Photometry and spectroscopy of the continuing rise in brightness of ASASSN-18pe

ATel #11875; U. Munari and L. Zampieri (INAF Padova), S. Moretti, A. Maitan and S. Tomaselli (ANS Collaboration)
on 20 Jul 2018; 11:19 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)

Tweet

We have obtained UBVRI photometry (Landolt system) and optical spectroscopy of recently discovered ASASSN-18pe currently undergoing a large amplitude outburst (Atel #11867). Photometry was obtained on UT July 19.888 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 (including transformation from the instantaneous local photometric system to the reference Landolt equatorial standards) less than 0.01 mag in all bands. We also obtain U=13.27, but with a larger uncertainty considering the sea level altitude of the observing site. This indicates a continuing (and slow) rise in brightness of ASASSN-18pe, that was reported in Atel #11867 at V=14.3 two days earlier. The overall color seems somewhat bluer that the SLOAN griz values listed for quiescence in the Pan-STARRS release 1 (PS1) Survey

Absolute optical spectrophotometry was obtained with the Asiago 1.22m telescope at 2.31 Ang/pix over the 3250-7950 Ang range on UT Jul 19.853. The spectrum rises steeply toward the short wavelengths in agreement with the blue photometric colors. Hbeta, gamma, delta, epsilon lines stands in clear absorption although with a minimal equivalent width, H9 being the highest Hydrogen line before those further up merge into the Balmer continuum in absorption. NaI 5890, 5896 doublet is also in clear absorption. Only an emission component is visible in Halpha with an integrated flux of 6.6 (+/- 0.6) x 10(-15) erg/cm2 sec. The strongest emission line is HeII 4686, radiating an integrated flux of 1.34 (+/-0.11) x 10(-14) erg/cm2 sec, twice larger than Halpha (both lines single peaked). No evident emission line from HeI is visible, with the possible exception of a feeble 5876 Ang. The radial velocity for all observable lines is pretty low within the limited sensitivity of the adopted spectral dispersion.

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.