SN 1996cr
Supernova 1996cr: SN 1987A's Wild Cousin?
Bauer, F. E.; Dwarkadas, V. V.; Brandt, W. N.; Immler, S.; Smartt, S.; Bartel, N.; Bietenholz, M. F.
The Astrophysical Journal, Volume 688, Issue 2, pp. 1210-1234.
12/2008
Abstract
We report on new VLT optical spectroscopic and multiwavelength archival observations of SN 1996cr, a previously identified ultraluminous X-ray source known as Circinus galaxy X-2. Our optical spectrum confirms SN 1996cr as a bona fide Type IIn supernova, while archival imaging from the Anglo-Australian Telescope archive isolates the explosion date to between 1995 February 28 and 1996 March 16. SN 1996cr is one of the closest SNe (~3.8 Mpc) in the last several decades, and in terms of flux ranks among the brightest radio and X-ray SNe ever detected. The wealth of optical, X-ray, and radio observations that exist for this source provide relatively detailed constraints on its postexplosion expansion and progenitor history, including a preliminary angular size constraint from VLBI. Archival X-ray and radio data imply that the progenitor of SN 1996cr evacuated a large cavity just prior to exploding: the blast wave likely spent ~1-2 yr in relatively uninhibited expansion before eventually striking the dense circumstellar material which surrounds SN 1996cr. The X-ray and radio emission, which trace the progenitor mass-loss rate, have respectively risen by a factor of >~2 and remained roughly constant over the past 7 years. This behavior is reminiscent of the late rise of SN 1987A, but 1000 times more luminous and much more rapid to onset. SN 1996cr may likewise provide us with a younger example of SN 1978K and SN 1979C, both of which exhibit flat X-ray evolution at late times. Complex oxygen line emission hints at a possible concentric shell or ringlike structure. The discovery of SN 1996cr suggests that a substantial fraction of the closest SNe observed in the last several decades have occurred in wind-blown bubbles, and argues for the phenomena being widespread.
Modeling the deep HETG observation of SN 1996cr
Dewey, Daniel; Bauer, F. E.; Dwarkadas, V. V.
Chandra's First Decade of Discovery, Proceedings of the conference held 22-25 September, 2009 in Boston, MA. Edited by Scott Wolk, Antonella Fruscione, and Douglas Swartz, abstract #85
Abstract
SN 1996cr appears to have exploded into a wind-blown bubble structure giving rise to an increasing X-ray flux as late as a decade after explosion. This behavior is rare among SNe and similar to that of SN 1987A. However, SN 1996cr's X-ray luminosity is several orders of magnitude greater than '87A's. Comparing simple 1D hydrodynamic models to the HETG spectra support a model where '96cr is exploding into a more compact and extensive circumstellar bubble-and-shell structure than '87A's.
Chandra High-Resolution Spectroscopy of SN 1996cr
Bauer, Franz E.; Dewey, D.; Dwarkadas, V.; Brandt, W. N.
American Astronomical Society, HEAD meeting #11, #18.11; Bulletin of the American Astronomical Society, Vol. 41, p.688
Abstract
SN1996cr is among the closest SNe to explode in the past 30 years, yet went undiscovered for many years. Due to its fortuitous location in the Circinus Galaxy at 3.7 Mpc, however, we have a wealth of serendipitous archival data available to piece together its early evolution. Like SN1987A, it appears to have exploded into a shell-like structure formed by the progenitor, sparking a unique temporal evolution. I will describe our current multi-wavelength constraints on SN1996cr, focusing on a new 485ks Chandra HETG spectrum, and discuss physical interpretations of the system with the aid of hydrodynamical simulations by our team.
Bursting SN 1996cr's bubble: hydrodynamic and X-ray modelling of its circumstellar medium
Dwarkadas, V. V.; Dewey, D.; Bauer, F.
