No progenitor detection for PTF11kly/SN2011fe in Hubble Space Telescope pre-explosion images

ATel #3623; S. J. Smartt, M. Fraser, R. Kotak (Queen's University Belfast) L. Magill (Queen's University Belfast, Issac Newton Group), K. Maguire (University of Oxford)
on 1 Sep 2011; 20:00 UT
Credential Certification: Stephen Smartt (s.smartt@qub.ac.uk)

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We observed PTF11kly=SN2011fe (Nugent et al. ATEL #3581) on 30 August 2011 with the William Herschel Telescope on La Palma and the ACAM imager (8 arcmin field, 0.25 arsec pixels). A set of 10x0.4s exposures were taken (SDSS-like r-band filter) to avoid the SN centroid saturating, followed by 3 x 200s exposures. The telescope was guided continuously throughout to minimise shifts. Ten stars were identified in both the 4s stack and 600s stack to co-align these images to within ~3 mas. The centroid position of SN 2011fe was measured to an accuracy of 16 mas on the 4s stack and hence its position on the deep 600s image is known to a total accuracy of 16 mas. The image quality was measured at 0.92" FWHM in the 600s stack and 0.78" in the 4s stack.

The ACAM 600s deep stack was aligned to the Hubble Space Telescope images of M101 with ACS/WFC in the F814W filter (720s exposure ; Program ID: 9490; the same data set as discussed by Li et al. ATel#3582,#3585). A total of 26 sources common to both images were used to determine a transformation between the two images (using IRAF geomap/geotran), with a total rms error of 46 mas (< 1 ACS pixel). Using this transformation, the SN coordinates were transformed from the 4s ACAM stack, via the 600s ACAM stack to the pre-explosion HST ACS F814W image. The transformed pixel coordinates in j8d6a2030_drz are (2997.140,1339.875), with a total uncertainty of +/- 0.98 pixels. We confirm the result of Li et al, finding no source coincident with the SN in the pre-explosion data. Furthermore, we find the two closest sources to the SN position in the F814W ACS image (the red two sources identified by Li et al ATel#3582,#3585) to be offset by 4.40 and 5.23 ACS pixels respectively, and hence can formally exclude these as candidate progenitor companions with a significance >3 sigma.

We further checked the other two HST images available - WFPC2 F606W (520s taken on 22 May 1998; GO7909) and WFC3 F469N (6106s taken on 8 Apr 2010; GO11635) and similarly find no detection of a progenitor candidate at the SN position. We thus confirm that there is no progenitor counterpart (e.g. red giant or otherwise) detected in any of the HST archive pre-explosion images. As noted by Li et al. (ATel#3582,#3585), their 3-sigma detection limit of M(I) > -3 is still not deep enough to place restrictive limits on the binary companion to the white dwarf. Low-mass red giants and main-sequence stars below about 5 solar masses would remain undetected.

An image of the SN position on the pre-explosion HST ACS F814W image is at http://star.pst.qub.ac.uk/~sne/2011fe.png The circle marks the transformed SN position, while the radius of the circle corresponds to three times the total uncertainty in the SN position. Scale and orientation are as indicated.