31 research outputs found
The bright supernova 1996cr in the circinus galaxy imaged with VLBI: shell structure with complex evolution
DATA AVAILABILITY STATEMENT : The raw data underlying this paper are available in the Australian Telescope National Facility archive, http://atoa.atnf.csiro.au, the South African Radio Astronomy Observatory archive https://archive.sarao.ac.za, and the ALMA archive https://almascience.nrao.edu/aq. Other data will be shared on reasonable request.Please read abstract in the article.The Australia Telescope Compact Array is part of the Australia Telescope National Facility which is funded by the Australian Government; the NASA/IPAC Extragalactic Database (NED), which is funded by the National Aeronautics and Space Administration; the National Sciences and Engineering Research Council of Canada and the National Research Foundation of South Africa.https://academic.oup.com/mnrashj2024PhysicsNon
AT 2018cow VLBI: No Long-Lived Relativistic Outflow
Abstract We report on VLBI observations of the fast and blue optical transient (FBOT), AT 2018cow. At âŒ62 Mpc, AT 2018cow is the first relatively nearby FBOT. The nature of AT 2018cow is not clear, although various hypotheses from a tidal disruption event to different kinds of supernovae have been suggested. It had a very fast rise time (3.5 d) and an almost featureless blue spectrum although high photospheric velocities (40,000 km sâ1) were suggested early on. The X-ray luminosity was very high, âŒ1.4 Ă 1043 erg sâ1, larger than those of ordinary SNe, and more consistent with those of SNe associated with gamma-ray bursts. Variable hard X-ray emission hints at a long-lived âcentral engine.â It was also fairly radio luminous, with a peak 8.4-GHz spectral luminosity of âŒ4 Ă 1028 erg sâ1 Hzâ1, allowing us to make VLBI observations at ages between 22 and 287 d. We do not resolve AT 2018cow. Assuming a circularly symmetric source, our observations constrain the average apparent expansion velocity to be <0.49 c by t = 98 d (3Ï limit). We also constrain the proper motion of AT 2018cow to be <0.51 c. Since the radio emission generally traces the fastest ejecta, our observations make the presence of a long-lived relativistic jet with a lifetime of more than one month very unlikely
Radio source calibration for the VSA and other CMB instruments at around 30 GHz
Accurate calibration of data is essential for the current generation of CMB
experiments. Using data from the Very Small Array (VSA), we describe procedures
which will lead to an accuracy of 1 percent or better for experiments such as
the VSA and CBI. Particular attention is paid to the stability of the receiver
systems, the quality of the site and frequent observations of reference
sources. At 30 GHz the careful correction for atmospheric emission and
absorption is shown to be essential for achieving 1 percent precision. The
sources for which a 1 percent relative flux density calibration was achieved
included Cas A, Cyg A, Tau A and NGC7027 and the planets Venus, Jupiter and
Saturn. A flux density, or brightness temperature in the case of the planets,
was derived at 33 GHz relative to Jupiter which was adopted as the fundamental
calibrator. A spectral index at ~30 GHz is given for each. Cas A,Tau A, NGC7027
and Venus were examined for variability. Cas A was found to be decreasing at
percent per year over the period March 2001 to August 2004.
In the same period Tau A was decreasing at percent per year. A
survey of the published data showed that the planetary nebula NGC7027 decreased
at percent per year over the period 1967 to 2003. Venus showed
an insignificant ( percent) variation with Venusian illumination.
The integrated polarization of Tau A at 33 GHz was found to be
percent at pa .}Comment: 13 pages, 15 figures, submitted to MNRA
High sensitivity measurements of the CMB power spectrum with the extended Very Small Array
We present deep Ka-band ( GHz) observations of the CMB made
with the extended Very Small Array (VSA). This configuration produces a
naturally weighted synthesized FWHM beamwidth of arcmin which covers
an -range of 300 to 1500. On these scales, foreground extragalactic
sources can be a major source of contamination to the CMB anisotropy. This
problem has been alleviated by identifying sources at 15 GHz with the Ryle
Telescope and then monitoring these sources at 33 GHz using a single baseline
interferometer co-located with the VSA. Sources with flux densities \gtsim 20
mJy at 33 GHz are subtracted from the data. In addition, we calculate a
statistical correction for the small residual contribution from weaker sources
that are below the detection limit of the survey.
