Wavelength dependence of interstellar polarization and ratio of total to selective extinction

Abstract

Wavelength dependence of interstellar linear polarization has been observed for about 180 stars, mostly southern, in the UBVR spectral regions. A multichannel polarimeter-photometer, in which spectral regions are separated by dichroic filters, was used. Normalized wavelength dependence of interstellar linear polarization p follows closely a single empirical curve p(X)lp ma , x = exp [-1.15 ln 2 (A max /A)], where the wavelength A max at which the maximum interstellar linear polarization /? raax occurs takes values from 0.45 /x to 0.8 p.Wavelength A max is well correlated with the ratios of color excesses Eand Ev-^Ev-r. These correlations indicate that the ratio R of total to selective interstellar extinction can be found for any individual star from the relationship R = 5.5 A max . Polarimetry seems to be the most practical method of estimating R. A map of distribution of A max on the sky, based on values for about 350 stars, indicates several well defined regions with A max , and hence R, clearly larger (or smaller) than the median value A max = 0.545 /x, corresponding to i? = 3.0.The predominance of larger than average values of A max among stars nearer than 0.4 kpc and the negative correlation between A max and E B -v are explained by selection effects. There is evidence of negative correlation between A max and p m eLxlE Bv suggested by Kruszewski. The lower limits for color excess of Praesepe, M67, and several globular clusters are set by their linear polarization. The largest known values of interstellar circular polarization, \q\ ^ 0.06 percent, were found in near-infrared for two stars with exceptionally small A max : star No. 12 in association VI Cygni and HD 204827. Subject headings: instruments -interstellar extinction -interstellar matter -open clustersphotometry -polarization

Polarized synchrotron emission in quiescent black hole X-ray transients

“…The line of sight extinction of Av = 0.124 towards the source is way too low to produce this level of polarization due to dust at NIR wavelengths. There is a linear relation between the maximum optical polarization caused by dust extinction; FLPmax = 3Av (Serkowski, Mathewson, & Ford 1975), which implies FLP dust < 0.4 per cent for Swift J1357.2-0933, and much lower in the NIR regime. Scattering on the disc surface also seems unlikely since the disc does not dominate the NIR emission, so the intrinsic polarization of the disc would have to be even higher than that measured, which is unlikely (scattering usually produces a few per cent FLP in LMXBs; e.g.…”

Mid-infrared imaging- and spectro-polarimetric subarcsecond observations of NGC 1068

“…This allows us to constrain the absorptive and emissive polarization components in the 2−13 µm wavelength range. For the absorptive component, we assume a Serkowski (Serkowski et al 1975) curve up to 8 µm, followed by the absorptive profile of the BN object in Orion (Section 5.2.1) in the 8-13 µm wavelength range. This composition takes into account the silicate fea-ture at MIR wavelengths, that, if not used, would lead to the expected MIR polarization being underestimated by a factor >100.…”

Optical Spectropolarimetry of the GRB 020813 Afterglow

“…The line of sight to GRB 020813 has a Galactic reddening of mag (Schlegel, Finkbeiner, & E(BϪV ) p 0.111 Davis 1998). Along most Galactic lines of sight, the maximum interstellar polarization observed is 9% per magnitude of (Serkowski, Mathewson, & Ford 1975). The ISP probe E(BϪV ) star has at over 5800-p p 0.67% ‫ע‬ 0.01% v p 167Њ .0 ‫ע‬ 0Њ .3 6800 Å , falling comfortably within the expected range, and its spectrum is consistent with a Serkowski interstellar pop(l) Note.-The results given above have been corrected for ISP using observations of the A2 V star HD 187330, as described in the text.…”