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1. Shocks and dust formation in nova V809 Cep

   https://doi.org/10.1093/mnras/stac1366  

   Babul, Aliya-Nur ; Sokoloski, Jennifer L. ; Chomiuk, Laura ; Linford, Justin D. ; Weston, Jennifer H. S. ; Aydi, Elias ; Sokolovsky, Kirill V. ; Kawash, Adam M. ; Mukai, Koji ( May 2022 , Monthly Notices of the Royal Astronomical Society)

   The discovery that many classical novae produce detectable GeV γ-ray emission has raised the question of the role of shocks in nova eruptions. Here, we use radio observations of nova V809 Cep (nova Cep 2013) with the Jansky Very Large Array to show that it produced non-thermal emission indicative of particle acceleration in strong shocks for more than a month starting about 6 weeks into the eruption, quasi-simultaneous with the production of dust. Broadly speaking, the radio emission at late times – more than 6 months or so into the eruption – is consistent with thermal emission from $10^{-4}\, {\rm M}_\odot$ of freely expanding, 104 K ejecta. At 4.6 and 7.4 GHz, however, the radio light curves display an initial early-time peak 76 d after the discovery of the eruption in the optical (t0). The brightness temperature at 4.6 GHz on day 76 was greater than 105 K, an order of magnitude above what is expected for thermal emission. We argue that the brightness temperature is the result of synchrotron emission due to internal shocks within the ejecta. The evolution of the radio spectrum was consistent with synchrotron emission that peaked at high frequencies before low frequencies, suggesting that the synchrotron from the shockmore »was initially subject to free–free absorption by optically thick ionized material in front of the shock. Dust formation began around day 37, and we suggest that internal shocks in the ejecta were established prior to dust formation and caused the nucleation of dust.

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2. Low-frequency radio observations of recurrent nova RS Ophiuchi with MeerKAT and LOFAR

   https://doi.org/10.1093/mnras/stad1418  

   de Ruiter, Iris ; Nyamai, Miriam M. ; Rowlinson, Antonia ; Wijers, Ralph A. M. J. ; O’Brien, Tim J. ; Williams, David R. A. ; Woudt, Patrick ( May 2023 , Monthly Notices of the Royal Astronomical Society)

   We report low-frequency radio observations of the 2021 outburst of the recurrent nova RS Ophiuchi. These observations include the lowest frequency observations of this system to date. Detailed light curves are obtained by MeerKAT at 0.82 and 1.28 GHz and LOFAR at 54 and 154 MHz. These low-frequency detections allow us to put stringent constraints on the brightness temperature that clearly favour a non-thermal emission mechanism. The radio emission is interpreted and modelled as synchrotron emission from the shock interaction between the nova ejecta and the circumbinary medium. The light curve shows a plateauing behaviour after the first peak, which can be explained by either a non-uniform density of the circumbinary medium or a second emission component. Allowing for a second component in the light-curve modelling captures the steep decay at late times. Furthermore, extrapolating this model to 15 yr after the outburst shows that the radio emission might not fully disappear between outbursts. Further modelling of the light curves indicates a red giant mass-loss rate of ∼5 × 10−8 M⊙ yr−1. The spectrum cannot be modelled in detail at this stage, as there are likely at least four emission components. Radio emission from stellar wind or synchrotron jets is ruled out as themore »possible origin of the radio emission. Finally, we suggest a strategy for future observations that would advance our understanding of the physical properties of RS Ophiuchi.

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3. Radio light curves and imaging of the helium nova V445 Puppis reveal seven years of synchrotron emission

