IR spectropolarimetry of SN2002ap with NICS-TNG (with data publicly available)

The spectropolarimetric mode of NICS has been successfully tested on the sky on the night of Feb 6, 2002. The targeted object was SN2002ap, a nearby peculiar type II supernova for which several requests for discretionary and ToO observations have been submitted to the TNG.

The observations were performed using a 0.5" slit and the Amici prism in combination with the LiNbO3 Wedged Double Wollaston expressively designed for this instrument, and following the normal procedure of "dithering along the slit". Four spectra of 300sec were collected under quite bad weather conditions: seeing of about 3" and thin cirrus. The figures below show a raw spectrum (left) and the result of the subtraction of two subsequent spectra with the star at different positions along the 50" slit (right). The four spectra, from bottom to top, correspond to measurements taken through a 90deg, 0deg, 45deg and 135deg ideal polarizer, respectively; wavelength increases from right to left. The higher noise and offset residual in the first (bottom left) quadrant is a relatively common and still not understood problem of NICS which, in this case, limits the accuracy of the measurements in the K band.

Instrumental transmission and polarization were both corrected for by measuring the nearby unpolarized standard HD14069 with the identical instrumental setup and with the slit always aligned along the parallactic angle. The resulting spectrum and polarization parameters are displayed in the figure below (please note that the absolute flux scale is uncertain). The most remarkable features in the spectrum are the broad P-Cygni profile of HeI 1.083Ám and the jump between the H and K band, which might be related to absorption by HeI 2.058Ám. The polarimetric measurements are compatible with zero polarization within a 3-sigma upper limit of 1.2%, 1.1%, 2.1% and 4.8% in the 1mic, J, H and K band, respectively. Interestingly, these values closely correspond to the limiting accuracies exected from the photon noise of the background, except for the point in K (see above).

Given the widespread interest for this supernova, the data are made immediately available to the community. Here you can find the spectral data and the polarization data, both in ASCII format. Credit to these data should be properly acknowledged.