| 1. |
- Björnsson, Claes-Ingvar
(författare)
-
Implications of the Low-frequency Turnover in the Spectrum of Radio Knot C in DG Tau
- 2021
-
Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 923:1
-
Tidskriftsartikel (refereegranskat)abstract
- The synchrotron spectrum of radio knot C in the protostellar object DG Tau has a low-frequency turnover. This is used to show that its magnetic field strength is likely to be at least 10 mG, which is roughly two orders of magnitude larger than previously estimated. The earlier, lower value is due to an overestimate of the emission volume together with an omission of the dependence of the minimum magnetic field on the synchrotron spectral index. Since the source is partially resolved, this implies a low volume-filling factor for the synchrotron emission. It is argued that the high pressure needed to account for the observations is due to shocks. In addition, cooling of the thermal gas is probably necessary in order to further enhance the magnetic field strength as well as the density of relativistic electrons. It is suggested that the observed spectral index implies that the energy of the radio-emitting electrons is below that needed to take part in first-order Fermi acceleration. Hence, the radio emission gives insights to the properties of its pre-acceleration phase. Attention is also drawn to the similarities between the properties of radio knot C and the shock-induced radio emission in supernovae.
|
|
| 2. |
- Björnsson, Claes-Ingvar
(författare)
-
Modeling of Radio Supernovae : Including the Effects of Inhomogeneities and Radiative Cooling
- 2024
-
Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 963:2
-
Tidskriftsartikel (refereegranskat)abstract
- The presence of inhomogeneities in a spatially unresolved source is often hard to establish. This limits the accuracy with which the source properties can be determined. It is shown how observed features not expected for a homogeneous model can be used to infer the properties of the inhomogeneities in radio supernovae. Furthermore, the observed consequences of radiative cooling can be seriously affected by inhomogeneities. It is shown that the deduced source properties are very sensitive to the observed value of the cooling frequency; even a lower limit is often useful to constrain its characteristics. It is argued that the main synchrotron emission region in SN 2003L has a small volume filling factor, possibly as low as a few percent. On the contrary, deviations from homogeneity are substantially smaller in SN 2002ap. The observed properties of Type Ib/c radio supernovae in general indicate the volume filling factor to remain rather constant with time for individual sources, but those peaking later at radio frequencies have lower filling factors. The conditions in the main synchrotron component in both SN 2003L and SN 2002ap are consistent with equipartition of energy between relativistic electrons and magnetic fields.
|
|
| 3. |
- Björnsson, Claes-Ingvar
(författare)
-
Radio Spectra of SN 2020oi : Effects of Radiative Cooling on the Deduced Source Properties
- 2022
-
Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 936:2
-
Tidskriftsartikel (refereegranskat)abstract
- Observations of radiative cooling in a synchrotron source offer a possibility to further constrain its properties. Inverse Compton cooling is indicated in the radio spectra during the early phases of SN 2020oi. It is shown that contrary to previous claims, observations are consistent with equipartition between relativistic electrons and magnetic field as well as a constant mass-loss rate of the progenitor star prior to the supernova explosion. The reason for this difference is the need to include cooling directly in the fitting procedure rather than estimating its effects afterward. It is emphasized that the inferred properties of the supernova ejecta are sensitive to the time evolution of the synchrotron self-absorption frequency; hence, great care should be taken when modeling spectra for which cooling and/or inhomogeneities are indicated. Furthermore, it is noted that the energies of the relativistic electrons in the radio emission regions in supernovae are likely too low for first-order Fermi acceleration to be effective.
|
|
| 4. |
- Lundqvist, Peter, et al.
(författare)
-
Atacama Compact Array observations of the pulsar-wind nebula of SNR 0540-69.3
- 2020
-
Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 496:2, s. 1834-1844
-
Tidskriftsartikel (refereegranskat)abstract
- We present observations of the pulsar-wind nebula (PWN) region of SNR 0540-69.3. The observations were made with the Atacama Compact Array (ACA) in Bands 4 and 6. We also add radio observations from the Australia Compact Array at 3 cm. For 1.449-233.50 GHz, we obtain a synchrotron spectrum F-nu proportional to nu(-alpha nu) with the spectral index alpha(nu) = 0.17 +/- 0.02. To conclude how this joins the synchrotron spectrum at higher frequencies, we include hitherto unpublished AKARI mid-infrared data, and evaluate published data in the ultraviolet (UV), optical, and infrared (IR). In particular, some broad-band filter data in the optical must be discarded from our analysis due to contamination by spectral line emission. For the UV/IR part of the synchrotron spectrum, we arrive at alpha(nu) = 0.87(-0.10)(+0.08). There is room for 2.5 x 10(-3) M-circle dot of dust with a temperature of similar to 55 K if there are dual breaks in the synchrotron spectrum, one around similar to 9 x 10(10) Hz and another at similar to 2 x 10(13) Hz. The spectral index then changes at similar to 9 x 10(10) Hz from alpha(nu) = 0.14 +/- 0.07 in the radio to alpha(nu) = 0.35(+0.05)(-0.07) in the millimetre-to-far-IR range. The ACA Band 6 data marginally resolve the PWN. In particular, the strong emission similar to 1 ''.5 south-west of the pulsar, seen at other wavelengths, and resolved in the 3 cm data with its 0 ''.8 spatial resolution, is also strong in the millimetre range. The ACA data clearly reveal the supernova remnant shell similar to 20-35 arcsec west of the pulsar, and for the shell we derive alpha(nu )= 0.64 +/- 0.05 for the range 8.6-145 GHz.
|
|
| 5. |
- Lundqvist, Peter, et al.
(författare)
-
The Deepest Radio Observations of Nearby SNe Ia : Constraining Progenitor Types and Optimizing Future Surveys
- 2020
-
Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 890:2
-
Tidskriftsartikel (refereegranskat)abstract
- We report deep radio observations of nearby Type Ia supernovae (SNe Ia) with the electronic Multi-Element Radio Linked Interferometer Network and the Australia Telescope Compact Array. No detections were made. With standard assumptions for the energy densities of relativistic electrons going into a power-law energy distribution and the magnetic field strength (epsilon(e) = epsilon(B)=.0.1), we arrive at upper limits on mass-loss rate for the progenitor system of SN.2013dy.(SN 2016coj, SN 2018gv, SN 2018pv, SN 2019np) of M less than or similar to 12 (2.8, 1.3, 2.1, 1.7) x 10(-8) M-circle dot yr(-1) (v(w)/100 km s-(1)), where v(w) is the wind speed of the mass loss. To SN.2016coj, SN 2018gv, SN 2018pv, and SN 2019np we add radio data for 17 other nearby SNe.Ia and model their nondetections. With the same model as described, all 21 SNe Ia have M less than or similar to 4 x 10(-8) M-circle dot yr(-1) (v(w)/ 100 km s(-1)). We compare those limits with the expected mass-loss rates in different single-degenerate progenitor scenarios. We also discuss how information on oe and oB can be obtained from late observations of SNe.Ia and the youngest SN.Ia remnant detected in radio, G1.9+0.3, as well as strippedenvelope core-collapse SNe. We highlight SN.2011dh and argue for epsilon e approximate to 0.1 and epsilon(B) approximate to 0.0033. Finally, we discuss strategies to observe at radio frequencies to maximize the chance of detection, given the time since explosion, the distance to the SN, and the telescope sensitivity.
|
|
