arXiv subjects

M. Gritsevich

Publications and source records attributed to M. Gritsevich.

6 recordsLinked to original sources

arXiv2026

EP251023a: A fast X-ray transient featuring a magnetar-powered optical internal plateau followed by a steep decay

EP251023a is an extragalactic fast X-ray transient (eFXT) detected solely by EP without a gamma-ray counterpart. The prompt emission consists of a main emission with a duration $T_{90}=292\pm19$ s, followed by a long-lasting tail emission that persists until the observation ends at $T_0+1571$ s. With the upper limit of Konus--Wind, we derived a conservative upper limit on the isotropic gamma-ray energy $E_{\gamma,\rm{iso}}$ of $5.7 \times 10^{52}$ erg for the main emission phase. A redshift of $z = 2.232\pm0.001$ is identified from strong absorption features in the Keck spectrum, which also indicate a relatively low host-galaxy HI column density. Based on the broadband spectral energy distribution, the late-time light curves show an achromatic plateau, followed by an extremely steep decay with a slope of 3.99 after a break at about 49 ks, which is consistent with a rapidly spinning millisecond magnetar engine. Under the isotropic wind scenario, we obtain the initial period $P_0<2.27$~ms and the magnetic field strength $B_p<8.33\times10^{14}$~G for the magnetar; whereas considering a jet collimation with a typical opening angle of 0.1 rad relaxes these constraints to $P_0<32.15$~ms and $B_p<1.18\times10^{16}$~G. Together with GRB\,070707, EP251023a may represent a rare class of optical magnetar-powered internal plateaus with little external-shock contamination, unlike previous examples detected primarily in X-rays. Future discoveries of similar events will help clarify the relationship between magnetar-powered internal emission observed in the optical band and that detected only in X-rays.

astro-ph.HE↗

arXiv2026

Ellipsoidal modulation and multi-wavelength activity in the pre-cataclysmic binary RX J1553.0+4457

RX J1553.0+4457 (TMTS J15530469+4457458) is a detached post-common-envelope binary containing a cool white dwarf and an active late-type companion. We present a multi-wavelength study combining BOOTES multi-band photometry, six sectors of public TESS full-frame imaging, Einstein Probe/FXT X-ray observations, CAFOS optical spectroscopy, and archival ultraviolet-to-mid-infrared photometry. The BOOTES data reveal two short optical flares separated by about 3 h, with amplitudes of roughly 1-1.5 mag and faster decay at shorter wavelengths. The combined TESS light curve shows a stable signal at P = 0.083782 d, consistent with the first harmonic of the known spectroscopic orbital period, and its folded morphology indicates ellipsoidal modulation from a tidally distorted late-type companion rather than an irradiation-dominated waveform. TESS flare energetics lie in the active M-dwarf regime. The Einstein Probe/FXT spectra show a factor of about 4 decline in the 0.3-10 keV flux, mainly associated with decreasing emission measures. The broadband SED is reproduced by a cool white dwarf plus a late-type M dwarf, with no clear hot-continuum or mid-infrared excess. RX J1553.0+4457 is therefore best interpreted as a detached pre-cataclysmic binary whose rapid optical variability is dominated by magnetic activity and whose orbital modulation is ellipsoidal, although a weak wind-fed or intermittent accretion contribution remains possible.

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arXiv2026

Stochastic Optical Variability and an rms-flux Relation in the Intermediate Polar EP240309a

Magnetic cataclysmic variables provide a natural laboratory for studying how accretion interacts with compact-object magnetospheres and generates stochastic variability. We present an optical variability study of the intermediate-polar candidate EP240309a, an Einstein Probe X-ray transient, using BOOTES photometry, high-cadence TESS light curves, and a SOAR/Goodman optical spectrum. Previous studies found a white-dwarf spin period of 3.97 min (Pspin ~ 238 s) and an orbital period of Porb = 3.7614(4) h. Power spectral densities from the BOOTES data are consistent with single power laws with slopes alpha ~ 1.2-1.8, with no statistically significant evidence for a bend across the sampled frequency range. Using red-noise simulations and injection-recovery tests, we place one-sided constraints on any putative break frequency, which translate, under standard dynamical identifications, into an upper limit on the magnetospheric radius of Rm <= few x 10^10 cm for MWD = 0.8 Msun. In the TESS data, we detect a linear rms-flux relation on hour timescales in three high-cadence sectors, while two other sectors do not show a robust detection, indicating epoch-dependent rms-flux behavior. The SOAR spectrum shows Balmer and He II emission lines with FWHM about 1000-1600 km s^-1; under a Keplerian interpretation, these imply characteristic radii of r about (0.9-3.4) x 10^10 cm, broadly comparable to the timing-based constraints. Overall, the data provide conservative, order-of-magnitude radius constraints consistent with accretion onto a magnetic white dwarf, but they do not establish the detailed accretion geometry or exclude stream-fed or mixed accretion scenarios.

