Supernova 1993J (SN 1993J) in M81, now classified as Type IIb, is a strongly emitting radio supernova whose size can be resolved and whose growth can be monitored with the technique of very long baseline interferometry (VLBI). The results could provide important information about the circumstellar matter and the degree of asymmetry of the explosion. For several models of the emission of the radio supernova, we report estimates of its angular sizes 29 and 36 days after explosion at the wavelengths of 3.6 and 1.3 cm, respectively. These results, which correspond to our first epochs in an ongoing effort to determine the supernova structure and its growth, slightly favor an optically thick uniform disk model, given the recently derived Cepheid distance to M81 and the estimated maximum supernova expansion speed. Further VLBI observations, combined with the expansion-speed data, may yield an independent estimate of the distance to M81.This work was supported in part by the Spanish DGICYT
grant PB89-009 and in part by NSF grant AST 9303527. We
thank Kurt Weiler for a very careful review of this paper and
N. Renzetti and P. Wolken for their extraordinary efforts in
arranging for our use of the DSN antenn

Alberdl, A

Elósegui, P

Guirado, J.C.

Mantovani, F

Marcaide, J.M.

Pérez, E

Ratner, M I

Rius, A

Rogers, A. E. E

Schmidt, Brian

Shapiro, I I

The Astrophysical Journal

The Australian National University

1994ApJ...424L..25MTHE AsTROPHYSICAL JOURNAL, 424:L25-L27, 1994 March 20 © 1994. The American Astronomical Society. All rights reserved. Printed in U.S.A. RADIO-SIZE ESTIMATES OF SN 1993J J.M. MARCAIDE,1 •2 A. ALBERDI,3 P. EL6sEGUI,2 J.C. GUIRAD0,3 F. MA~OVANI,4 E. PEREz,5 ~- I. RATNER,2 A. Rms,6 A. E. E. RoGERs,7 B. P. ScHMIDT,2 I. I. SHAPIRO, AND A. R. WHITNEY Received 1993 December 8; accepted 1994 January 3 ABSTRACT Supernova 1993J (SN 1993J) in M81, now classified as Type IIb, is a strongly emitting radio supern~va whose size can be resolved and whose growth can be monitored with the techniqu~ of very long basehne interferometry (VLBI). The results could provide important information a?o~t the circums!ellar matter and the degree of asymmetry of the explosion. For several models o~ the emission of the radio supernova, we report estimates of its angular sizes 29 and 36 days after expl~sion at th~ wavelengths of 3.~ and 1.3 cm, respectively. These results, which correspond to ou~ first ep~chs i? an o~gomg effort_ to determme the super-nova structure and its growth, slightly favor an optically thick umform dis~ model, given the recently derived Cepheid distance to M81 and the estimated maxim~m supe_rnova expansio_n speed. Furt~er VLBI observa-tions, combined with the expansion-speed data, may yield an mdependent estimate of the distance to M81. Subject headings: galaxies: individual (NGC 3031, M81) - r~dio co~tinuum: sta~s -supernovae: individual (SN 1993J) - techmques: mterferometnc 1. INTRODUCTION SN 1993J was discovered on 1993 March 28 in M81 (NGC 3031) (Ripero & Garcia 1993), most likely 2 days aft~r. the explosion (Schmidt et al. 1993). The supernova wa~ imtially classified (Filippenko et al. 1993) as Type II due to its broad Hix emission line. Its optical light curve peaked less than 2 days after discovery and passed through a second maximum about 2 weeks later (Schmidt et al. 1993; Wheeler et al. 1993). Based on detailed modeling of the optical light curves, several groups (Nomoto et al. 1993; Podsiadlowski et al. 1993) suggested that SN 1993J might be similar to Type lb supernovae, such as SN 1983N. These suggestions were supported by the (later) appearance of strong He I lines in the supernova spectrum, which seemed to indicate that the supernova had begun to transform into a type similar to lb (Filippenko & Matheson 1993· Schmidt et al. 1993; Swartz et al. 1993), such as reported earli~r for SN 1987K (Filippenko 1988). Now SN 1993J is classified as Type IIb (Filippenko 1988; Woosley 1991). Compared to many previous supernovae, the radio emission of SN 1993J was detected quite early (Weiler et al. 1993; Pooley & Green 1993) and increased rather rapidly (Weiler et al. 1994). Although the details of how, where, and when radio emission originates in supernovae are incompletely known, it is thought (Chevalier 1982, 1990; Weiler et al. 1986, 1990) that the emission is generated by the outgoing shock produced by the supernova blast wave interacting with the circumstellar matter enhancing the magnetic field and energizing the rela-tivistic' electrons responsible for the radio synchrotron radi-ation. The early detection of the radio emission indicates that the circumstellar material, presumably ejected by the progeni-tor as a wind has less optical depth than in radio-bright Type II supernov~ such as SN 1979C or SN 1986J, according to the 1 Departamento de Astronomia, Universitat de Valencia, E-46100 Burjas-sot, Spain. 2 Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138. 3 Instituto de Astrofisica de Andalucia-CSIC, E-18080 Granada, Spain. 4 Istituto di Radioastronomia, 1-40129, Bologna, Italy. 5 Instituto de Astrofisica de Canarias, E-38200 La Laguna, Spain. 6 Instituto de Astronomia y Geodesia-CSIC, E-28040 Madrid, Spain. 7 MIT Haystack Observatory, Westford, MA 01886. L25 circumstellar interaction model (Chevalier 1990). On the other hand, the flux density of the radio emission at wavelengths longer than 3.6 cm continues to gro~ e_ven_ 180 days af!er the explosion (Weiler et al. 1994), thus mdicatmg denser circum-stellar material than that surrounding SN 1983N but similar to the material around SN 1980K. Asymmetry in the shape of the supernova explosion has been suggested, based on the unusual_ shape of t~e absorption troughs observed in the He I and Htx hoes (Sc~midt et a~. 19?3) and on the detection of strong linear optical polarization (Trammell et al. 1993). The radio structure could also be asym-metric due to some combination of an intrinsic asymmetry of the geometry of the interaction shell, of its m~gnet~c ~eld ~s-tribution, and of its relativistic electron-density distribution. Using verv long baseline (radio)interferometry (VLBI), we should be ~ble to detect any such asymmetry: combining the estimated distance to M81 of 3.6 Mpc (Freedman et al. 1994) and the maximum supernova gas expansion velocity of about 20,000 km s - 1, estimated from the extent of the blue wing in the Htx line absorption (Schmidt et al. 1993), suggests that the radio supernova should grow at an angular rate of about 0.20 milliarcseconds (mas) month- 1, which is well matched to the beam sizes (point-spread functions) of worldwide VL~I arrays operating at centimeter wavelengths. Thus, a det~ct1_on of a possible asymmetry in radio structure should be w1thm reach after a few months of observations and might help to deter-mine whether the explosion was asymmetric or not. A detec-tion and measurement of such an asymmetry might, in turn, help understand whether the progenitor was a single sta~ or part of a multiple-star system (Nomoto et al. 1993;_ Pods1ad-lowski et al. 1993; Hoflich et al. 1993). Further, the cucumstel-lar interaction model (Chevalier 1990) predicts that the radio supernova should have a shell structure; with SN 1993J, we have the opportunity to test this prediction. VLBI measurements can also be used to estimate the dis-tance to M81, in much the same way as they were used to estimate the distance to SN 1979C in MlOO (Bartel et al. 1985), by comparing the evolution of the angular size of the super-nova, as observed by VLBI, to the maximum radial expansion rate measured from optical lines. For SN 1979C the lack of a © American Astronomical Society • Provided by the NASA Astrophysics Data System 1994ApJ...424L..25ML26 MARCAIDE ET AL. Vol. 424 detailed knowledge of the radio structure was an important source of systematic error, which may be greatly reduced for SN 1993J. 2. OBSERVATIONS AND DATA REDUCTION We carried out VLBI observations of the supernova on 1993 April 24 (day 29 after the explosion) and on May 1 (day 36) at the wavelengths of 3.6 and 1.3 cm, respectively. On April 24 we used two radio telescopes, one each in Robledo