Transcript Slide 1
Unusual Behavior in Radio Supernovae Poonam Chandra Jansky Fellow, National Radio Astronomy Observatory Astronomy Department , University of Virginia 22nd February 2007 Poonam Chandra SN 1993J Unusually well understood 22nd February 2007 Poonam Chandra SN 1993J: Why so special??? • Exploded just 3 Mpc away, extremely bright • First clear case of transition between supernova Type IIP to Ib. • Was detected in X-rays and Radio bands just 5 days after the explosion. • Maund et al (2004) detected unambiguously the signature of the massive binary companion in HST observations, 10 years after the explosion. • Excellent VLBI measurements, Radio emitting zone resolved. 22nd February 2007 Poonam Chandra Still lot more to learn from SN 1993J My talk is based on the new surprises, it is offering to us 22nd February 2007 Poonam Chandra Radio Emission in a Supernova Radio emission in a supernova arises due to synchrotron emission, which arises by the ACCELERATION OF ELECTRONS ? in presence of an ENHANCED MAGNETIC FIELD. 22nd February 2007 Poonam Chandra ? Radio data: We found synchrotron cooling break in combined GMRT and VLA (0.2 GHz to 44 GHz) near simultaneous observations on day 3200. 22nd February 2007 Poonam Chandra On Day 3200…… GMRT+VLA spectrum Chandra, P. et al. 2004 Synchrotron cooling break at 4 GHz GMRT VLA 22nd February 2007 Poonam Chandra Is synchrotron cooling break real? Unsettled Issue!!! 22nd February 2007 Poonam Chandra But on day 3770………….. Observations of SN 1993J on Day 3770 Date of observation Frequency GHz Flux density mJy GMRT VLA June 17, 03 0.239 58.2 ± 11.0 June 17, 03 0.619 33.4± 4.3 June 13, 03 1.280 20.2± 2.1 June 26, 03 4.885 8.35 ± 0.42 June 26, 03 14.940 0.98 ± 0.32 June 26, 03 22.485 0.82 ± 0.27 22nd February 2007 Poonam Chandra 1.5 years later…………. ~Day 3750 Synchrotron cooling break at ~5.5 GMRT VLA 22nd February 2007 Poonam Chandra GHz We predicted the evolution of break frequency with roughly 1.2 GHz per year. Our shift in the synchrotron cooling frequency is consistent with this calculation. (PC, Ray, Bhatnagar, 2004) 22nd February 2007 Poonam Chandra Fn=n-a a1a 1 aa2 2 Flux Density a3 a 1< = a 2< = a3 Frequency 22nd February 2007 a3 Poonam Chandra On Day 3200…… GMRT+VLA spectrum Chandra, P. et al. 2004 GMRT VLA 22nd February 2007 Poonam Chandra 1.5 years later…………. ~Day 3750 GMRT VLA 22nd February 2007 Poonam Chandra Synchrotron Aging in SN 1993J Synchrotron losses Adiabatic expansion Diffusive Fermi acceleration Energy losses due to adiabatic expansion V E dE - E R t dt Adia 22nd February 2007 Poonam Chandra R Ejecta velocity V Size of the SN Energy gain due to diffusive Fermi acceleration E EV E( R / t ) dE tc 20 20 dt Fermi 2 4( v1 - v 2 ) 3v 4 tc v 22nd February 2007 2 v1 Upstream velocity v 2Downstream velocity Spatial diffusion 1 1 v1 v 2 Poonam Chandra coefficient of the test particles across ambient magnetic field vParticle velocity E E 2 -2 2 2 -1 dE / dtTotal ( R t / 20 ) E - bB E - t E For t and B B0 / t (Fransson & Bjornsson, 1998, ApJ, 509, 861) Break frequency . n break . 22nd February 2007 R -1 / 2 1/ 2 B t - 2t 20 -3 0 2 Poonam Chandra 2 On day 3200 B=330 mG On day 3770 B=280 mG Magnetic Field follows 1/t decline trend 22nd February 2007 Poonam Chandra ISM magnetic field is few microGauss. Shock wave will compress magnetic field at most by a factor of 4, still few 10s of microGauss. Hence magnetic field inside the forward shock is highly enhanced, most probably due to instabilities Equipartition magnetic field is 10 times smaller than actual B, hence magnetic energy density is 4 order of magnitude higher than relativistic energy density 22nd February 2007 Poonam Chandra . n break . R -1 / 2 1/ 2 B t - 2t 20 -3 0 2 2 Acceleration diffusion constant Ball & Kirk 1992 22nd February 2007 Poonam Chandra For SN 1987A 21024 cm2 sec-1 (Ball & Kirk, 1992, ApJL) Scaled value of diffusion coefficient for 1993J 4 2 10 24 2.96 10 24 cm2 sec-1 2.7 22nd February 2007 Poonam Chandra dn break dt 2 2 R -1/ 2 R -3 / 2 -3 1/ 2 -1/ 2 B0 t - 2t t - 2t 20 20 From VLBI (Bartel, 2002, ApJ), nbreak(3200)=4 GHz on day 3200 R=2.65 x 1017 cm On day 3770 R=3.8 x 1017 cm 22nd February 2007 Poonam Chandra nbreak(3770)=5.5 GHz Diffusion acceleration coefficient =(5.3 +/- 3.0) x 1024 cm2 s-1 22nd February 2007 Poonam Chandra Result: First time direct measurement of ENHANCED MAGNETIC FIELD and ACCELERATION DIFFUSION COEFFICIENT responsible for radio emission by observing the synchrotron cooling break and its evolution with time 22nd February 2007 Poonam Chandra Radio Emission in a Supernova Take it with caution!!!! Radio emission in a supernova arises due to We needemission, at leastwhich 4-5arises simultaneous synchrotron by the datasets showing the synchrotron ACCELERATION OF ELECTRONS cooling break to feel more confident!!! in presence of an ENHANCED MAGNETIC FIELD. 22nd February 2007 Poonam Chandra Thanks to: Kurt Weiler: NRL, Washington Chris Stockdale: Marquette Univ. Alak Ray: TIFR, Mumbai, India Roger Chevalier: Univ Virginia Juan Uson: NRAO, Charlottesville 22nd February 2007 Poonam Chandra Thanks 22nd February 2007 Poonam Chandra