Supernova Polarimetry with the VLT: Lessons from Asymmetry L. WANG1, D. BAADE 2, P. HÖFLICH 3, J.C. WHEELER 3 1Lawrence Berkeley National Laboratory, California, USA 2European Southern Observatory, Garching, Germany 3Department of Astronomy and McDonald Observatory, The University of Texas at Austin, Austin, USA 1. Introduction Fritz Zwicky shortly after the discovery in this sort of supernova resulting from of the neutron in 1932. When the first the spin of the white dwarf, the motion Picture a 55-year-old telescope of neutron stars were discovered by of the orbit, a surrounding accretion modest two-metre aperture and the Jocelyn Bell they were manifested as disk, or the presence of the companion need to take multiple spectral expo- rapidly rotating pulsars with intense star. As for the case of core-collapse sures of the same object to elicit any magnetic fields. Pulsars have space supernovae, this was known, but there signal from the noise with an inexpen- velocities that average several hundred was no compelling observational rea- sive spectrograph jury rigged to do po- kilometres per second. This indicates son to consider departures from spher- larimetry, an effort requiring integra- that they are somehow “kicked” at birth ical symmetry. This, too, is changing. tions on a single object lasting a whole in a manner that requires a departure long winter night with a single observer from both spherical and up/down sym- 2. Polarization of Supernovae and no night assistant, if any transient metry. More recently, NTT and then target is available during the scheduled Hubble Space Telescope images of The question of the shape of super- time. Contrast that with ordering up Supernova 1987A in the Large Magel- novae has undergone a revolution in queue-scheduled target-of-opportunity lanic Cloud showed rings of gas that the last decade. The driving force has observations by a dedicated profes- had been ejected by the progenitor star been a new type of observation: meas- sional staff on an 8 metre telescope before it exploded. This means either urement of the polarization of the light with a state-of-the-art spectrograph and the progenitor star or its surroundings from supernovae. polarimeter. That is the leap that has possessed some sort of asymmetry. Light consists of oscillating electric occurred in our programme to obtain Further observations showed that the and magnetic fields. An ordinary beam spectropolarimetry of all accessible su- debris of the explosion were also asym- of light is a mix of photons with all ori- pernovae. The spectropolarimetry ob- metric. The supernova remnant Cas- entations equally present, a state that tained in a brief exposure on the VLT siopeia A shows signs of a jet and coun- leaves the light unpolarized on aver- with the FORS1 spectrograph is com- terjet that have punched holes in the age. Some processes of producing or parable to the total flux spectrum ob- expanding shell of debris and there are scattering light favour certain orienta- tained on those long nights on the 2.1- numerous other asymmetric supernova tions of the electric and magnetic fields m Struve Telescope at McDonald Ob- remnants. Each of these things has over others. One such process is the servatory where this programme be- been known. The question has been: reflection of light. When light scatters gan. While the data obtained at McDon- are they merely incidental or a vital clue through the expanding debris of a su- ald pointed the way to a revolution to how supernovae work? pernova, it retains information about in the way we think about super- The previous discussion pertained to the orientation of the scattering layers. novae, it is the quality of the data from core-collapse supernovae. These If the supernova is spherically symmet- the VLT that has led the programme to come in a variety of spectral classifica- ric, all orientations will be present flourish. tions, Type II, Type Ib or Type Ic, de- equally and will average out, so there Supernovae have been studied with pending on whether there is abundant will be no net polarization. If, however, modern scientific methods for nearly a hydrogen, helium or neither in the out- the gas shell is not round, a slight net century. During this time, it has been er layers. There is another kind of su- polarization will be imprinted on the traditional to assume that these cata- pernova known as Type Ia. These are light. strophic stellar explosions are, for all thought to be the result of the ther- Since we cannot spatially resolve the practical purposes, spherically sym- monuclear explosion of a white-dwarf average extragalactic supernova, po- metric. There were observational rea- star composed of carbon and oxygen. larization is the most powerful tool we sons for this. The Sun is essentially When