de Chavela (M, 70 m diameter), near Madrid, Spain, and in Westford (E, 18 m), Massachusetts, USA. At M and at E we had system tem-peratures of about 20 and 50 K, respectively. We observed from 21 :30 UT on April 24 through 01: 30 UT on April 25 with a fringe spacing of 1.4 mas. We alternated 5.5 minute observa-tions of SN 1993J (ix= 9h55m24!774, {J = +69°01'13''.70, J2000.0 [Marcaide et al. 1993]) with 1 minute observations of the calibrator source 0917 + 624 in a 10 minute cycle. We also observed the calibrators 0923 + 392 (4C 39.25) and 0954 + 658 several times each. On May 1, we used M, another radio-telescope (K, 37 m) in Westford, Massachusetts, and one in Medicina (L, 32 m), near Bologna, Italy. Typical system tem-peratures were 70, 160, and 120 K, respectively. We observed alternately SN 1993J (5.5 minutes) and 0954+ 658 (1 minute) with a 15 minute cycle from 11: 15 UT on May 1 through 01: 15 UT on May 2, with fringe spacings of about 0.6 and 2.0 mas; for the remainder of our 24 hr session we observed the calibrators 0316 + 413 (3C 84), 0917 + 624, 0923 + 392, 1641+399 (3C 345), 1901+319 (3C 395), and 2200+420 (BL Lac). The experiments were run with Mark IHA recording equipment (Rogers et al. 1983) at 144 and 224 megabits s - 1 sampling rates, corresponding to 72 and 112 MHz bandwidths, at 3.6 and 1.3 cm, respectively. The data were correlated at the Haystack Observatory, and fringes were detected for SN 1993J with signal-to-noise ratios in the range from 5-7 for the 3.6 cm data and in the range from 7-14 for the 1.3 cm data. The supernova flux densities at 3.6 cm on April 24 and at 1.3 cm on May 1 were 15.2 ± 0.8 and 55.6 ± 2.8 mJy, respectively (Weiler et al. 1994). We used the Caltech Package (Pearson 1991) to calibrate and fit a model to the data. In the first step of the calibration procedure, we used radiometric information obtained during the observations and a priori knowledge about the antenna gains. We then improved this calibration by incorporating antenna-gain corrections obtained from model-ing the calibrator source 0917 + 624 for the April 24 data and the calibrator sources 0954 + 658 and 1901 + 319 for the May 1 data. At 3.6 cm we relied on knowledge of the structure of 0917 + 624 at 3.6 cm from three epochs in 1991 (K. Standke, 1993, private communication). However, one should notice that 0917 +624 is the prototype of intraday variable sources (Quirrenbach et al. 1989), and its flux density may vary by almost 20% on hour timescales. For the 1.3 cm data, we relied for the calibration on flux density measurements, on published maps at 3.6 and 6 cm (Gabuzda et al. 1992; Lara et al. 1994), and on our inference of the 1.3 cm structures for 0954 + 658 and 1901 + 319 by extrapolating from the 3.6 cm maps using the spectral indices obtained from the 3.6 and 6 cm maps. From our amplitudes for the three baselines and correspond-ing closure phases, we were able to improve these brightness distributions. The data reduction for each calibrator source was carried out independently by different people to check for consistency of reduction. The resultant differences in antenna-gain corrections for both 0954 + 658 and 1901 + 319 were con-sistent with each other to 5%-10%. We adopted the average values and used them in the analysis of the SN 1993J data. Due to its low flux density, the supernova was not detected on the K-L baseline, and hence we have no closure phase available. From scans of 1901 + 319, we estimated that amplitude loss due to loss of coherence over the 5.5 minute integration time for SN 1993J was about 8%. 