Strong Neon Emission in CSS081007:030559+054715: A Possible Oxygen/Neon Nova

J. L. Prieto, K. Denney, O. Pejcha, R. M. Wagner

On 2008 October 30.4 we obtained low-resolution optical spectra (range 3350-4800 Angstroms) of the transient CSS081007:030559+054715 using the Boller & Chivens CCD Spectrograph on the 1.3m McGraw-Hill telescope at MDM Observatory. The most striking feature in this spectral region is very strong, double-peaked [Ne V] at 3426 Angstroms. The two peaks are offset by -1700 and 1100 km/s with respect to the rest wavelength. The equivalent width of this line is about five times higher than of Halpha in our previous spectrum (ATEL #1825). The HWZI of this line is 3100 km/s. Similar double-peaked structure with the same wavelength shifts are observed in substantially weaker lines of [Ne III] at 3869 and 3968 Angstroms. The bright emission line between 4650 and 4760 Angstroms, that we observed in ATEL #1825, is due to [Ne IV] 4721 blended with He II 4686.

Apart from Catalina Real-time Transient Survey, CSS081007:030559+054715 was observed by several other surveys. The All Sky Automated Survey (Pojmanski, 2002, Acta Astronomica, 52, 397) detected the object four times, specifically on 2008 Jan 14.19 UT at V=16.09(7), 2008 Jul 17.4 UT at V=14.26(5), 2008 Jul 24.4 UT at V=14.37(5), and 2008 Aug 3.4 UT at V=15.06(5). John Greaves brought to our attention that CSS081007:030559+054715 was measured by SDSS on 2004 Dec 14.2 UT to have u=18.61(2), g=18.56(1), r=18.52(1), i=18.57(1) and z=18.68(5).

Photometry on our RETROCAM images from 2008 Oct 27.35 UT give g=15.94(5), r=16.12(5) and i=16.28(5). Two other recent photometric measurements are provided by CRTS followup and indicate that CSS081007:030559+054715 is slowly fading, but with fairly rapid decline in the g-band.

Light curve of CSS081007:030559+054715. Green symbols denote ASAS V band measurements, detections are shown with crosses and arrows mark upper limits which we set rather arbitrarily to V=14.5. Red plus signs are unfiltered CRTS measurements. Blue squares are g band data from CRTS followup and our RETROCAM images. Yellow triangles show r band measurements from the same sources.

The ASAS measurements from 2008 Jul and Aug are very close to the limiting magnitude of that survey, but seem to indicate that the object was brighter during the seasonal gap. Our blue spectrum is similar to the late nebular phase spectrum of the very fast Nova Sgr 1991 (V4160 Sgr, see Figure 10 of Williams et al. 1994, ApJS, 90, 297), which developed very strong and broad forbidden neon emission lines. The exceptional strength of the forbidden Ne emission lines in the spectrum of CSS081007:030559+054715 suggests that it is a member of the class of neon novae which may originate on massive ONeMg white dwarfs and the outburst occurred during its seasonal gap, i.e., around JD 2454600.

We thank K. Z. Stanek for useful discussions and suggestions.

Optical spectroscopy of transient CSS081007:030559+054715

Astronomer's telegram #1825

O. Pejcha, J. L. Prieto & K. Denney

Department of Astronomy, The Ohio State University

We report on observations of a transient CSS081007:030559+054715 discovered by the Catalina Real-time Transient Survey (CRTS). According to CRTS photometry the object brightened by about 4 mag during the last year and is currently at about 16th magnitude. We obtained low resolution spectrum (range 4200-7500 Angstroms) on 2008 Oct 27.32 UT using Modspec instrument on 2.4m Hiltner telescope at the MDM Observatory. The spectrum shows blue continuum with very wide and prominent emission lines. The Halpha line shows three well-separated components with radial velocities of -1740, -160 and 1580 km/s. The total HWZI is 2900 km/s. Apparently similar structure is seen at the position of He II and [OIII]. Hbeta is detected but weak. There is a wide unresolved line or blend of lines at the position of Hgamma.

In addition, between 2008 Oct 27.41 and 27.48 UT we obtained 130 r band images with RETROCAM instrument also on the MDM 2.4m telescope. During that time the transient remained constant to within ~0.05 mag.

The transient is located at galactic coordinates of (l,b)=(172.627,-43.727), i.e. away from the Galactic center. The Schlegel et al. (1998) value of the interstellar extinction at this position is AV = 0.48 mag.

Based on the similarity of the observed spectral features with spectra of recurrent nova CI Aql (see, e.g., Fig. 5 in Matsumoto et al. 2001, A&A, 378, 487), including large expansion velocities, we suggest that the transient CSS081007:030559+054715 represents a nova-like phenomenon, although with very unusual, gradual rise of the light curve. For any plausible distance within our Galaxy (i.e. less than 50 kpc), the absolute magnitude of CSS081007:030559+054715 is fainter than about -3 magnitudes, significantly less luminous than classical novae. However, note the large gap in the temporal coverage of the event, and the fact that it is still ongoing.

Further observations of this interesting transient are strongly encouraged.

We thank K. Z. Stanek and R. M. Wagner for useful discussions and suggestions.