Theses Doctoral

Uncovering Shocking Mysteries Buried in the Ejecta of Classical Novae and Magnetars

Babul, Aliya Nur Virji

This dissertation uses shocks to explain both the prevalence of radio synchroton emission and dust formation in classical novae, as well as the origin of fast radio bursts. First, we examine the radio lightcurves of nova V809 Cep and find that the peak brightness temperature exceeded 10⁵𝘒, 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. We then examine the radio lightcurves of seven novae with radio evidence for shocks (QU Vul, V1723 Aql, V5668 Sgr, V809 Cep, V357 Mus, V1324 Sco, PGIR20fbf) and IR/optical evidence for dust formation. We demonstrate that dust formation generally precedes the rise of radio non-thermal emission, and present evidence to suggest that shocks occur prior to the onset of dust formation but that the radio shock emission is initially being absorbed by a layer of photo-ionized gas ahead of the shock.

We model the optical depth of the photo-ionized gas to demonstrate that the time required for the photo-ionized gas to become optically thin to radio frequencies can be longer than the time required for dust nucleation; thus, dust appears to form before the shock emission is visible. We further demonstrate that the radio spectral evolution in novae with no evidence for dust formation is markedly different from novae with evidence for shocks, suggesting that in novae without velocity or distance estimates, the radio spectral evolution could be used to constrain the presence of shocks. Finally, we demonstrate that novae with evidence for dust absorption are preferentially inclined edge, on suggesting that both shocks and dust form in the equatorial plane. Since internal shocks in nova ejecta are thought to lead to dust formation, localizing both phenomenon to the equatorial plane strengthens the connection between the two phenomena.

We then use Particle-In-Cell (PIC) simulations to explore the synchroton maser instability as a potential mechanism for the formation of Fast Radio Bursts. Electromagnetic precursor waves generated by the synchrotron maser instability at relativistic magnetized shocks have been recently invoked to explain the coherent radio emission of Fast Radio Bursts. By means of two-dimensional particle-in-cell simulations, we explore the properties of the precursor waves in relativistic electron-positron perpendicular shocks as a function of the pre-shock magnetization σ ≳1 (i.e., the ratio of incoming Poynting flux to particle energy flux) and thermal spread Δᵧ ≡ 𝑘𝑇/𝑚𝑐² = 10⁻⁵−10⁻¹. We measure the fraction 𝑓𝜉 of total incoming energy that is converted into precursor waves, as computed in the post-shock frame.

At fixed magnetization, we find that 𝑓𝜉 is nearly independent of temperature as long as Δᵧ ≲ 10¹·⁵ (with only a modest decrease of a factor of three from Δᵧ = 10⁻⁵ to Δᵧ = 10¹·⁵, but it drops by nearly two orders of magnitude for Δᵧ ≳ 10⁻¹. For our reference σ = 1, the power spectrum of precursor waves is relatively broad (fractional width ∼ 1−3) for cold temperatures, whereas it shows pronounced line-like features with fractional width ∼ 0.2 for 10⁻³ ≲ Δᵧ ≲ 10¹·⁵. For σ ≳ 1, the precursor waves are beamed within an angle ≃ σ -⁻¹/² from the shock normal (as measured in the post-shock frame), as required so they can outrun the shock. Our results can provide physically-grounded inputs for FRB emission models based on maser emission from relativistic shocks.


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More About This Work

Academic Units
Thesis Advisors
Sokoloski, Jennifer Lynn
Sironi, Lorenzo
Ph.D., Columbia University
Published Here
February 2, 2022