Long term VLBI imaging and monitoring of the SNR in M82

2004

The high resolution obtained through the use of VLBI gives an unique opportunity to directly observe the interaction of an expanding radio supernova with its surrounding medium. We present here results from our VLBI observations of the young supernovae SN 1979C, SN 1986J, and SN 2001gd.

2005

The high resolution obtained through the use of VLBI gives an unique opportunity to directly observe the interaction of an expanding radio supernova with its surrounding medium. We present here results from our VLBI observations of the young supernovae SN 1979C, SN 1986J, and SN 2001gd.

Springer Proceedings in Physics

S u m m a r y. We report on recent MERLIN, VLA and VLBI observations of the compact radio sources in the nearby starburst M82, with angular resolutions ranging from arcseconds to milliarcseconds. The spectral properties of the compact sources have been investigated by 15 GHz VLA-Pie Town observations which show that 16 of the less luminous compact sources are, in fact, HII regions. However the steep non-thermal spectrum, parsec size and small variability of the remaining sources is consistent with their being supernova remnants. Several show clear shell structures at MERLIN resolution (~50 mas) and 5 have been resolved further using VLBI. Measurements of the most compact source (41.95+575) shows an expansion velocity of ~ 2000 km s" 1 , and one of the best defined SNR shells (43.31+592) shows an expansion velocity of ~ 10 4 km s _ 1. Recent VLBI and MERLIN measurements confirm this expansion velocity and show little evidence for deceleration. We comment on the discrepancy between this measured expansion velocity and the low expansion velocities predicted theoretically for remnants in M82.

High resolution radio observations of the nearby starburst galaxy M82 at both 1.6 GHz and 5 GHz are presented. The combination of simultaneous Global VLBI and MERLIN observations of M82, made on the 3rd Mar. 2005, have produced the first detailed observations of the majority of the supernova remnants within the central starburst of M82 at a frequency of 1.6 GHz. These data also provide the fifth epoch of milliarcsec resolution images of the four most compact sources, allowing monitoring of their evolution over a 19 year timeline. In addition, MERLIN 5 GHz observations over an 8 day period have been used to study the expansion of ten of the supernova remnants within the central starburst.

New Astronomy Reviews, 1999

We have used the European VLBI Network (EVN) at 18 cm to study 5 young supernova remnants in the starburst galaxy M82. We have found clear evidence for expansion in the one of the shell sources (43.311592) by comparing images at epochs separated by more than a decade. Between 1986 and 1997 this source has increased its diameter by 13.662 mas 21 corresponding to an average expansion velocity of 985061500 km s . If we assume the remnant is in free expansion, this is consistent with a birth in the early 1960s. Hence this remnant is almost certainly younger than the strongest, most compact 21 source (41.951575) which is showing a much lower expansion velocity ( , 4000 km s ).

The Astrophysical Journal, 2009

The aftermath of supernova (SN) 1987A continues to provide spectacular insights into the interaction between a SN blastwave and its circumstellar environment. We here present 36 GHz observations from the Australia Telescope Compact Array of the radio remnant of SN 1987A. These new images, taken in 2008 Apr and 2008 Oct, substantially extend the frequency range of an ongoing monitoring and imaging program conducted between 1.4 and 20 GHz. Our 36.2 GHz images have a diffraction-limited angular resolution of 0. ′′ 3 − 0. ′′ 4, which covers the gap between high resolution, low dynamic range VLBI images of the remnant and low resolution, high dynamic range images at frequencies between 1 and 20 GHz. The radio morphology of the remnant at 36 GHz is an elliptical ring with enhanced emission on the eastern and western sides, similar to that seen previously at lower frequencies. Model fits to the data in the Fourier domain show that the emitting region is consistent with a thick inclined torus of mean radius 0. ′′ 85, and a 2008 Oct flux density of 27 ± 6 mJy at 36.2 GHz. The spectral index for the remnant at this epoch, determined between 1.4 GHz and 36.2 GHz, is α = −0.83. There is tentative evidence for an unresolved central source with flatter spectral index.

