AT 2024lwu is a likely UGSU variable

ATel #16676; Christopher Lloyd (University of Sussex), Maxim Usatov (Alnitak Remote Observatory)
on 27 Jun 2024; 22:31 UT
Credential Certification: Christopher Lloyd (c.lloyd@sussex.ac.uk)

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We report V-band photometry of the recently discovered transient AT 2024lwu and note at least four previous outbursts recorded in the ATLAS archive data.

AT 2024lwu was originally reported as a likely CV by the GEOTS Group at an unfiltered magnitude of 16.6 on 2024 June 20.27 (JD = 2460481.77). A series of observations by ATLAS centred 4 hours later give a mean orange magnitude of o = 16.8, compared with o = 19.6, two days earlier. A brief fade to o = 17.8 on June 22.53 (JD = 2460484.03) is apparently confirmed by Sloan g and r photometry from Balam & Hess, with g = 17.24±0.01 and g-r = 0.08±0.02 on June 21.41 (JD = 2460482.91), assuming that the colours are broadly close to zero. Since then, the star has remained brighter than o = 16.6 up to the most recent observation on June 26.57 (JD = 2460487.97).

A series of V-band observations using the 0.43-m Dall-Kirkham telescope at the Alnitak Remote Observatory, Sierra del Segura, Spain yields a provisional mean magnitude of V = 15.55±0.01 at 2024 June 26.95 (JD = 2460488.45), and confirm that the system is still in outburst. Observations over 30 minutes show a total range of 0.03 mag, but without any obvious coherence.

ATLAS has observed AT 2024lwu since 2015 at a median cadence of 2.0 d in the orange band and 4.0 d in the cyan (c) band. Observations in 2015 and 2016 are extremely sparse, but the bulk of the coverage is relatively even from 2017. The median magnitudes are o = 20.0 and c = 20.5, with a range of about one magnitude in both bands. There are three coherent groups of observations brighter than 18.0 prior to the current outburst, plus one other single night with multiple observations. All of these runs point to additional outbursts with durations from 4 to at least 10 d, and appear to be consistent with SU UMa-type normal and superoutbursts. Three of the maxima are between o = 16.5 and 17.0. The current outburst has been in progress for 6 d, and is still near maximum. It also appears to have a pre-cursor outburst so is very likely another superoutburst.

Further observations, particularly time series, are encouraged in an effort to identify any possible superhump period.

This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project. The Asteroid Terrestrial-impact Last Alert System (ATLAS) project is primarily funded to search for near earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogs from the survey area. This work was partially funded by Kepler/K2 grant J1944/80NSSC19K0112 and HST GO-15889, and STFC grants ST/T000198/1 and ST/S006109/1. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy, the Queen's University Belfast, the Space Telescope Science Institute, the South African Astronomical Observatory, and The Millennium Institute of Astrophysics (MAS), Chile.