High Energy Astrophysical Phenomena (astro-ph.HE)

Stellar X-Ray Variability and Planetary Evolution in the DS Tucanae System

2025-03-28T17:00:32Z

Stellar X-Ray Variability and Planetary Evolution in the DS Tucanae System King, George W.; Corrales, Lía R.; Bourrier, Vincent; Dos Santos, Leonardo A.; Doyle, Lauren; Lavie, Baptiste; Ramsay, Gavin; Wheatley, Peter J. We present an analysis of four Chandra observations of the 45 Myr old DS Tuc binary system. We observed X-ray variability of both stars on timescales from hours to months, including two strong X-ray flares from star A. The implied flaring rates are in agreement with past observations made with XMM-Newton, though these rates remain imprecise due to the relatively short total observation time. We find a clear, monotonic decline in the quiescent level of the star by a factor of 1.8 across 8 months, suggesting stellar variability that might be due to an activity cycle. If proven through future observations, DS Tuc A would be the youngest star for which a coronal activity cycle has been confirmed. The variation in our flux measurements across the four visits is also consistent with the scatter in empirical stellar X-ray relationships with Rossby number. In simulations of the possible evolution of the currently super-Neptune-sized planet DS Tuc A b, we find a range of scenarios for the planet once it reaches a typical field age of 5 Gyr, from Neptune size down to a completely stripped super-Earth. Improved constraints on the planet's mass in the future would significantly narrow these possibilities. We advocate for further Chandra observations to better constrain the variability of this important system.

Optical evolution of AT 2024wpp: the high-velocity outflows in Cow-like transients are consistent with high spherical symmetry

2025-03-28T17:00:30Z

Optical evolution of AT 2024wpp: the high-velocity outflows in Cow-like transients are consistent with high spherical symmetry Pursiainen, M.; Killestein, T. L.; Kuncarayakti, H.; Charalampopoulos, P.; Warwick, B.; Lyman, J.; Kotak, R.; Leloudas, G.; Coppejans, D.; Kravtsov, T.; Maeda, K.; Nagao, T.; Taguchi, K.; Ackley, K.; Dhillon, V. S.; Galloway, D. K.; Kumar, A.; O'Neill, D.; Ramsay, G.; Steeghs, D. We present the analysis of optical/near-infrared (NIR) data and host galaxy properties of a bright, extremely rapidly evolving transient, AT 2024wpp, which resembles the enigmatic AT 2018cow. AT 2024wpp rose to a peak brightness of

$c=-21.9$

mag in 4.3 d and remained above the half-maximum brightness for only 6.7 d. The blackbody fits to the photometry show that the event remained persistently hot (

$T\gtrsim 20\, 000$

K) with a rapidly receding photosphere (

$v\sim 11\, 500$

km s

$^{-1}$

), similarly to AT 2018cow albeit with a several times larger photosphere.

$JH$

photometry reveals an NIR excess over the thermal emission at

$\sim +20$

d, indicating a presence of an additional component. The spectra are consistent with blackbody emission throughout our spectral sequence ending at

$+21.9$

d, showing a tentative, very broad emission feature at

$\sim 5500$

Å - implying that the optical photosphere is likely within a near-relativistic outflow. Furthermore, reports of strong X-ray and radio emission cement the nature of AT 2024wpp as a likely Cow-like transient. AT 2024wpp is the second event of the class with optical polarimetry. Our

$BVRI$

observations obtained from

$+6.1$

$+14.4$

d show a low polarization of

$P\lesssim 0.5$

per cent across all bands, similar to AT 2018cow that was consistent with

$P\sim 0$

per cent during the same outflow-driven phase. In the absence of evidence for a preferential viewing angle, it is unlikely that both events would have shown low polarization in the case that their photospheres were aspherical. As such, we conclude that the near-relativistic outflows launched in these events are likely highly spherical, but polarimetric observations of further events are crucial to constrain their ejecta geometry and stratification in detail.