   https://doi.org/10.1093/mnras/staa3712  

   Nyamai, M M ; Chomiuk, L ; Ribeiro, V A ; Woudt, P A ; Strader, J ; Sokolovsky, K V ( December 2020 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT V445 Puppis is the only helium nova observed to date; its eruption in late 2000 showed high velocities up to 8500 km s−1, and a remarkable bipolar morphology cinched by an equatorial dust disc. Here we present multifrequency radio observations of V445 Pup obtained with the Very Large Array (VLA) spanning 1.5–43.3 GHz, and between 2001 January and 2008 March (days ∼89–2700 after eruption). The radio light curve is dominated by synchrotron emission over these 7 yr, and shows four distinct radio flares. Resolved radio images obtained in the VLA’s A configuration show that the synchrotron emission hugs the equatorial disc, and comparisons to near-IR images of the nova clearly demonstrate that it is the densest ejecta – not the fastest ejecta – that are the sites of the synchrotron emission in V445 Pup. The data are consistent with a model where the synchrotron emission is produced by a wind from the white dwarf impacting the dense equatorial disc, resulting in shocks and particle acceleration. The individual synchrotron flares may be associated with density enhancements in the equatorial disc and/or velocity variations in the wind from the white dwarf. This overall scenario is similar to a common picture of shock production in hydrogen-rich classical novae,more »but V445 Pup is remarkable in that these shocks persist for almost a decade, much longer than the weeks or months for which shocks are typically observed in classical novae.« less
4. The multiwavelength view of shocks in the fastest nova V1674 Her

   https://doi.org/10.1093/mnras/stad887  

   Sokolovsky, K. V. ; Johnson, T. J. ; Buson, S. ; Jean, P. ; Cheung, C. C. ; Mukai, K. ; Chomiuk, L. ; Aydi, E. ; Molina, B. ; Kawash, A. ; et al ( March 2023 , Monthly Notices of the Royal Astronomical Society)

   Classical novae are shock-powered multiwavelength transients triggered by a thermonuclear runaway on an accreting white dwarf. V1674 Her is the fastest nova ever recorded (time to declined by two magnitudes is t2 = 1.1 d) that challenges our understanding of shock formation in novae. We investigate the physical mechanisms behind nova emission from GeV γ-rays to cm-band radio using coordinated Fermi-LAT, NuSTAR, Swift, and VLA observations supported by optical photometry. Fermi-LAT detected short-lived (18 h) 0.1–100 GeV emission from V1674 Her that appeared 6 h after the eruption began; this was at a level of (1.6 ± 0.4) × 10−6 photons cm−2 s−1. Eleven days later, simultaneous NuSTAR and Swift X-ray observations revealed optically thin thermal plasma shock-heated to kTshock = 4 keV. The lack of a detectable 6.7 keV Fe Kα emission suggests super-solar CNO abundances. The radio emission from V1674 Her was consistent with thermal emission at early times and synchrotron at late times. The radio spectrum steeply rising with frequency may be a result of either free-free absorption of synchrotron and thermal emission by unshocked outer regions of the nova shell or the Razin–Tsytovich effect attenuating synchrotron emission in dense plasma. The development of the shock inside the ejecta is unaffected by the extraordinarily rapid evolution and the intermediate polar host of this nova.
5. Internal shocks from variable outflows in classical novae

   https://doi.org/10.1093/mnras/stz3300  

   Steinberg, Elad ; Metzger, Brian D. ( November 2019 , Monthly Notices of the Royal Astronomical Society)

   We present one-dimensional hydrodynamical simulations including radiative losses, of internal shocks in the outflows from classical novae, to explore the role of shocks in powering multiwavelength emission from radio to gamma-ray wavelengths. Observations support a picture in which the initial phases of some novae generate a slow, equatorially focused outflow (directly from the outer Lagrange point, or from a circumbinary disc), which then transitions to, or is overtaken by, a faster more isotropic outflow from the white dwarf which collides and shocks the slower flow, powering gamma-ray and optical emission through reprocessing by the ejecta. However, the common occurrence of multiple peaks in nova light curves suggests that the outflow’s acceleration need not be monotonic, but instead can involve successive transitions between ‘fast’ and ‘slow’ modes. Such a time-fluctuating outflow velocity naturally can reproduce several observed properties of nova, such as correlated gamma-ray and optical flares, expansion of the photosphere coincident with (though lagging slightly) the peak flare luminosity, and complex time evolution of spectral lines (including accelerating, decelerating, and merging velocity components). While the shocks are still deeply embedded during the gamma-ray emission, the onset of ∼keV X-ray and ∼10 GHz radio synchrotron emission is typically delayed until themore »forward shock of the outermost monolithic shell (created by merger of multiple internal shock-generated shells) reaches a sufficiently low column through the dense external medium generated by the earliest phase of the outburst.

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