arXiv subjects

Montana N. Williams

Publications and source records attributed to Montana N. Williams.

5 recordsLinked to original sources

arXiv2026

TESS detection of periodic brightness variations during the rise of classical nova PGIR22akgylf

Classical novae are transient events powered by thermonuclear burning in a layer of hydrogen-rich material accreted by a white dwarf from its binary companion. Most classical novae reach optical maximum within ~1 d, but a rare few rise far more slowly. We probe the envelope structure and ejection mechanism of the slowly-rising nova PGIR22akgylf with TESS photometry spanning 3 to 16 d after the nova discovery, supplemented by ground-based observations that cover its full ~133 d ascent to maximum. We detect a 0.1802 +/-0.0012 d periodic brightness modulation with a peak-to-peak amplitude of ~0.02 mag, identified with PGIR22akgylf via temporal and spatial coincidence. The period is stable over the two weeks of TESS coverage, suggesting an orbital origin. Whether this period corresponds to the full or half orbital period, it implies a dwarf donor companion. At the time of the TESS observations the nova was >~6 mag above quiescence (but still 4 mag below peak), so its light should be dominated by the expanding photosphere. We interpret the periodic signal as arising from the binary orbital motion distorting the nova envelope while its size remains comparable to the binary separation. This interpretation points to common-envelope interaction as a contributor to shell ejection in PGIR22akgylf and demonstrates that the slow-rise phenomenon is not exclusive to thermonuclear eruptions in symbiotic binaries, where the large orbital separation of the giant companion inhibits such interaction.

astro-ph.SR↗

arXiv2026

Shocks in the Symbiotic Recurrent Nova V3890 Sgr: VLBI Radio Imaging and Fermi GeV Gamma-Rays

We present very long baseline interferometric (VLBI) radio imaging and Fermi/LAT GeV $γ$-ray observations of the 2019 eruption of the symbiotic recurrent nova V3890 Sgr.The VLBI imaging spans 8 -- 51 days after eruption, synchronous with the detected $γ$-rays. VLBI imaging shows the eruption starts out asymmetric on day 8 with an eastern component brighter than a western component. By day 32 the blast is rather circularly symmetric, and on day 49, the nova shell is brighter along the north--south axis. This morphological evolution is explained by interaction with circumstellar material (CSM) comprised of a spherical wind plus an over-density in the orbital plane. Comparing radio images to optical line widths gives an expansion parallax distance of 6.8 kpc. In the first 32 days or eruption, VLBI images capture $>$80 per cent of the integrated flux (as measured by the VLA), implying that synchrotron emission dominates. A second peak in the VLA light curve is explained by an image on day 48 that reveals the nova shell surrounded by a diffuse halo, powered by synchrotron emission from particles that have diffused upstream of the shock. The $γ$-rays appear around optical maximum and remain detectable for 23 days; marginally significant $γ$-rays reappear around day 60, concurrent with the second radio peak. Modelling indicates radio and $γ$-ray emission arise in distinct shock regions: $γ$-rays from dense CSM in the orbital plane, radio from the more spherical CSM component. X-ray observations constrain the spherical CSM density, which is higher than in other symbiotic recurrent novae. Assuming equipartition, we estimate the fraction of the post-shock pressure in magnetic fields, $ε_B = 3 \times 10^{-4} - 2 \times 10^{-3}$.

astro-ph.HE↗

arXiv2025

Multiple outflows and delayed ejections revealed by early imaging of novae

Novae are thermonuclear eruptions on accreting white dwarfs in interacting binaries. Although most of the accreted envelope is expelled, the mechanism -- impulsive ejection, multiple outflows or prolonged winds, or a common-envelope interaction -- remains uncertain. GeV $γ$-ray detections from $>20$ Galactic novae establish these eruptions as nearby laboratories for shock physics and particle acceleration, underscoring the need to determine how novae eject their envelopes. Here we report on near-infrared interferometry, supported with multiwavelength observations, of two $γ$-ray detected novae. The images of the very fast 2021 nova V1674~Her, taken just 2--3 days after discovery, reveal the presence of two perpendicular outflows. The interaction between these outflows likely drives the observed $γ$-ray emission. Conversely, the images of the very slow 2021 nova V1405~Cas suggest a delay in the ejection of the bulk of the accreted envelope of more than 50 days after the start of eruption, as the nova slowly rises to visible peak and during which the envelope engulfed the system in a common envelope phase. These unprecedented images offer direct observational evidence that the mechanisms driving mass ejection from the surfaces of accreting white dwarfs are not as simple as previously thought, revealing multiple outflows and delayed ejections.