the mass of the white dwarf have to judge the shape of the ejecta. spherically symmetric and most stars closely approaches the Chandrasekhar The method used is called spectropo- are thought to be. The assumption that limit of about 1.4 solar masses, the car- larimetry. This technique both spreads stars are round is quite reasonable. bon ignites. The resulting thermonu- the light out into its spectrum of colours Self-gravity will tend to pull any large clear explosion is thought to complete- and determines the net orientation of body into a sphere which is the mini- ly disrupt the star, leaving no compact the electric field at each wavelength. mum-energy configuration. There were remnant. This sort of explosion has re- This way both the overall shape of the also practical reasons. Theoretical ceived prominence recently because emitting region and the shape of re- study of stellar explosions has been dif- they have been the tool to discover the gions composed of particular chemical ficult enough even with the assumption accelerating Universe and the dark en- elements can be determined. We note of spherical symmetry. There has been ergy that drives the expansion. The that the effective spatial resolution at- no commanding observational need to progenitor white dwarf has long been tained by polarimetry of a supernova of abandon that simplifying assumption. treated as basically spherically sym- radius 1015 cm at 10 Mpc is 10 mi- Now there is. metric even though the popular model croarcsec. This is a factor of 100 better The evidence that supernovae may is that the explosion must take place in resolution than VLTI or other compara- depart a little or even drastically from a binary system where the white dwarf ble optical interferometer installations – spherical symmetry has been growing grows to the critical mass by accretion and at a tiny fraction of the cost. for years. Neutron stars were predicted of mass and, inevitably, angular mo- There were systematic and stimulat- to form and to power supernovae by mentum. There could be asymmetries ing observations of the polarization of 47 the light of SN 1987A that are still being studied and interpreted. Another event that was modestly well studied was the hydrogen-depleted event SN 1993J in M81. These two events just illustrated how poor the overall data base of su- pernova spectropolarimetry was. In 1994 we began a programme to obtain spectropolarimetry of as many super- novae as possible that were visible from McDonald Observatory. At the time, only a handful of events had been examined at all and there were virtually no statistics. The data reduction was tricky, if only because the intervening interstellar medium can impose a polar- ization signal that has nothing to do with the supernova. The data were also difficult to inter- pret. There are, in principle, many rea- sons why the light from a supernova could be polarized. The supernova could be aspherical, it could be spheri- cal but have off-centre sources of light, or other matter in the vicinity could be asymmetrically distributed, blocking part of the scattering surface and yield- ing a net polarization signal from even a spherical surface. To make matters worse, the first few supernovae our group studied (and those in the previ- ous sparse record like SN 1987A and SN 1993J) were classified as “peculiar” in some way, so we did not know whether we were seeing incidental pe- Figure 1: Polarimetry of the Type II SN 1999em on the Q-U plane. The wavelength of differ- culiarities or something truly significant. ent data points are colour encoded. The data points are rebinned to 100 Å for clarity. The 1999 Nov 8 data (clustered in the lower-right quadrant) are clearly separated from the 2000 As data accumulated, however, this Jan. 9 data (clustered in the upper-right quadrant), suggesting strong polarization evolution. uncertainty was removed, and signifi- The data points of both epochs fall roughly on the line denoted AB. Line AB defines the axis cant new insights were revealed. With of symmetry if the object is axially symmetric. Note that the blue and red dots are well sepa- more data and better statistics, we rated for each epoch with the blue points preferentially located at the lower-right of the data identified the first key trend. In 1996, we cluster of each observations. The circles are the upper limits to the interstellar polarization realized that the data were bi-modal. assuming E(B-V) toward the supernova to be 0.05 (inner circle) and 0.1 (outer circle). The Type Ia supernovae showed little or no approximate location of the component due to interstellar dust is shown as a solid circle. polarization signal (we will talk about some significant exceptions below). Supernovae thought to arise by core and allows us a view deeper inside.