Further Brightening of MASTER OT J132501.00+431846.1

ATel #10480; D. Denisenko (Sternberg Astronomical Institute, Lomonosov Moscow State University)
on 10 Jun 2017; 08:17 UT
Credential Certification: Denis Denisenko (d.v.denisenko@gmail.com)

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Large amplitude optical transient MASTER OT J132501.00+431846.1 was discovered by MASTER robotic telescopes network (P. Balanutsa et al., ATel #10470). The following unfiltered magnitude in MASTER system (calibrated against USNO-B1.0 0.2B+0.8R) was reported:

2017-06-05.955 UT 15.4m MASTER-IAC

Follow up observations were performed remotely using iTelescope.Net instruments in Nerpio, Spain (T18), Auberry, CA, USA (T24) and Mayhill, NM, USA (T11) under the full Moon conditions. The following magnitudes (unfiltered with V zero point) were obtained from selected combined images (6x60-sec exposures):

Obs. date, UT	Mag (CV)	Instrument (optics)	CCD camera
2017-06-07.305	14.83	T24 (0.61-m f/6.5)	FLI-PL09000
2017-06-07.901	14.62	T18 (0.32-m f/7.9)	SBIG-STXL-6303E
2017-06-10.190	14.26	T11 (0.50-m f/6.8)	FLI-PL11002M

V magnitudes for two reference stars were taken from APASS DR9 (A. Henden et al., 2016): GSC 03031-00475 (V=12.914) and GSC 03031-00608 (V=13.909) labeled on the chart as Ref1 and Ref2, respectively.

A total of twenty four images spanning 38 minutes were taken with T24, twenty four images in 30 minutes with T18 and twelve images in 16 minutes with T11. Object does not show any rapid variation between the individual images at the 0.02m level. The outburst amplitude is at least 9 magnitudes (assuming 23.5m limit from SDSS and PanSTARRS combined images). This is larger than even the extreme WZ Sge type outbursts. The slow rise (0.6m in 3 days) and the absence of early superhumps (confirmed by K. Kasai observations in [vsnet-alert 21107]) are also in contradiction with dwarf nova scenario.

Astrometric position (J2000.0) measured from iTelescope images is 13 25 00.89 +43 18 45.1 (+/-0.2"), in agreement with CRTS coordinates listed in VSX (13 25 00.87 +43 18 44.9). The object was detected by CRTS (A. Drake et al. 2009, ApJ, 696, 870) as MLS170606:132501+431845 at 15.4m on 2017-06-06.240-06.252 UT (~7 hours after the discovery by MASTER-IAC).

The OT is located 3.6' NW of NGC 5145 galaxy whose formal size is 55"x32", but the spiral arms can be traced to at least 1.5' from the nucleus in SDSS images (and to 2' in PanSTARRS-1 images). Values of distance modulus for NGC 5145 in HyperLeda are ranging from modz=31.70+/-0.74 (cosmological) to mod0=33.62+/-1.01 (distance measurements). If MASTER OT J132501.0+431846.1 is at the same distance as NGC 5145, its absolute magnitude would be consistent with a Supernova before the maximum light.

Spectroscopic confirmation is required to identify the nature of this remarkable OT.