3. RESULTS After correcting upward the visibility amplitudes at 1.3 cm by 8% to account for the coherence loss, we fitted three models to the resultant SN 1993J visibility data at both 3.6 and 1.3 cm: An optically thin uniform sphere, a uniform disk, and a uniform ring. To estimate the effects of calibration errors on the models, we scaled up and down our amplitudes at 3.6 and 1.3 cm by 15% (total range of 30%). The results are given in Table 1. The fits to the data are very similar for all models; hence, in Figure 1, a comparison with the data is shown only for the uniform disk model. We turn now to using the estimate of the distance to M81 (Freedman et al. 1994) to distinguish among these radio super-nova models. Let us assume that the maximum supernova gas speed equals the speed of the shock causing the radio emission and that the shock front does not decelerate appreciably during, at least, the first 40 days following the explosion of SN 1993J. Given the maximum inferred gas speed of about 20,000 km s- 1 (Schmidt et al. 1993), the radius of the super-nova gas sphere should be about 5.2 x 1010 and 6.4 x 1010 --;; !:!, SN1993J A ril 24, 1993 A 3.6 cm ~ ME f;i 0.020 11 I I I I I I I r::i I I ~ 0.015 I I I I I I I I I ~ 0.010 ~ 0.005 E--< j 0.000 r.:i 22:00 23:00 00:00 01:00 P: P: UNIVERSAL TIME 0 u FIG. la SN1993J May 1, 1993 A 1.3 cm -~ 0.060 ~ 0.045 ~ 0.030 r.:i r::i 0.015 MK I I 11111 I I I I I I I I I I I 111 I II I I I II I I I I I I I I 'I I II I I I I I I I I I . g 0.000 1----t-----+-----+-----,1------1 ii. ML ~ 0.120 E--< j 0.090 r.:i ~ 0.060 ~ 0 u 0.030 12:00 16:00 20:00 UNIVERSAL TIME FIG. lb 00:00 FIG. 1.-Fits of the uniform disk models given in Table 1 to the observed correlated flux densities at (a) 3.6 cm and (b) 1.3 cm. © American Astronomical Society • Provided by the NASA Astrophysics Data System 1994ApJ...424L..25MNo. 1, 1994 RADIO-SIZE ESTIMATES OF SN 1993J L27 TABLE 1 EsTIMATES OF SIZE OF SN 1993J Observing wavelength (cm) Model 3.6 • 1,3:b Diameter• (mas) Optically thin uniform sphere 0.3i:!t;~ 0.31 ± 0.06 Uniform disk 0.25:!t~~ 0.28 ± 0.06 Uniform ring 0.19:!t~i 0.20 ± 0.04 • Day 29 after explosion. Optically estimated size is 0.19 mas (see text). b Day 36 after explosion. Optically estimated size is 0.24 mas (see text). ' Quoted errors cover diameter estimates obtained by scaling up and down the best calibrated data by 15% (see text). km, 29 and 36 days after the explosion, respectively. At a dis-tance to M81 of 3.6 Mpc (Freedman et al. 1994), the angular diameter of such spheres would be about 0.19 and 0.24 mas, respectively. Our estimates in Table 1 for day 29 after explo-sion are too uncertain to be useful. The estimates for day 36 after explosion, however, favor slightly the uniform disk model. This conclusion is not in contradiction with the circumstellar interaction model-a shell model-because if the shell emis-sion were optically thick during the early times, as suggested by measurements of the spectral index ex [S(v) oc v«:] of 1.17 ± 0.15 and 0. 76 ± 0.15, on April 24 and May 1, respec-tively (Weiler et al. 1994), the radio emission would appear to emanate from a uniform disk. Thus, while the present stage of VLBI data accumulation and analysis for SN 1993J does not allow us to obtain a useful independent estimate of the distance to M81, the currently accepted distance to M81 proves useful in constraining the models of SN 19931. This work was supported in part by the Spanish DGICYT grant PB89-009 and in part by NSF grant AST 9303527. We thank Kurt Weiler for a very careful review of this paper and N. Renzetti and P. Wolken for their extraordinary efforts in arranging for our use of the DSN antenna. REFERENCES Bartel, N., et al. 1985, Nature, 318, 25 Chevalier, R. 1982, ApJ, 285, L63 --. 1990, in Supernovae, ed. A.G. Petschek (New York: Springer), 90 Filippenko, A. V. 1988, AJ, 96, 1941 Filippenko, A. V., et al. 1993, IAU Circ., No. 5731 Filippenko, A. V., & Matheson, T. 1993, IAU Circ., No. 5787 Freedman, W. L., et al. 1994, ApJ, in press Gabuzda, D. C., et al. 1992, ApJ, 388, 40 Hoflich, P., et al. 1993, A&A, 275, L29 Lara, L., et al. 1994, A&A, in press Marcaide, J. M., et al. 1993, IAU Circ., No. 5820 Nomoto, K., et al. 1993, Nature, 364,507 Pearson, T. J. 1991, BAAS, 23, 991 Podsiadlowski, Ph., et al. 1993, Nature, 364, 509 Pooley, G. G., & Green, D. 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Radio-Size Estimates of SN 1993J

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Radio-Size Estimates of SN 1993J

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