Mini-HaWKS discovery of an eclipsing nova-like and a Green Pea galaxy at redshift z=0.31

ATel #17442; S. Navarro Umpierrez, J. Casares, G. J. Escobar, M. A.P. Torres, P. Rodriguez-Gil, R. Alonso, M. Armas Padilla, D. Mata Sanchez and T. Munoz-Darias (IAC), A. Lumbreras-Calle, J. A. Fernandez-Ontiveros, C. Lopez-Sanjuan, A. Marin-Franch and J. Zaragoza-Cardiel (CEFCA) and A. Cabrera-Lavers (GTC)
on 11 Oct 2025; 17:59 UT
Credential Certification: Jorge Casares (jcv@iac.es)

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We report the discovery of a cataclysmic variable (CV) and a Green Pea galaxy in the preliminary analysis of the Mini-HaWKS commissioning field. Mini-HaWKS is the pathfinder of HaWKS (the Halpha Width Kilo-degree Survey), which uses three Ha interference filters of different widths to enable the photometric recovery of the full-width at half-maximum (FWHM) and equivalent width (EW) of galactic Ha emitting objects (Casares, 2018, MNRAS, 473, 5195). The observing strategy consisted of 40 cycles with short exposures to reach a signal-to-noise ratio (SNR) of ~50 at R~20 in each filter, equivalent to an effective limiting magnitude (SNR=5) of 22.0 in rSDSS. The observations were performed on 2024 July 30 and 31 (~3.5 h per night) using the JAST80 Telescope at the Observatorio Astronomico de Javalambre in Spain.

The source named as HAWKS J195146.06+193011.0 was selected as a potential broad Ha emitter with FWHM~800 km/s, based on its location in the Mini-HaWKS narrow-band color-color diagram, and later confirmed through spectroscopic follow-up with OSIRIS at the 10.4 m Gran Telescopio Canarias (GTC). Its J2000 coordinates are RA=19:51:46.06 and Dec=+19:30:11.04, with an astrometric accuracy of 0.1". The Ha light curves show evidence of shallow eclipses on both nights, with a depth of less than 0.5 mag. A Phase Dispersion Minimization analysis reveals a potential orbital period of ~3.4 h, although other periods in the range ~2-5 h cannot be excluded. Sloan broad-band photometry obtained on the second night gives g=21.09+/-0.20, r=20.33+/-0.07, and i=20.11+/-0.09.

Two 1800-s spectra were obtained with OSIRIS/GTC on 2025 August 02 and September 13, using the R1000R and R1000B grisms, respectively. Both spectra display broad single-peaked Balmer and He I emission lines, with FWHM(Ha)=1150 km/s and EW(Ha)=91 A. Weak broad emissions from He II 4686 and the CIII/NIII Bowen blend are also detected with a combined EW=5 A, which is 10% smaller than that of Hb. The OSIRIS acquisition image on 2025-08-02 gives r= 20.46+/-0.03. Based on the photometric and spectroscopic characteristics, including the broad single-peaked emission lines, the presence of eclipses and the estimated orbital period, as well as the absence of dwarf nova outbursts in public surveys, we classify the source as a nova-like CV, possibly of the SW Sextantis type. For a period of 3.4 h, the empirical Mv-Porb calibration (Warner, 1987, MNRAS, 227, 23) predicts Mv~4.8. Together with our observed magnitude g = 21.09 and the 3D dust maps from Pan-STARRS/2MASS/Gaia (Green+2019, ApJ, 887, 93), this yields a tentative distance of ~1.3 kpc and an interstellar reddening of E(B-V)~0.25.

The Green Pea galaxy, identified as HAWKS J195354.95+191109.0, was selected based on its Mini-HaWKS colours, which suggest large FWHM~2000 km/s and EW~270 A values. Two 1800-s OSIRIS/GTC spectra acquired on 2025 Aug 14 reveal strong narrow emission lines of [OIII] 4959+5007, Hb, Ha, [NII] 6548+6584 and [SII] 6716+6731 at a redshift of z=0.31. The line ratios log([OIII]5007 / Hb) ~0.45 and log([NII]6584 / Ha) ~-0.76 indicate that the object is not an AGN and that star formation is the dominant source of ionization. The large EWs at rest frame, EW(Ha)=174 A and EW(OIII5007)=128 A, imply that the star formation activity is very strong, typical of a Green Pea galaxy (Cardamone+2009, MNRAS, 399, 1191). The object is not detected in the Sloan gri images obtained during the Mini-HaWKS campaign. We measure r=21.87+/-0.08 in the OSIRIS acquisition images. Its J2000 coordinates are RA=19:53:54.95 and Dec=+19:11:08.96, with an astrometric accuracy better than 0.1".

The HaWKS project acknowledges the support of Centro de Estudios de Fisica del Cosmos de Aragon, Observatorio Astronomico de Javalambre, Gran Telescopio Canarias, and Instituto de Astrofisica de Canarias. The GTC spectra were obtained as part of a Director's Discretionary Time proposal GTC04-24BDDT.