Christopher John Stockdale

An early and comprehensive millimetre and centimetre wave and X-ray study of SN 2011dh: a non-equipartition blast wave expanding into a massive stellar wind

Only a handful of supernovae (SNe) have been studied in multiwavelengths from the radio to X-rays, starting a few days after the explosion. The early detection and classification of the nearby Type IIb SN 2011dh/PTF 11eon in M51 provides a unique opportunity to conduct such observations. We present detailed data obtained at one of the youngest phase ever of a core-collapse SN (days 3–12 after the explosion) in the radio, millimetre and X-rays; when combined with optical data, this allows us to explore the early evolution of the SN blast wave and its surroundings. Our analysis shows that the expanding SN shock wave does not exhibit equipartition (ϵ_e/ϵ_B ∼ 1000), and is expanding into circumstellar material that is consistent with a density profile falling like R^(−2). Within modelling uncertainties we find an average velocity of the fast parts of the ejecta of 15u2009000 ± 1800u2009kmu2009s^(−1), contrary to previous analysis. This velocity places SN 2011dh in an intermediate blast wave regime between the previously defined compact and extended SN Type IIb subtypes. Our results highlight the importance of early (∼1u2009d) high-frequency observations of future events. Moreover, we show the importance of combined radio/X-ray observations for determining the microphysics ratio ϵ_e/ϵ_B.

SN 2007bg: the complex circumstellar medium around one of the most radio-luminous broad-lined Type Ic supernovae

In this paper we present the results of the radio light curve and X-ray observations of broad-lined Type Ic SN 2007bg. The light curve shows three distinct phases of spectral and temporal evolution, implying that the SNe shock likely encountered at least 3 different circumstellar medium regimes. We interpret this as the progenitor of SN 2007bg having at least two distinct mass-loss episodes (i.e., phases 1 and 3) during its final stages of evolution, yielding a highly-stratified circumstellar medium. Modelling the phase 1 light curve as a freely-expanding, synchrotron-emitting shell, self-absorbed by its own radiating electrons, requires a progenitor mass-loss rate of dot{M}~1.9x10^{-6}(v_{w}/1000 km s^{-1}) Solar masses per year for the last t~20(v_{w}/1000 km s^{-1}) yr before explosion, and a total energy of the radio emitting ejecta of Esim1x10^{48} erg after 10 days from explosion. This places SN 2007bg among the most energetic Type Ib/c events. We interpret the second phase as a sparser gap region between the two winds stages. Phase 3 shows a second absorption turn-on before rising to a peak luminosity 2.6 times higher than in phase 1. Assuming this luminosity jump is due to a circumstellar medium density enhancement from a faster previous mass-loss episode, we estimate that the phase 3 mass-loss rate could be as high as dot{M}<~4.3x10^{-4}(v_{w}/1000 km s^{-1}) Solar masses per year. The phase 3 wind would have transitioned directly into the phase 1 wind for a wind speed difference of ~2. In summary, the radio light curve provides robust evidence for dramatic global changes in at least some Ic-BL progenitors just prior (~10-1000 yr) to explosion. The observed luminosity of this SN is the highest observed for a non-gamma-ray-burst broad-lined Type Ic SN, reaching L_{8.46 GHz}~1x10^{29} erg Hz^{-1} s^{-1}, ~567 days after explosion.

Recovery of the Historical SN1957D in X-rays with Chandra

SN1957D, located in one of the spiral arms of M83, is one of the small number of extragalactic supernovae (SNe) that has remained detectable at radio and optical wavelengths during the decades after its explosion. Here, we report the first detection of SN1957D in X-rays, as part of a 729 ks observation of M83 with Chandra .T he X-ray luminosity (0.3‐8 keV) is 1.7 +2.4 −0.3 ×10 37 erg s −1 . The spectrum is hard and highly self-absorbed compared to most sources in M83 and to other young supernova remnants (SNRs), suggesting that the system is dominated at X-ray wavelengths by an energetic pulsar and its pulsar wind nebula. The high column density may be due to absorption within the SN ejecta. Hubble Space Telescope WFC3 images resolve the SNR from the surrounding emission and the local star field. Photometry of stars around SN1957D, using WFC3 images, indicates an age of less than 10 7 years and a main-sequence turnoff mass more than 17 M� . New spectra obtained with Gemini-South show that the optical spectrum continues to be dominated by broad [Oiii] emission lines, the signature of fast-moving SN ejecta. The width of the broad lines has remained about 2700 km s −1 (FWHM). The [Oiii] flux dropped precipitously between 1989 and 1991, but continued monitoring shows the flux has been almost constant since. In contrast, radio observations over the period 1990‐2011 show a decline rate Sν ∼ t −4.0 , far steeper than the rate observed earlier, suggesting that the primary shock has overrun the edge of a pre-SN wind.

