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Alert Notice 642: ASASSN-18pe observing campaign

July 24, 2018: Dr. Matt Darnley (Astrophysics Research Institute, Liverpool John Moores University) and the AAVSO are requesting monitoring of ASASSN-18pe, the cataclysmic variable (possible long period dwarf nova) discovered 2018 July 10 by the ASAS-SN program (J. Strader et al. ATel #11867) at magnitude V = 15.8.

ASASSN-18pe brightened slowly, reaching V = 14.3 on July 17 (ASAS-SN). It remains bright and may be continuing to brighten; the most recent observations in the AAVSO International Database show it at magnitude B = 14.259 +/-0.028 on 2018 July 23.94263 UT and V = 14.135 +/-0.018 on July 23.94665 UT (O. Nickel, Mainz, Germany).

Strader et al. report that "ASAS-SN shows no previous variable or transient source at this location. The PS1 photometry for the source (Chambers et al 2016, arXiv:1612.05560) implies a mean quiescent mag of V~18.2, so a rise of at least 3.9 mag from quiescence so far and a total rise time of > 1 week."

They also observed the source with Swift/XRT for 1 ksec on 2018 July 17.7, when ASASSN-18pe was clearly detected but faint, with 13 events.

Strader et al. further report:"The optical spectrum and probable low X-ray luminosity would normally suggest that the most likely explanation for ASASSN-18pe is a dwarf nova rising to outburst. However, the slow rise is unusual for a dwarf nova (e.g., Szkody & Mattei 1984, PASP, 96, 988). The rare systems with slow rises, such as GK Per, are generally interpreted to have long orbital periods and evolved secondaries (e.g., Crampton et al. 1986, ApJ, 300, 788)...We emphasize that, while several aspects of the transient resemble a dwarf nova, the slow rise time and possible high luminosity suggest that other, more exotic possibilities, such as an unusual X-ray binary outburst, should be considered."

U. Munari et al. (ATel #11875) obtained UBVRI photometry (Landolt system) on 2018 July 19.888 UT with the ANS Collaboration telescope 1205 (42cm), "providing B=13.961, V=13.895, R=13.801, and I=13.678, with total error budgets...less than 0.01 mag in all bands. We also obtain U=13.27, but with a larger uncertainty...The overall color seems somewhat bluer that the SLOAN griz values listed for quiescence in the Pan-STARRS release 1 (PS1) Survey." They also obtained absolute optical spectrophotometry with the Asiago 1.22-m telescope on July 19.853 UT, and found that "the spectrum rises steeply toward the short wavelengths in agreement with the blue photometric colors. Overall, the photometric and spectral appearances are compatible with a CV eruption. Worth noticing are the slow rising time (commented upon also in ATel #11867), the unusual strength of HeII emission, and the possibly bluer colors compared to quiescence."

E. Aydi et al. (ATel #11878) obtained high-resolution optical spectroscopy of ASASSN-18pe under the SALT Large Science Program on transients on 2018 July 19.2 (HJD 2458319.2), using the High Resolution Spectrograph mounted on the Southern African Large Telescope (SALT), which showed a very blue continuum with an unusually strong He II 4686 emission.

BVRI observations are requested. The nature of the object and event not being known, a specific cadence is not requested. It is recommended that different cadences be tried to see what behavior(s) may be seen. Visual observations are welcome.

Coordinates (2000): R.A. 17 15 21.59  Dec. +06 00 28.3  (from VSX)

Charts with a comparison star sequence for ASASSN-18pe may be created using the AAVSO Variable Star Plotter (VSP). An 'f' scale chart will show the sequence extending to V = 18.0.

Please submit observations to the AAVSO International Database using the name ASASSN-18PE.

AAVSO Forums: ASASSN-18pe is the topic of the AAVSO Campaigns and Observation Reports forum thread https://www.aavso.org/asassn-18pe-outburst

This AAVSO Alert Notice was compiled by Elizabeth O. Waagen.
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