Pieter Meintjes | Searcharxiv

arXiv2025

Exploring Hard X-ray Properties of $γ$-ray Emitting Narrow Line Seyfert-I Galaxies through NuSTAR Observations

We studied the six gamma-ray-detected Narrow Line Seyfert 1 (NLSy1) galaxies using the hard X-ray observations from Nuclear Spectroscopic Telescope Array (NuSTAR) and optical g- \& r-band from Zwicky Transient Facility (ZTF). The X-ray spectra corresponding to all objects are well-fitted with a power-law spectral model, and a strong "redder-when-brighter" trend is seen, which is mostly seen in Blazars. The X-ray light curves were produced for all the available observations, and the F$_{var}$ is estimated. In 1H 0323+342, we found that F$_{var}$ lies between 9$\%$ to 22$\%$, suggesting significant variability in the source. Similarly, for PKS 2004-447, we found F$_{var}$ lies between 10$\%$ to 21$\%$. We see a strong X-ray and $γ$-ray spectral index correlation among these objects, suggesting that these are produced through a similar process. Comparing the X-ray spectral index with other class objects, we see that NLSy1 galaxies are similar to LBL and IBL types. We see a negative trend of X-ray flux with the $γ$-ray luminosity in these objects, suggesting an anti-correlation between them. A similar trend is seen between the X-ray flux, total jet power, and disk luminosity. The X-ray spectral index also shows a negative trend with total jet power and disk luminosity. The optical variability amplitude (in magnitude) lies between 0.90 to 2.32, and the fractional variability varies from 13\% to 40\%. The color-magnitude plot shows mostly the redder-when-brighter (RWB) trend, suggesting $γ$-NLSy1 are much closer to FSRQs than BL Lacs. Our results, overall, summarize how the various parameters in gamma-ray-detected NLSy1 are connected.

astro-ph.HE↗

arXiv2024

Millinovae: A New Class of Transient Supersoft X-ray Sources without a Classical Nova Eruption

Some accreting binary systems containing a white dwarf (such as classical novae or persistent supersoft sources) are seen to emit low-energy X-rays with temperatures of ~10^6 K and luminosities exceeding 10^35 erg/s. These X-rays are thought to originate from nuclear burning on the white dwarf surface, either caused by a thermonuclear runaway (classical novae) or a high mass-accretion rate that sustains steady nuclear burning (persistent sources). The discovery of transient supersoft X-rays from ASASSN-16oh challenged these ideas, as no clear signatures of mass ejection indicative of a classical nova eruption were detected, and the origin of these X-rays remains controversial. It was unclear whether this star was one of a kind or representative of a larger, as yet undiscovered, group. Here, we present the discovery of 29 stars located in the direction of the Magellanic Clouds exhibiting long-duration, symmetrical optical outbursts similar to that seen in ASASSN-16oh. We observed one of these objects during an optical outburst and found it to be emitting transient supersoft X-rays, while no signatures of mass ejection (indicative of a classical nova eruption) were detected. We therefore propose that these objects form a homogeneous group of transient supersoft X-ray sources, which we dub ``millinovae'' because their optical luminosities are approximately a thousand times fainter than those of ordinary classical novae.

astro-ph.SR↗