Monthly Notices of the Royal Astronomical Society, Volume 407, Issue 2, pp. 812-829. 09/2010
Abstract
SN1996cr is one of the five closest supernovae (SNe) to explode in the past 30 yr. Due to its fortuitous location in the Circinus galaxy at ~3.7 Mpc, there is a wealth of recently acquired and serendipitous archival data available to piece together its evolution over the past decade, including a recent 485-ks Chandra high-energy transmission grating spectrum. In order to interpret these data, we have explored hydrodynamic simulations, followed by computations of simulated spectra and light curves under non-equilibrium ionization conditions, and directly compared them to the observations. Our simulated spectra manage to fit both the X-ray continuum and lines at four epochs satisfactorily, while our computed light curves are in good agreement with additional flux-monitoring data sets. These calculations allow us to infer the nature and structure of the circumstellar medium (CSM), the evolution of the SN shock wave, and the abundances of the ejecta and surrounding medium. The data imply that SN 1996cr exploded in a low-density medium before interacting with a dense shell of material about 0.03 pc away from the progenitor star. We speculate that the shell could be due to the interaction of a blue supergiant or Wolf-Rayet wind with a previously existing red supergiant (RSG) wind. The shock wave has now exited the shell and is expanding in the medium exterior to it, possibly the undisturbed continuation of the dense RSG wind. The narrow lines that earned SN 1996cr its IIn designation possibly arise from dense, shocked clumps in the CSM. Although the possibility for a luminous blue variable progenitor for this Type IIn SN cannot be completely excluded, it is inconsistent with much of the data. These calculations allow us to probe the stellar mass-loss in the very last phases (<104 yr) of a massive star's life (>106 yr), and provide another means to deducing the progenitor of the SN.
Modeling The Morphology Of SN 1996cr From X-ray Lines At High Resolution
Bauer, Franz E.; Dewey, D.; Dwarkadas, V
AA(Space Science Institute), AB(MIT Kavli Institute), AC(University of Chicago)
American Astronomical Society, AAS Meeting #217, #434.26 01/2011
Abstract
SN1996cr ( 3.7 Mpc) was X-ray "dim" up to 1000 days yet brightened to 4e39 erg/s (0.5-8 keV) after 10 years (Bauer et al. 2008). A 1-D hydro model of the ejecta-CSM interaction produces good agreement with the measured X-ray light curves and spectra at multiple epochs, suggesting that SN 1996cr was most likely a massive star, M > 30 solar masses, which went from an RSG to a brief W-R phase before exploding within its r 0.04 pc wind-blown shell (Dwarkadas et al. 2010). Further analysis of a 485ks Chandra HETG observation allows velocity-profile fitting to a handful of bright emission lines in the spectrum (e.g., Si and Fe). The line shapes are well fit with axisymmetric emission models that put the higher temperature Fe XXVI emission at high latitudes. The axis orientation is well constrained to be 55 degrees from our line-of-sight. The latitude variation may be explained either by higher CSM densities near the equator, or by the ejecta having greater velocity along the poles. SN1996cr demonstrates how X-ray emission lines can provide important diagnostics of SN shock structure.
Performing a stellar autopsy using the radio-bright remnant of SN 1996cr
Meunier, C.; Bauer, F. E.; Dwarkadas, V. V.; Koribalski, B.; Emonts, B.; Hunstead, R. W.; Campbell-Wilson, D.; Stockdale, C.; Tingay, S. J.
Monthly Notices of the Royal Astronomical Society, Volume 431, Issue 3, p.2453-2463
We present newly reduced archival radio observations of SN 1996cr in the Circinus Galaxy from the Australia Telescope Compact Array and the Molonglo Observatory Synthesis Telescope, and attempt to model its radio light curves using recent hydrodynamical simulations of the interaction between the supernova (SN) ejecta and the circumstellar material (CSM) at X-ray wavelengths. The radio data within the first 1000 d show clear signs of free-free absorption (FFA), which decreases gradually and is minimal above 1.4 GHz after day ˜3000. Constraints on the FFA optical depth provide estimates of the CSM free electron density, which allows insight into the ionization of SN 1996cr's CSM and offers a test on the density distribution adopted by the hydrodynamical simulation. The intrinsic spectral index of the radiation shows evidence for spectral flattening, which is characterized by α = 0.852 ± 0.002 at day 3000 and a decay rate of Δα = -0.014 ± 0.001 yr-1. The striking similarity in the spectral flattening of SN 1987A, SN 1993J and SN 1996cr suggests this may be a relatively common feature of SNe/CSM shocks. We adopt this spectral index variation to model the synchrotron radio emission of the shock, and consider several scalings that relate the parameters of the hydrodynamical simulation to the magnetic field and electron distribution. The simulated light curves match the large-scale features of the observed light curves, but fail to match certain tightly constraining sections. This suggests that simple energy density scalings may not be able to account for the complexities of the true physical processes at work, or alternatively, that the parameters of the simulation require modification in order to accurately represent the surroundings of SN 1996cr.