The CMB power spectrum corrected for Galactic foregrounds and extragalactic
point sources is presented. A total -range of 150-1500 is achieved by
combining the complete extended array data with earlier VSA data in a compact
configuration. Our resolution of allows the first 3
acoustic peaks to be clearly delineated. The is achieved by using mosaiced
observations in 7 regions covering a total area of 82 sq. degrees. There is
good agreement with WMAP data up to where WMAP data run out of
resolution. For higher -values out to , the agreement in
power spectrum amplitudes with other experiments is also very good despite
differences in frequency and observing technique.Comment: 16 pages. Accepted in MNRAS (minor revisions
Seven years of coordinated ChandraâNuSTAR observations of SN 2014C unfold the extreme mass-loss history of its stellar progenitor
We present the results from our 7 yr long broadband X-ray observing campaign of SN 2014C with Chandra and NuSTAR. These coordinated observations represent the first look at the evolution of a young extragalactic SN in the 0.3â80 keV energy range in the years after core collapse. We find that the spectroscopic metamorphosis of SN 2014C from an ordinary type Ib SN into an interacting SN with copious hydrogen emission is accompanied by luminous X-rays reaching L x â 5.6 Ă 1040 erg sâ1 (0.3â100 keV) at âŒ1000 days post-explosion and declining as L x â t â1 afterwards. The broadband X-ray spectrum is of thermal origin and shows clear evidence for cooling after peak, with T(t)â20keV(t/tpk)â0.5 . Soft X-rays of sub-keV energy suffer from large photoelectric absorption originating from the local SN environment with NHint(t)â3Ă1022(t/400days)â1.4cmâ2 . We interpret these findings as the result of the interaction of the SN shock with a dense (n â 105 â 106 cmâ3), H-rich disk-like circumstellar medium (CSM) with inner radius âŒ2 Ă 1016 cm and extending to âŒ1017 cm. Based on the declining NHint(t) and X-ray luminosity evolution, we infer a CSM mass of âŒ(1.2 fâ2.0 f)Mâ , where f is the volume filling factor. We place SN 2014C in the context of 121 core-collapse SNe with evidence for strong shock interaction with a thick circumstellar medium. Finally, we highlight the challenges that the current mass-loss theories (including wave-driven mass loss, binary interaction, and line-driven winds) face when interpreting the wide dynamic ranges of CSM parameters inferred from observations
Radio analysis of SN2004C reveals an unusual CSM density profile as a harbinger of core collapse
We present extensive multifrequency Karl G. Jansky Very Large Array (VLA) and Very Long Baseline Array (VLBA) observations of the radio-bright supernova (SN) IIb SN 2004C that span âŒ40â2793 days post-explosion. We interpret the temporal evolution of the radio spectral energy distribution in the context of synchrotron self-absorbed emission from the explosion's forward shock as it expands in the circumstellar medium (CSM) previously sculpted by the mass-loss history of the stellar progenitor. VLBA observations and modeling of the VLA data point to a blastwave with average velocity âŒ0.06 c that carries an energy of â1049 erg. Our modeling further reveals a flat CSM density profile ÏCSM â Râ0.03±0.22 up to a break radius Rbr â (1.96 ± 0.10) Ă 1016 cm, with a steep density gradient following ÏCSM â Râ2.3±0.5 at larger radii. We infer that the flat part of the density profile corresponds to a CSM shell with mass âŒ0.021 Mâ, and that the progenitor's effective mass-loss rate varied with time over the range (50â500) Ă 10â5 Mâ yrâ1 for an adopted wind velocity vw = 1000 km sâ1 and shock microphysical parameters epsilone = 0.1, epsilonB = 0.01. These results add to the mounting observational evidence for departures from the traditional single-wind mass-loss scenarios in evolved, massive stars in the centuries leading up to core collapse. Potentially viable scenarios include mass loss powered by gravity waves and/or interaction with a binary companion