The Fading Radio Emission from SN 1961V: Evidence for a Type II Peculiar Supernova?

Using the Very Large Array (VLA), we have detected radio emission from the site of SN 1961V in the Sc galaxy NGC 1058. With a peak flux density of 0.063 ± 0.008 mJy beam-1 at 6 cm and 0.147 ± 0.026 mJy beam-1 at 18 cm, the source is nonthermal, with a spectral index of -0.79 ± 0.23. Within errors, this spectral index is the same value reported for previous VLA observations taken in 1984 and 1986. The radio emission at both wavelengths has decayed since the mid-1980s observations with power-law indices of β20 cm = -0.69 ± 0.23 and β6 cm = -1.75 ± 0.16. We discuss the radio properties of this source and compare them with those of Type II radio supernovae and luminous blue variables.

Narrow Lines in Type II Supernovae: Probing the Circumstellar Nebulae of the Progenitors

We have carried out a high-dispersion (R ~ 30,000) echelle spectroscopic survey of 16 Type II supernovae (SNe) to search for narrow emission lines from circumstellar nebulae ejected by their massive progenitors. Circumstellar nebulae, if detected, provide invaluable opportunities to probe SN progenitors. Of the 16 SNe observed, SN ejecta are clearly detected in four SNe and possibly in another two SNe, interstellar gas is detected in 12 SNe, and circumstellar material is detected only in SN 1978K and SN 1998S. In the case of SN 1978K, we are able to place an upper limit of ~2.2 pc for the size of the circumstellar ejecta nebula and note that this is more consistent with the typical sizes observed for ejecta nebulae around luminous blue variables, rather than Wolf-Rayet stars. In the case of SN 1998S, our observations of the narrow lines ~1 yr after the SN explosion show variations compared to early epochs. The nebular lines we observe from SN 1998S either originate from the low-density outer region of a circumstellar nebula or have become dominated by an interstellar component.

The Continuing Radio Evolution of SN 1970G

Using the Very Large Array, we have detected radio emission from the site of SN 1970G in the Sc galaxy M101. These observations are 31 yr after the supernova event, making SN 1970G the longest monitored radio supernova. With flux densities of 0.12 ± 0.020 mJy at 6 cm and 0.16 ± 0.015 mJy at 20 cm, the spectral index of -0.24 ± 0.20 appears to have flattened somewhat when compared with the previously reported value of -0.56 ± 0.11, taken in 1990. The radio emission at 20 cm has decayed since the 1990 observations with a power-law index of β20 cm = -0.28 ± 0.13. We discuss the radio properties of this source and compare them to those of other Type II radio supernovae.

Discovery of a Nuclear X-Ray Source in NGC 7331: Evidence for a Massive Black Hole

Using ROSAT, we have made the first deep X-ray observations of NGC 7331, discovering a nuclear X-ray source coincident with the previously determined radio and optical nuclear centers. The positions, luminosities, and fluxes of X-ray sources in the field are compared with previously identified radio and optical sources. The nucleus of NGC 7331 has been analyzed to discern any new evidence supporting the presence of a massive black hole, and comparisons are made with other low-ionization nuclear emission-line region (LINER) galaxies. The flux ratio of core radio to soft X-ray emission in NGC 7331 is lower than in other LINERs included in our sample.

The fading radio emission from supernova 1961V

Using the Very Large Array (VLA), we have detected radio emission from the site of SN 1961V in the Sc galaxy NGC 1058. With a peak flux density of 0.084±0.011u200amJy/beam at 6 cm and 0.145±0.028u200amJy/beam at 18 cm, the source is nonthermal, with a spectral index of −0.51±0.21. Within errors, this index is the same value reported for previous VLA observations taken in 1984 and 1986. The radio emission at both wavelengths has decayed since the mid 1980’s observations with an averaged power-law index of β20cm&6cm=−0.98±0.13. We discuss the radio properties of this source and compare them with those of Type II radio supernovae and luminous blue variables.