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arXiv2025

An extremely soft and weak fast X-ray transient associated with a luminous supernova

Long gamma-ray bursts (LGRBs), including their subclasses of low-luminosity GRBs (LL-GRBs) and X-ray flashes (XRFs) characterized by low spectral peak energies, are known to be associated with broad-lined Type Ic supernovae (SNe Ic-BL), which result from the core collapse of massive stars that lose their outer hydrogen and helium envelopes. However, the soft and weak end of the GRB/XRF population remains largely unexplored, due to the limited sensitivity to soft X-ray emission. Here we report the discovery of a fast X-ray transient, EP250108a, detected by the Einstein Probe (EP) in the soft X-ray band at redshift $z = 0.176$, which was followed up by extensive multiband observations. EP250108a shares similar X-ray luminosity as XRF\,060218, the prototype of XRFs, but it extends GRBs/XRFs down to the unprecedentedly soft and weak regimes, with its $E_{\rm peak} \lesssim 1.8\,\mathrm{keV}$ and $E_{\rm iso} \lesssim 10^{49}\, \mathrm{erg}$, respectively. Meanwhile, EP250108a is found to be associated with SN\,2025kg, one of the most luminous and possibly magnetar-powered SNe Ic-BL detected so far. Modeling of the well-sampled optical light curves favors a mildly relativistic outflow as the origin of this event. This discovery demonstrates that EP, with its unique capability, is opening a new observational window into the diverse outcomes of death of massive stars.

astro-ph.HE↗

arXiv2023

Revealing characteristics of dark GRB 150309A: dust extinguished or high-z?

Dark GRBs constitute a significant fraction of the GRB population. In this paper, we present the multiwavelength analysis of an intense two-episodic GRB 150309A observed early on to ~114 days post-burst. Despite the strong gamma-ray emission, no optical afterglow was detected for this burst. However, we discovered near-infrared afterglow ($K_{\rm S}$-band), ~5.2 hours post burst, with the CIRCE instrument mounted at the 10.4m GTC. We used Fermi observations of GRB 150309A to understand the prompt emission mechanisms and jet composition. We performed the early optical observations using the BOOTES robotic telescope and late-time afterglow observations using the GTC. A potential faint host galaxy is also detected at optical wavelength using the GTC. We modelled the potential host galaxy of GRB 150309A in order to explore the environment of the burst. The time-resolved spectral analysis of Fermi data indicates a hybrid jet composition consisting of a matter-dominated fireball and magnetic-dominated Poynting flux. GTC observations of the afterglow revealed that the counterpart of GRB 150309A was very red, with H-$K_{\rm S}$ > 2.1 mag (95 $\%$ confidence). The red counterpart was not discovered in any bluer filters of Swift UVOT, indicative of high redshift origin. This possibility was discarded based on multiple arguments, such as spectral analysis of X-ray afterglow constrain z < 4.15 and a moderate redshift value obtained using spectral energy distribution modelling of the potential galaxy. The broadband afterglow SED implies a very dusty host galaxy with deeply embedded GRB (suggesting $A_{\rm V}$ $\gtrsim$ 35 mag). The environment of GRB 150309A demands a high extinction towards the line of sight, demanding dust obscuration is the most probable origin of optical darkness and the very red afterglow of GRB 150309A. This result makes GRB 150309A the highest extinguished GRB known to date.

astro-ph.HE↗

arXiv2023

Ultra-High-Energy Cosmic Rays: The Intersection of the Cosmic and Energy Frontiers

The present white paper is submitted as part of the "Snowmass" process to help inform the long-term plans of the United States Department of Energy and the National Science Foundation for high-energy physics. It summarizes the science questions driving the Ultra-High-Energy Cosmic-Ray (UHECR) community and provides recommendations on the strategy to answer them in the next two decades.

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