Probable progenitor detection of PSN J12304185+4137498

ATel #3574; M. Fraser, R. Kotak, L. Magill, S. J. Smartt (Queens University Belfast), A. Pastorello (University of Padova)
on 22 Aug 2011; 12:41 UT
Credential Certification: Stephen Smartt (s.smartt@qub.ac.uk)

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We obtained 6x180s R-filter exposures of PSN J12304185+4137498 (CBET 2789) with the Nordic Optical Telescope + ALFOSC on 20 August 2011. After stacking the individual frames, we aligned the image to the archival mosaic pre-explosion HST+WFPC2 F606W filter image of the region (pipeline-drizzled). Using 21 sources common to both images, we derived a geometric transformation between the two images with an rms error of 0.08". The position of PSN J12304185+4137498 was mapped onto the pre-explosion WFPC2 image using this transformation, where it was found to be within 0.14" (~1 WF pixel) of a faint source. While this is slightly outside the uncertainty of our transformation, we consider it likely that the transformation uncertainty has been underestimated (as the resolution of the ALFOSC image is much poorer than that of WFPC2). We hence suggest that the source may be the progenitor of PSN J12304185+4137498.

Photometry of the progenitor candidate with the HSTPhot package (Dolphin 2000) gives a magnitude of F606W=23.66±0.09 in the WFPC2 flight system. Given the distance (D=9.2 Mpc) and foreground line-of-sight extinction (A_V=0.07) towards NGC 4490, this corresponds to an absolute magnitude in F606W of -6.2.

The absolute magnitude of the progenitor candidate is fainter than the typical unextinguished magnitude of a SN impostor outside of eruption (for comparison, SN 2009ip had a V band absolute magnitude of -10.3, while UGC 2773 OT2009-1 had V=-7.5, Foley et al. 2011). It is however, brighter in the optical than the progenitor of the dust-enshrouded SN 2008S (Botticella et al. 2009), which was only detected prior to explosion at mid-infrared wavelengths. Diffuse emission is seen in archival Spitzer images (PID 3360 from 3.6-8.0 microns) at the location of the transient, but there is no obvious point source at this position. The absolute magnitude of the progenitor candidate is also deemed to be too bright for that of a typical nova progenitor system.

A finder chart for the progenitor candidate is available at http://star.pst.qub.ac.uk/~sne/NGC4490_progenitor.jpg The circle in the left panel is at the transformed coordinates of PSN J12304185+4137498, the radius of the circle corresponds to 3 times the uncertainty in the alignment. The cross marks show the position of the progenitor candidate as measured in the WFPC2 image.