On the 3rd March 2005, global Very Long Baseline Interferometry (VLBI) observations were made of the central kiloparsec of M82 at 1.6 GHz (18 cm), with an angular resolution of 4 mas. We present detailed images of four of the brightest, most compact supernova remnants as seen in these observations. These data are the fifth epoch of 1.6 GHz VLBI observations of M82, the first of which was made in December 1986, enabling study of these compact sources over a 19 year timescale. We will also discuss MERLIN 1.6GHz observations made simultaneously, which have been combined with the global VLBI data, to produce high fidelity images of a large number of the known supernova remnants and HII regions in M82.

Monthly Notices of the Royal Astronomical Society, 2006

We present the results of a second epoch of 18 cm global Very Long-Baseline Interferometry (VLBI) observations, taken on 23 February 2001, of the central kiloparsec of the nearby starburst galaxy Messier 82. These observations further investigate the structural and flux evolution of the most compact radio sources in the central region of M82. The two most compact radio objects in M82 have been investigated (41.95+575 and 43.31+592). Using this recent epoch of data in comparison with our previous global VLBI observations and two earlier epochs of European VLBI Network observations we measure expansion velocities in the range of 1500-2000 km s −1 for 41.95+575, and 9000-11000 km s −1 for 43.31+592 using various independent methods. In each case the measured remnant expansion velocities are significantly larger than the canonical expansion velocity (500 km s −1 ) of supernova remnants within M82 predicted from theoretical models.

Astronomy & Astrophysics, 2009

We report on phase-referenced 23 GHz Very-Long-Baseline-Interferometry (VLBI) observations of the type IIb supernova SN 2008ax, made with the Very Long Baseline Array (VLBA) on 2 April 2008 (33 days after explosion). These observations resulted in a marginal detection of the supernova. The total flux density recovered from our VLBI image is 0.8$\pm$0.3 mJy (one standard deviation). As it appears, the structure may be interpreted as either a core-jet or a double source. However, the supernova structure could be somewhat confused with a possible close by noise peak. In such a case, the recovered flux density would decrease to 0.48$\pm$0.12 mJy, compatible with the flux densities measured with the VLA at epochs close in time to our VLBI observations. The lowest average expansion velocities derived from our observations are $(1.90 \pm 0.30) \times 10^5$ km s$^{-1}$ (case of a double source) and $(5.2 \pm 1.3) \times 10^4$ km s$^{-1}$ (taking the weaker source component as a spurious, close by, noise peak, which is the more likely interpretation). These velocities are 7.3 and 2 times higher, respectively, than the maximum ejecta velocity inferred from optical-line observations.

Monthly Notices of the Royal Astronomical Society, 2001

Observations of the starburst galaxy, M82, have been made with a 20-station global VLBI array at λ18cm. Maps are presented of the brightest young supernova remnants (SNR) in M82 and the wide-field mapping techniques used in making images over a field of view of ∼1 arcminute with 3 milliarcsecond resolution are discussed. A limit has been placed on the power law deceleration of the young SNR, 43.31+592 with an index greater than 0.73 ± 0.11 from observations with the European VLBI Network. Using the global array we have resolved compact knots of radio emission in the source which, with future global observations, will enable better constraints to be placed on the expansion parameters of this SNR.

2016

The European VLBI Network (EVN) has been used at two epochs in 2003 and 2009 to obtain multi-frequency high-resolution images of the merger galaxy NGC 6240 in order to study the radio properties of all compact high-brightness components in the galaxy. Our observations at milli-arcsecond resolution detected the northern and southern nuclei and two radio components, which we interpret as long-lived luminous supernovae associated with the circum-nuclear starburst activity at the southern nucleus. The new VLBI data support the presence of an active galactic nucleus (AGN) together with starburst activity at the southern nucleus and provides some evidence for an AGN at the northern nucleus. The two nuclei both display an inverted spectrum at lower GHz frequencies. The spectrum of the southern nucleus indicates thermal free-free absorption on parsec scales, consistent with the presence of an AGN.

