An 8.4 min period in the archival ZTF light curve of Nova Herculis 2021

ATel #14720; P. Mroz, K. Burdge, J. van Roestel, T. Prince (Caltech), A. K.H. Kong (NTHU), K.-L. Li (NCKU), on behalf of the Zwicky Transient Facility collaboration
on 15 Jun 2021; 19:06 UT
Credential Certification: Przemek Mroz (pmroz@astrouw.edu.pl)

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The Zwicky Transient Facility (ZTF; Bellm et al. 2019; Graham et al. 2019) has been observing the field of Nova Herculis 2021 (ATel #14704, #14705, #14707, #14710, #14713, #14718) since March 2018. In total, 475 exposures were collected in the r-band, 191 -- in the g-band, from 2018 March 26 until 2021 June 14.

We obtained the archival ZTF light curve of the nova using the ZTF forced-photometry service (Masci et al. 2018) on images processed with the ZTF reduction and image subtraction pipelines at IPAC (Masci et al. 2019) using the position of the nova from ATel #14710.

We searched for periodic signals in the archival r-band light curve in the frequency range 0.01-400 c/day using the analysis of variance and Lomb-Scargle algorithms. The most significant period is 0.00580356 d = 8.357 min. There are not any other significant independent periodicities in the data.

Additionally, the object was independently recovered in a systematic search of 1,213,165,975 objects across the northern hemisphere using Lomb-Scargle (frequency range 0.004-720 c/day). Among this search, the system was among the 700 highest significance sources with a best period under 10 minutes.

The light curve folded with the 8.4-min period shows differential minima and some additional scatter from flickering. The most likely explanation of this period is the spin period of a white dwarf in an intermediate polar system. The period is unlikely the orbital period of the binary - orbital periods of cataclysmic binaries with hydrogen-rich donors (spectra of the nova show strong Balmer lines, ATel #14704, #14710, #14718) are typically longer than ~78 minutes (Hellier 2001).

Based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Grant No. AST-2034437 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, Deutsches Elektronen-Synchrotron and Humboldt University, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, Trinity College Dublin, Lawrence Livermore National Laboratories, and IN2P3, France. Operations are conducted by COO, IPAC, and UW. The ZTF forced-photometry service was funded under the Heising-Simons Foundation grant #12540303 (PI: Graham).

Light curve of the nova progenitor folded with the 8.4-min period