VLA radio detection and continued optical activity of Nova Cygni 2019 (PGIR19brv)

ATel #13653; Kirill Sokolovsky (MSU), Aliya-Nur Babul, Jennifer Sokoloski (Columbia), Justin Linford (NRAO), Laura Chomiuk, Adam Kawash, Elias Aydi, Jesse Leahy-McGregor, Jay Strader (MSU), Koji Mukai (NASA/GSFC), Kwan-Lok Li (NTHU), Stella Kafka (AAVSO), Klaus Wenzel (BAV), Mansi Kasliwal, Kishalay De (Caltech)
on 22 Apr 2020; 04:38 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)

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Nova Cygni 2019 (PGIR19brv, AT2019qwf, V2891 Cyg) was discovered as a J band transient on 2019-09-17.25 UT by the Palomar Gattini-IR survey (De et al. 2020, PASP, 132, 1008) and spectroscopically classified as a FeII nova (ATel #13130, #13149, #13258, #13283, #13301, #13340). The ZTF (Masci et al. 2018, PASP, 131, 995) photometry shows that the outburst might have started as early as 2019-09-14.22 (t0) and after an initial rapid increase in optical brightness, the optical light curve switched to a much slower rise displaying irregular variations with an amplitude of more than two magnitudes. The recent observations by the AAVSO as well as ZTF and ASAS-SN (Shappee et al. 2014, ApJ, 788, 48) surveys show that as of 2020-04-15 (t0 + 214 days), the nova remains bright at 15.4 (unfiltered magnitude with V zero-point).

We initiated a radio monitoring of Nova Cygni 2019 with the Karl G. Jansky Very Large Array (VLA) starting on 2019-10-05 (t0 + 21 days) that, until recently, resulted in non-detections at frequencies between 1.26 and 35 GHz, with image RMS at 20 to 80 microJy level depending on the observing band and the array configuration. The latest VLA observation performed on 2020-03-31.7 (t0 + 200 days) resulted in a clear detection of the nova with the following flux densities

 
# f(GHz) F(uJy) eF(uJy)  
  35      878   46  
  29.5    639   38  
  16.5    235   21  
  13.5    184   20  

The previous observation, performed six days earlier, resulted in a marginal detection at 5 and 7 GHz and upper limits at lower frequencies:

 
# f(GHz) F(uJy) eF(uJy)  
   7       74   27  
   5       68   26  
   1.74   ...   72  
   1.26   ...   93  

The inverted radio spectrum (the positively-defined spectral index is 1.6) is consistent with an optically thick free-free emission. The radio detection is likely associated with thermal emission from the expanding nova ejecta, and if so, we expect the radio flux to rise over the next months. The long delay of approximately 200 days between the optical and radio brightenings is consistent with the slow expansion of the ejecta indicated by the low velocities in optical spectra (initially 820 km/s ATel #13149, #13258, while ATel #13340 reported a "principal component" velocity of 650 km/s). Using the 5 GHz flux of 68 uJy on day 194 and assuming a distance of 8 kpc, we estimate the maximum velocity of the radio-emitting material is about 650 km/s, consistent with the optical estimate (ATel #13340). We encourage continued multiwavelength observations of Nova Cygni 2019.

We thank the AAVSO observers BMU, DEY, FJQ, FRF, IMA, LFEA, LPEB, SDM, SREB, STHD, TRE, TRT, VDE, VMT for contributing valuable photometry of the nova. The NRAO is a facility of the NSF operated under cooperative agreement by Associated Universities, Inc. Based on observations obtained with the Samuel Oschin 48-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the NSF under Grant No. AST-1440341 and a collaboration including Caltech, IPAC, the Weizmann Inst. for Science, the Oskar Klein Center at Stockholm Uni., the Uni. of Maryland, the Uni. of Washington, DESY and Humboldt Uni., LANL, the TANGO Consortium of Taiwan, the Uni. of Wisconsin at Milwaukee, and Berkeley Lab. Operations are conducted by COO, IPAC, and UW. ANTARES is supported by the NSF through a cooperative agreement with the AURA for the operation of the NSF's National Optical-Infrared Astronomy Research Laboratory, through an NSF INSPIRE grant to the Uni. of Arizona (CISE AST-1344024, PI: R. Snodgrass), and through a grant from the Heising-Simons Foundation. Palomar Gattini-IR acknowledges generous support from the Mt Cuba Foundation, the Heising-Simons Foundation, the Bi-national Science Foundation, ANU and Caltech.

Lightcurve and radio spectrum plots