30th Annual Report 2017-18

31 (May 23, 2017) AstroSat Discovery of Strange Polarisation in the Crab Nebula ( 10.1038/s41550-017-0293-z) The AstroSat CZT Imager was used to make the most sensitive measurement so far of the polarisation of hard X-ray emission of the Crab Nebula and Pulsar at 100-380 keV band. The significance of the detection allowed phase-resolved polarimetry at these bands for the first time (see Figure 1). Radiation from the Crab Pulsar is produced by charged particles gyrating around the local magnetic field, and is expected to be polarised in a direction perpendicular to the projection of this magnetic field. The results show that the position angle of the polarisation is primarily aligned with the projection of the pulsar's spin axis on the sky over the entire phase range, strongly suggesting that most of the pulsed emission originates outside the boundary of the pulsar's magnetosphere, where the magnetic field becomes azimuthal and is frozen into the outgoing equatorial wind. This is contrary to the usual belief that the radiation is generated from poloidal magnetic field regions within the magnetosphere. These data also hint a variation of polarisation through the off-pulse region, indicating that a significant fraction of this emission may in fact be associated with the pulsar (S. Vadawale, et al, including DipankarBhattacharya,AjayVibhute, GulabC. Dewangan andRanjeevMisra ). Nature Astronomy publication doi: The Indian multi-wavelength astronomy satellite AstroSat was launched on 28 September 2015, and was placed into a 98 minutes period circular orbit at an altitude of 650km. The co-aligned instruments onboard theAstroSat are: the UV Imaging Telescopes (UVIT), a Soft X-ray focusing Telescope (SXT), the LargeArea X-ray Proportional Counters (LAXPC), and a Cadmium-Zinc Telluride Imager (CZTI). UVIT is the long wavelength eye of AstroSat. It is primarilymeant for imaging in FUV (130-180 nm) and NUV (200- 300 nm); within each of these band, a narrower band can be selected by a set of filters. This instrument has been used to image a large variety of galactic and extra-galactic objects, which include extra-solar-planets, evolved stars, globular clusters, active galactic nuclei, interacting galaxies, and star forming galaxies at redshifts up to about 1.5. The instrument has performed well in the orbit. The key parameters of the performance can be summarized as: FWHM of the images is 1.3 -1.5 arc-seconds, relative astrometric accuracy within the field (about 28 arcmin) is better than half arc-second, and effective area in FUV (NUV) is about 12 (45) square cm. The SXT provides CCD- quality X-ray imaging spectra in the 0.3-8.0keV range by focusing X-rays on to a cooled Charge Coupled Device (CCD). It is capable of imaging 40 arcmin x 40 arcmin field, moderate resolution X- r a y spectroscopy, and timing studies with a resolution of 2.4s. The LAXPC payload consists of three co-aligned large-area proportional counter units for X-ray timing with 10 micro-sec resolution, and spectral studies over the energy range of 3-80 keV. The CZTI is a position sensitive hard X-ray detector consisting of 16,384 semiconductor pixels, it is equipped with a CodedAperture Mask, and provides spectroscopic/timing measurements in the 20-100 keV region and hard X-ray (above 100 keV) polarization measurement of the brightest cosmic sources. In the last two years and a half, AstroSat has made a number of scientific contributions.Ahighlight of IUCAA's contribution toAstroSat research is given below. ASTROSAT RESEARCH HIGHLIGHTS OF RESEARCH DURING 2017-2018