WHT, DIPOL-2 polarimetry of Nova Sgr 2015b

ATel #7986; Eamonn Harvey (National University of Ireland, Galway) Andrei Berdyugin (University of Turku, Finland), Matt Redman (National University of Ireland, Galway)
on 3 Sep 2015; 11:44 UT
Credential Certification: Matt Redman (matt.redman@nuigalway.ie)

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We report polarimetry data from three nights observing of Nova Sgr 2015b (also PNV J18365700-2855420 or V5668 Sgr) with the William Herschel Telescope in the BVR passbands using the DIPOL-2 instrument. Spectra of the nova are reported in ATel #7230 #7265, confirming it to be of Fe II type. The three nights in question are July 4.024, 5.085 and 7.087, after the formation of dust ATel #7643 #7862 and during the local minimum in the transitional part of the optical light curve. The polarimetry data is suggestive of scattering off of small dust grains: % in B | % in V | % in R 1.6991±0.0169 | 0.6008±0.0137 | 0.3441±0.0072 1.4713±0.0152 | 0.5655±0.0115 | 0.3300±0.0063 1.3375±0.0183 | 0.5650±0.0231 | 0.3569±0.0087 An increase in the position angle of ~4 degrees over the three nights from ~ 144 degrees was observed. The error in the polarization angle was on average 0.3 in B, 0.8 in V and 0.6 in R. There was also a decrease in percentage polarisation in B and V indicative of possible grain and ejecta geometry evolution, R is quite likely dominated by Halpha (private communication Steve Shore). As the X-ray counts increase over time (ATel #7953) we continue to observe the nova intermittently with 60cm class telescopes and encourage others to do so also, especially in the U band.