astro-ph.HE↗

arXiv2025

TESS photometry of the nova eruption in V606 Vul: asymmetric photosphere and multiple ejections?

Lightcurves of many classical novae deviate from the canonical "fast rise - smooth decline" pattern and display complex variability behavior. We present the first TESS-space-photometry-based investigation of this phenomenon. We use Sector 41 full-frame images to extract a lightcurve of the slow Galactic nova V606 Vul that erupted nine days prior to the start of the TESS observations. The lightcurve covers the first of two major peaks of V606 Vul that was reached 19 days after the start of the eruption. The nova reached its brightest visual magnitude V=9.9 in its second peak 64 days after the eruption onset, following the completion of Sector 41 observations. To increase the confidence level of the extracted lightcurve, we performed the analysis using four different codes implementing the aperture photometry (Lightkurve, VaST) and image subtraction (TESSreduce, tequila_shots) and find good agreement between them. We performed ground-based photometric and spectroscopic monitoring to complement the TESS data. The TESS lightcurve reveals two features: periodic variations (0.12771 d, 0.01 mag average peak-to-peak amplitude) that disappeared when the source was within 1 mag of peak optical brightness and a series of isolated mini-flares (with peak-to-peak amplitudes of up to 0.5 mag) appearing at seemingly random times. We interpret the periodic variations as the result of azimuthal asymmetry of the photosphere engulfing the nova-hosting binary that was distorted by and rotating with the binary. Whereas we use spectra to associate the two major peaks in the nova lightcurve with distinct episodes of mass ejection, the origin of mini-flares remains elusive.

astro-ph.SR↗

arXiv2024

The Symbiotic Recurrent Nova V745 Sco at Radio Wavelengths

V745 Sco is a Galactic symbiotic recurrent nova with nova eruptions in 1937, 1989 and 2014. We study the behavior of V745 Sco at radio wavelengths (0.6-37,GHz), covering both its 1989 and 2014 eruptions and informed by optical, X-ray, and $γ$-ray data. The radio light curves are synchrotron-dominated. Surprisingly, compared to expectations for synchrotron emission from explosive transients such as radio supernovae, the light curves spanning 0.6-37 GHz all peak around the same time ($\sim$18-26 days after eruption) and with similar flux densities (5-9 mJy).We model the synchrotron light curves as interaction of the nova ejecta with the red giant wind, but find that simple spherically symmetric models with wind-like circumstellar material (CSM) cannot explain the radio light curve. Instead, we conclude that the shock suddenly breaks out of a dense CSM absorbing screen around 20 days after eruption, and then expands into a relatively low density wind ($\dot{M}_{out} \approx 10^{-9}-10^{-8}$ M$_{\odot}$ yr$^{-1}$ for $v_w = 10$ km s$^{-1}$) out to $\sim$1 year post-eruption. The dense, close-in CSM may be an equatorial density enhancement or a more spherical red giant wind with $\dot{M}_{in} \approx [5-10] \times 10^{-7}$ M$_{\odot}$ yr$^{-1}$, truncated beyond several $\times 10^{14}$ cm. The outer lower-density CSM would not be visible in typical radio observations of Type Ia supernovae: V745 Sco cannot be ruled out as a Type Ia progenitor based on CSM constraints alone.Complementary constraints from the free-free radio optical depth and the synchrotron luminosity imply the shock is efficient at accelerating relativistic electrons and amplifying magnetic fields, with $ε_e$ and $ε_B \approx 0.01-0.1$.

astro-ph.HE↗