SN 2023tsz: a helium-interaction-driven supernova in a very low-mass galaxy

2025-02-11T21:42:32Z

SN 2023tsz: a helium-interaction-driven supernova in a very low-mass galaxy Warwick, B.; Lyman, J.; Pursiainen, M.; Coppejans, D. L.; Galbany, L.; Jones, G. T.; Killestein, T. L.; Kumar, A.; Oates, S. R.; Ackley, K.; Anderson, J. P.; Aryan, A.; Breton, R. P.; Chen, T. W.; Clark, P.; Dhillon, V. S.; Dyer, M. J.; Gal-Yam, A.; Galloway, D. K.; Gutiérrez, C. P.; Gromadzki, M.; Inserra, C.; Jiménez-Ibarra, F.; Kelsey, L.; Kotak, R.; Kravtsov, T.; Kuncarayakti, H.; Magee, M. R.; Matilainen, K.; Mattila, S.; Müller-Bravo, T. E.; Nicholl, M.; Noysena, K.; Nuttall, L. K.; O'Brien, P.; O'Neill, D.; Pallé, E.; Pessi, T.; Petrushevska, T.; Pignata, G.; Pollacco, D.; Ragosta, F.; Ramsay, G.; Sahu, A.; Sahu, D. K.; Singh, A.; Sollerman, J.; Stanway, E.; Starling, R.; Steeghs, D.; Teja, R. S.; Ulaczyk, K. SN 2023tsz is a Type Ibn supernova (SN Ibn), an uncommon subtype of stripped-envelope core-collapse supernovae (SNe), discovered in an extremely low-mass host. SNe Ibn are characterized by narrow helium emission lines in their spectra and are believed to originate from the collapse of massive Wolf-Rayet (WR) stars, though their progenitor systems still remain poorly understood. In terms of energetics and spectrophotometric evolution, SN 2023tsz is largely a typical example of the class, although line profile asymmetries in the nebular phase are seen, which may indicate the presence of dust formation or unshocked circumstellar material. Intriguingly, SN 2023tsz is located in an extraordinarily low-mass host galaxy that is in the second percentile for stripped-envelope SN host masses and star formation rates (SFRs). The host has a radius of 1.0 kpc, a g-band absolute magnitude of

$-12.72 \pm 0.05$

, and an estimated metallicity of

$\log (Z_{*}/{\rm Z}_{\odot }) \approx -1.6$

. The SFR and metallicity of the host galaxy raise questions about the progenitor of SN 2023tsz. The low SFR suggests that a star with sufficient mass to evolve into a WR would be uncommon in this galaxy. Further, the very low metallicity is a challenge for single stellar evolution to enable H and He stripping of the progenitor and produce an SN Ibn explosion. The host galaxy of SN 2023tsz adds another piece to the ongoing puzzle of SNe Ibn progenitors, and demonstrates that they can occur in hosts too faint to be observed in contemporary sky surveys at a more typical SN Ibn redshift.

Stellar expansion or inflation?

2025-02-19T01:07:18Z

Stellar expansion or inflation? Sabhahit, Gautham N.; Vink, Jorick S. While stellar expansion after core-hydrogen exhaustion related to thermal imbalance has been documented for decades, the physical phenomenon of stellar inflation that occurs close to the Eddington limit has only come to the fore in recent years. We aim to elucidate the differences between these physical mechanisms for stellar radius enlargement, especially given that additional terms such as 'bloated' and 'puffed-up' stars have been introduced in the recent massive star literature. We employ single and binary star MESA structure and evolution models for constant mass, as well as models allowing the mass to change due to winds or binary interaction. We find cases that were previously attributed to stellar inflation in fact to be due to stellar expansion. We also highlight that while the opposite effect of expansion is contraction, the removal of an inflated zone should not be referred to as contraction but rather deflation, as the star is still in thermal balance.