Observational Evidence of Activity in Galaxies, 1987

VLBI observations show that the center of the nearby (^3.3 Mpc distant) spiral galaxy, M81, consists of a single elongated radio core, of dimensions 1000x4000 AU, with the major axis aligned, in projection, within 3° (<la) of the galaxy's rotation axis. This morphology can be interpreted in terms of an active galactic nucleus (AGN) with either a core-jet structure residing in M81 f s center or an accretion disk filling out the broadline region of this center. In contrast, the radio structure in the companion galaxy, M82, is very complex. VLBI observations of M82 yield the diameters and spectral index distribution of the hot spots, and the morphology and expansion velocity of the brightest hot spot, 41.9+58. Our results argue against the hot spots being core and jet condensations, or young supernovae (SNe), or typical supernova remnants (SNRs). We suggest that the hot spots in M82 are the remnants of stellar events that yielded some combination of SNRs and "exotic" objects.

The Astrophysical Journal, 2010

We present Wide Field Camera 3 images taken with the Hubble Space Telescope within a single field in the southern grand design star-forming galaxy M83. Based on their size, morphology, and photometry in continuum-subtracted Hα, [S ii], Hβ, [O iii], and [O ii] filters, we have identified 60 supernova remnant (SNR) candidates, as well as a handful of young ejecta-dominated candidates. A catalog of these remnants, their sizes and, where possible, their Hα fluxes are given. Radiative ages and pre-shock densities are derived from those SNRs that have good photometry. The ages lie in the range 2.62 < log (τ rad /yr) < 5.0, and the pre-shock densities at the blast wave range over 0.56 < n 0 /cm −3 < 1680. Two populations of SNRs have been discovered. These divide into a nuclear and spiral arm group and an inter-arm population. We infer an arm to inter-arm density contrast of 4. The surface flux in diffuse X-rays is correlated with the inferred pre-shock density, indicating that the warm interstellar medium (ISM) is pressurized by the hot X-ray plasma. We also find that the ISM in the nuclear region of M83 is characterized by a very high porosity and pressure, and infer an SNR rate of 1 per 70-150 yr for the nuclear (R < 300 pc) region. On the basis of the number of SNRs detected and their radiative ages, we infer that the lower mass of Type II SNe in M83 is M min = 16 +7 −5 M . Finally, we give evidence for the likely detection of the remnant of the historical supernova, SN1968L.

Monthly Notices of the Royal Astronomical Society, 2009

We present observations between 14.2 and 17.9 GHz of 12 reported supernova remnants (SNRs) made with the Arcminute Microkelvin Imager Small Array (AMI SA). In conjunction with data from the literature at lower radio frequencies, we determine spectra of these objects. For well-studied SNRs (Cas A, Tycho's SNR, 3C58 and the Crab Nebula), the results are in good agreement with spectra based on previous results. For the less well-studied remnants the AMI SA observations provide higher-frequency radio observations than previously available, and better constrain their radio spectra. The AMI SA results confirm a spectral turnover at ≃ 11 GHz for the filled-centre remnant G74·9+1·2. We also see a possible steepening of the spectrum of the filled-centre remnant G54·1+0·3 within the AMI SA frequency band compared with lower frequencies. We confirm that G84·9+0·5, which had previously been identified as a SNR, is rather an HII region and has a flat radio spectrum. ) that 'shell' type remnants, which typically have α in the range 0.3 to 0.7, show spectral flattening at higher frequencies.

2007

We report on 8.4GHz Very Long Baseline Interferometry (VLBI) observations of the type II-P supernova SN2004et in the spiral galaxy NGC 6946, made on 20 February 2005 (151 days after explosion). The Very Large Array (VLA) flux density was 1.23$\pm$0.07 mJy, corresponding to an isotropic luminosity at 8.4GHz of (4.45$\pm$0.3)$\times10^{25}$ erg s$^{-1}$ Hz$^{-1}$ and a brightness temperature of (1.3$\pm$0.3)$\times10^{8}$ K. We also provide an improved source position, accurate to about 0.5 mas in each coordinate. The VLBI image shows a clear asymmetry. From model fitting of the size of the radio emission, we estimate a minimum expansion velocity of 15,700$\pm$2,000 km s$^{-1}$. This velocity is more than twice the expected mean expansion velocity estimated from a synchrotron self-absorbed emission model, thus suggesting that synchrotron self-absorption is not relevant for this supernova. With the benefit of an optical spectrum obtained 12 days after explosion, we favor an emission model which consists of two hot spots on an underlying expanding shell of width comparable to that of SN 1993J.