Chile. The transient is not detected at UT 2018-03-16.0316 with an upper-limit of 11.9 +/- 0.1 mag (g'). Beginning at UT 2018-03-16.227, they detected a new source at 10.21 +/- 0.05 mag (g'). (click on the images below for a bigger version).

Credit: Evryscope
Credit: Evryscope

An animation showing a comparison between my confirmation image and the archive POSS2/UKSTU Red plate (1991-02-10). Click on it for a bigger version:

Spectroscopy by  P. Luckas (see ATel #11460) seems to indicate that ASASSN-18fv is a classical nova brightening and in the optically thick (Fe curtain) phase. 

But according to L. Izzo et al. (ATel #11468): "The lack of a strong blue continuum, that is however typical of classical nova outburst, and the low expansion velocities suggest a possible different nature for this object. The presence of many narrow absorptions also suggests a similarity with other peculiar explosions, like the luminous red variable V4332 Sgr (Martini et al. 1999), the possible luminous red novae V838 Mon and V1309 Sco (Tylenda et al. 2011, Mason et al. 2010) or the 'helium-flash' explosion observed in the Sakurai object (Duerbeck and Benetti 1996)". Doubts about the nova nature of this object were expressed also by J. Strader et al. (ATel #11456)

So further spectroscopic observations are important to clarify the nature of this very interesting transient.

by Ernesto Guido

Monday, March 19, 2018

New Comet: C/2018 E1 (ATLAS)

CBET nr. 4494, issued on 2018, March 16, announces the discovery of an apparently asteroidal object (magnitude ~17) in the course of the "Asteroid Terrestrial-impact Last Alert System" (ATLAS) search program on CCD images obtained with a 0.5-m f/2 Schmidt reflector at Haleakala, Hawaii. Posted on the Minor Planet Center's PCCP webpage, it has been reported as showing cometary activity by CCD astrometrists elsewhere. The new comet has been designated C/2018 E1 (ATLAS). 

I performed follow-up measurements of this object, while it was still on the PCCP webpage. Stacking of 5 unfiltered exposures, 60 seconds each, obtained remotely on 2018, March 12.4 from Q62 (iTelescope network) through a 0.70-m f/6.6 reflector + CCD, shows that this object is a comet with a diffuse coma about 5 arcsec in diameter. The FWHM of this object was measured about 20% wider than that of nearby field stars of similar brightness.

My confirmation image (click on it for a bigger version)

"Pre-discovery" Panstarrs observations (2015 & 2016) were identified by R. Weryk. M.P.E.C. 2018-F10 assigns the following elliptical orbital elements to comet C/2018 E1: T 2018 Apr. 17.3; e= 0.95; Peri. =  299.47; q = 2.70;  Incl.= 72.48

by Ernesto Guido