Shashi Bhushan Pandey

We present a multiwavelength study of GRB 260310A, a nearby long-duration gamma-ray burst (GRB) at z ≃ 0.153 associated with a broad-lined Type Ic supernova. Despite its modest prompt gamma-ray output, E _{γ ,iso} ≃ 3.5 × 10 ⁵⁰ erg, GRB 260310A exhibits one of the brightest afterglows ever observed in the X-ray, optical, and radio bands. Its apparent brightness is not its only remarkable feature. The optical afterglow displays a delayed onset, characterized by a slow rising phase, with slope α ≈ −1, and a late peak at ∼0.1 day. We argue that the combination of weak prompt emission, hard peak energy, and late afterglow onset is naturally explained by a GRB jet viewed off-axis. The radio spectral energy distributions are consistent with synchrotron radiation and indicate the presence of both reverse- shock (RS) and forward-shock (FS) components, thus providing a first test of RS models in an off-axis geometry. The X-ray afterglow displays a prominent rebrightening, monitored for up to ∼68 days with no evidence of spectral evolution. A low level of linear polarization, Π ≈ 1.7%, is measured at 15 GHz at T ₀ + 55 days and suggests that, at these late times, the FS is the dominant emission component from radio to X-rays. This late-time rebrightening represents a critical test for the two-component jet model. If interpreted as the emergence of a narrow jet core viewed further off-axis, it would imply extreme luminosities and energetics for an on-axis observer.

Context. RX J1553.0+4457 (TMTS J15530469+4457458) is a detached post-common-envelope binary with a cool white dwarf and an active late-type companion. The origins of its optical modulation, rapid activity, and ultraviolet-to-mid-infrared spectral energy distribution (SED) remain unclear. Aims. We tested whether the dominant long-baseline Transiting Exoplanet Survey Satellite (TESS) waveform is ellipsoidal, characterised the rapid optical activity, and determined whether the SED requires a non-stellar component. Methods. We combined high-cadence Burst Observer and Optical Transient Exploring System (BOOTES) photometry, six sectors of TESS, Einstein Probe X-ray data, optical spectroscopy, and archival photometry to analyse orbital and flare variability, test timing significance with red-noise simulations and a moving-block bootstrap, fit the X-ray spectra, and model the SED. Results. BOOTES detected two optical flares separated by 3 h, with amplitudes of roughly 1–1.5 mag and faster decay at shorter wavelengths. The dominant TESS signal occurs at 2 f _orb and yields P _phot = 0.16756417 ± 0.00000118 d, consistent with spectroscopy. Its double-wave morphology identifies ellipsoidal modulation from a tidally distorted companion. We identify 19 conservative TESS flares, 14 of which exceed the 80% recovery-completeness threshold of 38.36 s in equivalent duration. Above this threshold, the calibrated bolometric energies span 2.2 × 10 ³³ − 2.8 × 10 ³⁴ erg, consistent with energetic flares from active M dwarfs. The Einstein Probe/Follow-up X-ray Telescope spectra show an approximately four-fold decline in the 0.3–10 keV flux, driven mainly by decreasing emission measures. The two stellar photospheres reproduce the SED without a hot-continuum or mid-infrared excess. A steady optically thick disc must have Ṁ ≲ 2 × 10 ⁻¹² M _⊙ yr ⁻¹ to remain hidden. Conclusions. RX J1553.0+4457 is a near-Roche-lobe-filling pre-cataclysmic binary with a stable optical waveform dominated by ellipsoidal modulation and rapid flares mainly tracing magnetic activity on the late-type companion. Weak wind-fed or intermittent accretion cannot be excluded, but the data do not require sustained Roche-lobe overflow, a luminous accretion disc, or an irradiation-powered optical component.

EP251023a is an extragalactic fast X-ray transient (eFXT) that was only detected by the Einstein Probe (EP) and that lacks a gamma-ray counterpart. The prompt emission consisted of a main emission with a duration T ₉₀ = 292 ± 19 s, followed by a long-lasting tail emission that persisted until the observation ended 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 _{γ ,iso} of 5.7 × 10 ⁵² erg for the main emission phase. We identified a redshift of z = 2.232 ± 0.001 based on the 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. This is consistent with a rapidly spinning millisecond magnetar engine. Under the isotropic wind scenario, we obtained an initial period P ₀ < 2.27 ms and a magnetic field strength B _p < 8.33 × 10 ¹⁴ G for the magnetar. If a jet collimation with a typical opening angle of 0.1 rad is considered, these constraints are relaxed to P ₀ < 32.15 ms and B _p < 1.18 × 10 ¹⁶ G. Together with GRB 070707, EP251023a might represent a rare class of optical magnetar-powered internal plateaus with little external-shock contamination. This is different from previous examples that were primarily detected in X-rays. Future discoveries of similar events will help us to clarify the relation between magnetar-powered internal emission observed in the optical band and the emission that is only detected in X-rays.

