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Rapid decline in optical and UV brightness of ASAS-SN 17pf

ATel #11301; A. Bahramian, L. Chomiuk, J. Strader (Michigan State), P. Kuin (MSSL/UCL), M. J. Darnley (LJMU), K. Page (Leicester)
on 14 Feb 2018; 18:13 UT
Credential Certification: Arash Bahramian (bahramian@pa.msu.edu)

Subjects: Ultra-Violet, Cataclysmic Variable, Nova, Transient

ASAS-SN 17pf is a newly discovered nova in the LMC (ATel #11132). We have been monitoring this transient weekly with Swift. The source shows significant variability in optical and UV bands up to 2 magnitudes.

Most recent ASAS-SN and Swift/UVOT observations over the last week indicates a rapid substantial decline. On Feb. 4th, 2018 the source has shown magnitudes of V=13.25(+/-0.02) as seen by ASAS-SN and UVW1=13.76(+/-0.02), UVW2=14.13(+/-0.02), and UVM2=14.02(+/-0.02) in Swift filters. However in observations performed between Feb. 10th and 12th, 2018 the source shows magnitudes of V=15.08(+/-0.06), UVW1=14.76(+/-0.02), UVW2=14.857(+/-0.02) and UVM2=14.84(+/-0.03), indicating a ~2 magnitude decline in V band over a week (all magnitudes in AB magnitude).

We have also looked at Swift/XRT images (all taken in PC mode) to search for a possible X-ray counterpart. Stacking all the available Swift observations of this source (taken since the discovery, in early December 2017) provides a total exposure of ~20 ks. We find no evidence of an X-ray counterpart for this source and estimate a 95% upper limit of 7.5e-4 ct/s on Swift/XRT count rate in the 0.3-10 keV band. Assuming a power law spectrum with photon index of 2, hydrogen column density of 7.4e20 cm^-2 for this direction (Dickey and Lockman, 1990, ARA&A, 28, 215), and a distance of ~49 kpc (to LMC, e.g., see Pietrzynski et al. 2013, Nature, 495, 76), gives an upper limit of 9.5e33 erg/s on X-ray luminosity.

Observations across the spectrum are encouraged. Further UV and optical observations are planned. We thank the Swift team for scheduling our observations.