arXiv subjects

N. E. Nunez

Publications and source records attributed to N. E. Nunez.

4 recordsLinked to original sources

arXiv2026

A search for new symbiotic stars in the Milky Way: Using machine learning techniques applied to photometric databases

Symbiotic stars (SySts) are interacting binaries composed of a red giant transferring material to a hot compact star, typically a white dwarf. Although only about 300 systems are confirmed, the Galactic population is estimated at 1.2 x 10^3 - 1.5 x 10^4, indicating that most remain undiscovered. We identify new SySts using a machine-learning approach that combines Gaia DR3, 2MASS, and WISE photometry, parallaxes, and the pseudo-equivalent width of H alpha. A Random Forest model was trained on 166 confirmed S-type SySts and 1600 non-symbiotic stars, applying SMOTE to mitigate class imbalance. The model achieved an F1-score of 89% for the symbiotic class. Applied to 2.5 x 10^6 color-selected sources, it identified 990 candidates with probabilities more than 70%. We further refined the sample using physically motivated cuts on effective temperature, surface gravity, metallicity, and SkyMapper photometry, yielding 12 high-confidence candidates. These objects show cool temperatures, low surface gravities, near-solar metallicity, H alpha emission, moderate-to-high luminosities, and UV excess consistent with S-type SySts. Validation on recently confirmed systems recovered 92.3%, demonstrating the robustness and generalizability of our method.

astro-ph.SR↗

arXiv2018

Dramatic change in the boundary layer in the symbiotic recurrent nova T Coronae Borealis

A sudden increase in the rate at which material reaches the most internal part of an accretion disk, i.e. the boundary layer, can change its structure dramatically. We have witnessed such change for the first time in the symbiotic recurrent nova T CrB. Our analysis of XMM-Newton, Swift Burst Alert Telescope (BAT)/ X-Ray Telescope (XRT) / UltraViolet Optical Telescope (UVOT) and American Association of Variable Stars Observers (AAVSO) V and B-band data indicates that during an optical brightening event that started in early 2014 ($Δ$ V$\approx$1.5): (i) the hard X-ray emission as seen with BAT almost vanished; (ii) the XRT X-ray flux decreased significantly while the optical flux remained high; (iii) the UV flux increased by at least a factor of 40 over the quiescent value; and (iv) the X-ray spectrum became much softer and a bright, new, blackbody-like component appeared. We suggest that the optical brightening event, which could be a similar event to that observed about 8 years before the most recent thermonuclear outburst in 1946, is due to a disk instability

astro-ph.HE↗

arXiv2016

Swift observations of the 2015 outburst of AG Peg -- from slow nova to classical symbiotic outburst

Symbiotic stars often contain white dwarfs with quasi-steady shell burning on their surfaces. However, in most symbiotics, the origin of this burning is unclear. In symbiotic slow novae, however, it is linked to a past thermonuclear runaway. In June 2015, the symbiotic slow nova AG Peg was seen in only its second optical outburst since 1850. This recent outburst was of much shorter duration and lower amplitude than the earlier eruption, and it contained multiple peaks -- like outbursts in classical symbiotic stars such as Z And. We report Swift X-ray and UV observations of AG Peg made between June 2015 and January 2016. The X-ray flux was markedly variable on a time scale of days, particularly during four days near optical maximum, when the X-rays became bright and soft. This strong X-ray variability continued for another month, after which the X-rays hardened as the optical flux declined. The UV flux was high throughout the outburst, consistent with quasi-steady shell burning on the white dwarf. Given that accretion disks around white dwarfs with shell burning do not generally produce detectable X-rays (due to Compton-cooling of the boundary layer), the X-rays probably originated via shocks in the ejecta. As the X-ray photo-electric absorption did not vary significantly, the X-ray variability may directly link to the properties of the shocked material. AG Peg's transition from a slow symbiotic nova (which drove the 1850 outburst) to a classical symbiotic star suggests that shell burning in at least some symbiotic stars is residual burning from prior novae.

astro-ph.HE↗

arXiv2016

SU Lyncis, a hard X-ray bright M giant: Clues point to a large hidden population of symbiotic stars

Symbiotic star surveys have traditionally relied almost exclusively on low resolution optical spectroscopy. However, we can obtain a more reliable estimate of their total Galactic population by using all available signatures of the symbiotic phenomenon. Here we report the discovery of a hard X-ray source, 4PBC J0642.9+5528, in the Swift hard X-ray all-sky survey, and identify it with a poorly studied red giant, SU Lyn, using pointed Swift observations and ground-based optical spectroscopy. The X-ray spectrum, the optical to UV spectrum, and the rapid UV variability of SU Lyn are all consistent with our interpretation that it is a symbiotic star containing an accreting white dwarf. The symbiotic nature of SU Lyn went unnoticed until now, because it does not exhibit emission lines strong enough to be obvious in low resolution spectra. We argue that symbiotic stars without shell-burning have weak emission lines, and that the current lists of symbiotic stars are biased in favor of shell-burning systems. We conclude that the true population of symbiotic stars has been underestimated, potentially by a large factor.

astro-ph.SR↗