A decade of SN 1993J: discovery of radio wavelength effects in the expansion rate

Abstract

We studied the growth of the shell-like radio structure of supernova SN 1993J in M 81 from September 1993 to October 2003 with very-long-baseline interferometry (VLBI) observations at the wavelengths of 3.6, 6, and 18 cm. We developed a method to accurately determine the outer radius (R) of any circularly symmetric compact radio structure such as SN 1993J. The source structure of SN 1993J remains circularly symmetric (with deviations from circularity under 2%) over almost 4000 days. We characterize the decelerated expansion of SN 1993J until approximately day 1500 after explosion with an expansion parameter m = 0.845 ± 0.005 (R ∝ t m ). However, from that day onwards the expansion differs when observed at 6 and 18 cm. Indeed, at 18 cm, the expansion can be well characterized by the same m as before day 1500, while at 6 cm the expansion appears more decelerated, and is characterized by another expansion parameter, m 6 = 0.788 ± 0.015. Therefore, since about day 1500 onwards, the radio source size has been progressively smaller at 6 cm than at 18 cm. These findings differ significantly from those of other authors in the details of the expansion. In our interpretation, the supernova expands with a single expansion parameter, m = 0.845 ± 0.005, and the 6 cm results beyond day 1500 are caused by physical effects, perhaps also coupled to instrumental limitations. Two physical effects may be involved: (a) a changing opacity of the ejecta to the 6 cm radiation; and (b) a radial decrease of the magnetic field in the emitting region. We also found that at 6 cm about 80% of the radio emission from the backside of the shell behind the ejecta is absorbed (our average estimate, since we cannot determine any possible evolution of the opacity), and the width of the radio shell is (31 ± 2)% of the outer radius. The shell width at 18 cm depends on the degree of assumed absorption. For 80% absorption, the width is (33.5 ± 1.7)%, and for 100% absorption, it is (37.8 ± 1.3)%. A comparison of our VLBI results with optical spectral line velocities shows that the deceleration is more pronounced in the radio than in the optical. This difference might be due to a progressive penetration of ejecta instabilities into the shocked circumstellar medium, as also suggested by other authors.

Detection of jet precession in the active nucleus of M 81

“…SN 1993J was observed using M 81* as the phase calibrator at the frequencies of 1.7,2.3,5.0,8.4,15,and 22 GHz. (There are only a few low-quality observations during years 1993 and 1994 at the two highest frequencies, which have not been used in our analysis.) The results of this observing campaign of SN 1993J are reported in several publications (e.g., Bartel et al 2002;Marcaide et al 2009;Martí-Vidal et al 2011a,b, and references therein), where the technical details of all these observations are given. Basically, one epoch typically lasted ∼12 h, with the participation of 10-15 antennas.…”

“…7 are obtained from fits of a circularly symmetric shell to the SN 1993J visibilities. The shell of SN 1993J does not have a perfect circular symmetry; there are deviations with respect to a perfect circular source in both the shape of the shell (of the order of 2%) and the intensity distribution inside it (of the order of 20%) (Bartel et al 2002;Bietenholz et al 2000;Marcaide et al 2009;Martí-Vidal et al 2011a). Moreover, the inhomogeneities in the shell evolve systematically in time.…”

VLBI observations of SN 2011dh: imaging of the youngest radio supernova

“…However, it is definitely worth monitoring any potential radio emission from this kind of events, in order to perform detailed studies of the physical conditions in the expanding supernova shocks. The case of supernova SN 1993J is the best example of such a study (see, e.g., Bartel et al 2002;Marcaide et al 2010;Martí-Vidal et al 2011a; and references therein). The intense VLBI/VLA observing campaign of this supernova allowed these authors to constrain much of the parameter space of the models (density profiles of ejecta and circumstellar medium, hydrodynamical instabilities and their role in the magnetic-field amplification, energy equipartition, etc.)…”

On the spherical-axial transition in supernova remnants

“…(1997); Dyson (1983) ;Cantó, Raga, and Adame (2006). The very-long-baseline interferometry (VLBI) observations of SN 1993J (wavelengths of 3.6, 6, and 18 cm) show that R ∝ t 0.82 over a 10 year period, see Marcaide et al (2009). This observational fact does not agree with the current models because the radius of SN 1993J grows slower than the free expansion and faster…”

“…The SNRs can also be classified at the light of the observed symmetry. A first example is SN 1993J which presented a circular symmetry for 4000 days, see Marcaide et al (2009). An example of weak departure from the circular symmetry is SN 1006 in which a ratio of 1.2 between maximum and minimum radius has been measured, see Reynolds and Gilmore (1986).…”