The launch of the Einstein probe (EP) mission has revolutionized the detection and follow-up observations of fast X-ray transients (FXTs) by providing prompt and timely access to their precise localizations. In the first year of its operation, the EP mission reported the discovery of 72 high signal-to-noise FXTs. Subjected to the visibility in the sky and weather conditions, we search for the optical counterparts of 42 EP-discovered FXTs from the Lulin Observatory. We successfully detected the optical counterparts of 12 FXTs, and five of those were first discovered by us from the Lulin Observatory. We find that the optical counterparts are generally faint ( r > 20 mag) and decline rapidly (>0.5 mag day ⁻¹ ). We also find that 12 out of 42 FXTs show direct evidence of their association with gamma-ray bursts (GRBs) through significant temporal and spatial overlapping. Furthermore, the luminosities and redshifts of FXTs with confirmed optical counterparts in our observations are fully consistent with the faintest end of the GRB population. However, the nondetection of any associated optical counterpart with a significant fraction of FXTs suggests that EP FXTs are likely a subset of the so-called “dark FXTs,” similar to “dark GRBs.” Additionally, the luminosities of two FXTs with confirmed redshifts are also consistent with jetted tidal disruption events (TDEs). However, we find that the optical luminosities of FXTs differ significantly from typical supernova shock breakout or kilonova emissions. Thus, we conclude that a significant fraction of EP-discovered FXTs are associated with events having relativistic jets; either a GRB or a jetted TDE.

The brightest gamma-ray burst (GRB) ever, GRB 221009A, displays ultralong GRB (ULGRB) characteristics, with a prompt emission duration exceeding 1000 s. To constrain the origin and central engine of this unique burst, we analyze its prompt and afterglow characteristics and compare them to the established set of similar GRBs. To achieve this, we statistically examine a nearly complete sample of Swift-detected GRBs with measured redshifts. We categorize the sample to bronze, silver, and gold by fitting a Gaussian function to the log-normal of T ₉₀ duration distribution and considering three subsamples respectively to 1, 2, and 3 times of the standard deviation to the mean value. GRB 221009A falls into the gold subsample. Our analysis of prompt emission and afterglow characteristics aims to identify trends between the three burst groups. Notably, the gold subsample (a higher likelihood of being ULGRB candidates) suggests a collapsar scenario with a hyperaccreting black hole as a potential central engine, while a few GRBs (GRB 060218, GRB 091024A, and GRB 100316D) in our gold subsample favor a magnetar. Late-time near-IR observations from 3.6 m Devasthal Optical Telescope rule out the presence of any bright supernova associated with GRB 221009A in the gold subsample. To further constrain the physical properties of ULGRB progenitors, we employ the tool MESA to simulate the evolution of low-metallicity massive stars with different initial rotations. The outcomes suggest that rotating (Ω ≥ 0.2 Ω_c) massive stars could potentially be the progenitors of ULGRBs within the considered parameters and initial inputs to MESA.

The 4K × 4K CCD Imager is the first light instrument for the 3.6[ ]m Devasthal Optical Telescope and is producing broad-band imaging observations of many Galactic and extra-galactic sources since 2015–2016. Capabilities of the CCD Imager are demonstrated recently through several publications using the well-calibrated multi-band deep photometric results as expected from other similar facilities globally. In this paper, we summarize some of the recent up-gradations made to improve the Imager, i.e. mounting the new filter wheel casing, replacing stray light baffles and discussing the fringe pattern corrections in redder filters. Some of the new science initiatives like galaxy-embedded faint point sources including WR stars and the observations of low surface brightness galaxy clusters are also discussed.

The 4K$\times$4K CCD Imager is the first light instrument for the 3.6m Devasthal Optical Telescope and is producing broad-band imaging observations of many Galactic and extra-galactic sources since 2015-2016. Capabilities of the CCD Imager are demonstrated recently through several publications using the well-calibrated multi-band deep photometric results as expected from other similar facilities globally. In this article, we summarize some of the recent up-gradations made to improve the Imager, i.e., mounting the new filter wheel casing, replacing stray light baffles and discussing the fringe pattern corrections in redder filters. Some of the new science initiatives like galaxy-embedded faint point sources including WR stars and the observations of low surface brightness galaxy clusters are also discussed.

We present a detailed prompt emission and early optical afterglow analysis of the two very-high-energy (VHE) detected bursts GRB 201015A and GRB 201216C, and their comparison with a subset of similar bursts. Time-resolved spectral analysis of multistructured GRB 201216C using the Bayesian binning algorithm revealed that during the entire duration of the burst, the low-energy spectral index (α _pt) remained below the limit of the synchrotron line of death. However, statistically some of the bins supported the additional thermal component. Additionally, the evolution of spectral parameters showed that both the peak energy (E _p) and α _pt tracked the flux. These results were further strengthened using the values of the physical parameters obtained by synchrotron modeling of the data. Our earliest optical observations of both bursts using the F/Photometric Robotic Atmospheric Monitor Observatorio del Roque de los Muchachos and Burst Observer and Optical Transient Exploring System robotic telescopes displayed a smooth bump in their early optical light curves, consistent with the onset of the afterglow due to synchrotron emission from an external forward shock. Using the observed optical peak, we constrained the initial bulk Lorentz factors of GRB 201015A and GRB 201216C to Γ₀ = 204 and Γ₀ = 310, respectively. The present early optical observations are the earliest known observations constraining outflow parameters and our analysis indicate that VHE detected bursts could have a diverse range of observed luminosity within the detectable redshift range of present VHE facilities.