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BRITE nanosatellite serendipitously captures oscillatory rise and fall of ASASSN-18fv

ATel #11508; R. Kuschnig (TUGraz), A. Pigulski (UWroclaw), A. F.J. Moffat (UdeM), J. M. Matthews (UBC), K. Zwintz (UInnsbruck), D. Baade (ESO), G. Handler (CAMK), W. W. Weiss (UVienna), G. A. Wade (RMC), S. M. Rucinski (UofT), H. Pablo (AAVSO), O. Koudelka (TUGraz), R. Smolec (CAMK), A. Popowicz (KPLabs/FP pace), C. Neiner (OBSPMeudon), J. Daszynska-Daszkiewicz (UWroclaw), C. Lovekin (MTA), N. St-Louis (UdeM), A. A. Pamyatnykh (CAMK), J. Rowe (UBishops), P. Orleanski (SRC), S. Mochnacki (UofT), A. Schwarzenberg-Czerny (CAMK)
on 6 Apr 2018; 10:46 UT
Credential Certification: Rainer Kuschnig (rainer.kuschnig@tugraz.at)

Subjects: Optical, Nova, Transient

Referred to by ATel #: 11546, 11553, 11677

One of the five satellites in BRITE-Constellation (http://www.brite-constellation.at/) was obtaining time-series optical photometry of the star HD 92063, only 2 arcmin from the 'Possible, Very Bright Galactic Nova ASASSN-18fv' reported on 21 March 2018 (2018-03-20.32) by ATel #11454. The satellite BRITE-Toronto began monitoring HD 92063 on 15 Feb 2018 (2018-02-15) and captured the rise of light of ASASSN-18fv on 18 March 2018 (2018-03-18); its photometry pre-dates the discovery announcement. BRITE observations of this target are scheduled to continue until 31 July 2018 (2018-07-31).

The light curve consists of 4-sec exposures through a custom red filter (550-700 nm) and obtained 3 times per minute during a 15-min portion of each 100-min orbit of the BRITE-Toronto satellite, resulting in about 600 measurements per day. The detector scale of the instrument is about 27 arcsec/pixel and the PSF shape is round and measures about 11 pixels (FWHM) at the field location of HD 92063. As a result, the PSFs of HD 92063 and ASASSN-18fv significantly overlap in the BRITE-Toronto images.

BRITE photometry reveals a discernible variation due to ASASSN-18fv starting on 2018-03-18.2 . The light curve of the mean signal level of the blended stars exhibits a general rise but with strong dips during both rise and decline on timescales of days. The highest peak in the light curve thus far (71420 e/sec) was recorded on 2018-03-26.2. The latest flux measurement at the time of this report (2018-04-03.2) was 62280 e/sec. Here is a list of the measured combined (HD92063 + ASASSN-18fv) flux values averaged per day. It does not sample the superimposed oscillatory behavior.

 
UT 			Flux [e/sec]	dFLux [%]	Comments 
2018-03-13.02		52039		0.0		mean flux of HD92063 
2018-03-18.02		52921		1.7		significant flux increase 
2018-03-19.02		53543		2.9 
2018-03-20.02		53919		3.6 
2018-03-21.02		55481		6.6 
2018-03-22.02		60002		15.3 
2018-03-23.02		58743		12.9 
2018-03-24.02		62904		20.9 
2018-03-25.02		65475		25.8 
2018-03-26.02		71420		37.2		highest flux value 
2018-03-27.02		70734		35.9 
2018-03-28.02		69301		33.2 
2018-03-29.02		66302		27.4 
2018-03-30.02		65349		25.6 
2018-03-31.02		64505		24.0 
2018-04-01.02		62586		20.3 
2018-04-02.02		62194		19.5 
2018-04-03.02		62280		19.7 

BRITE-Constellation photometry is usually checked daily by R. Kuschnig for quality and integrity by means of a fast reduction and analysis pipeline. Photometry of HD 92063 (K1 III, V = 5.09) during the first 30 days of monitoring was consistent with expectations for a red giant of this brightness: photometric scatter of about 0.4% with a mean signal level of 52039 e/sec. No trends or excursions greater than 3% were seen before the rise (up to a peak of about +54% seen so far) associated with ASASSN-18fv.