We present the most densely sampled multi-band optical photometric observations of the peculiar BL Lacertae object OJ 287 from 2015 to 2025 with a focus on its optical activity on diverse timescales. We present a total of 2296, 10927, 11484, and 2982 data points in B, V, R, and I bands, respectively. The densely sampled observations allow us to keep track of the source evolution that it has exhibited since the start of the predicted major optical flaring activity at the end of 2015. The study reveals clear and persistent bluer when brighter trends in both the long-term and short-term variations. Different bands were cross-correlated with discrete correlation functions, which peak at zero lag, implying co-spatial emission. Using eight optical spectra in the low flux states of OJ 287 taken from 2017 October 21 to 2017 November 22, from Steward Observatory, we estimate the central black hole mass to be at least 3.89 × 10^9 M_⊙ from the [O III] line width. The emission mechanism of the binary black hole blazar, and its possible implication in various aspects of multi-messenger astronomy are briefly discussed.

We report a multi-wavelength study of two flaring episodes of the blazar BL Lacertae during MJD 60500-60800 (9 July 2024 - 5 May 2025). The source reached a daily-averaged γ-ray flux of (1.03 ± 0.05) × 10^-5 ph cm^-2 s^-1 (E > 100 MeV) on MJD 60588 (5 October 2024). Using orbit-binned data from the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope, we identify a minimum flux halving timescale of τ= 1.33 ± 0.29 hr. This constrains the upper limit on the γ-ray emitting region size to R ≤ 2.0 × 10^15 cm, as well as its distance from the central supermassive black hole to R_H≤ 5.9 × 10^16 cm, assuming a Doppler factor of δ= 14.8 derived from the spectral energy distribution (SED) modeling. We find tentative evidence for sub-minute γ-ray variability with a minimum doubling time of 0.7 ± 0.2 min (p-value = 0.03). This may originate from an extremely compact region with a size of R ≤ 1.8 × 10^13 cm, suggesting that the emission arises from magnetohydrodynamic substructures, such as plasmoids within a magnetic reconnection zone. Spectral analysis reveals a significant “softer-when-brighter” trend (r = 0.96, p = 4.5 × 10^-4) during the minute-scale flare peaks, indicating a complex interplay between particle acceleration and radiative cooling. The SED is reproduced using a one-zone leptonic model, in which synchrotron self-Compton (SSC) and external Compton (EC) scattering effectively account for the high-energy emissions. The reduced magnetic field strengths and hard electron injection spectral indices observed during the flaring states suggest enhanced particle acceleration efficiency, possibly associated with relativistic magnetic reconnection.

Neutrino astronomy provides another window to exploring the Universe, exemplified by the detection of a megaelectronvolt neutrino burst from the core-collapse supernova (CCSN) SN 1987A (refs. ). Commonly discussed theories suggest that some CCSNe could produce neutrinos with energies a thousand times more than those of SN 1987A , which has been probed with new-generation facilities . The interaction of SN ejecta with a dense circumstellar medium (CSM) or a jet, launched in a CCSN, being choked in the stellar envelope of the progenitor or an outside CSM are both well-accepted scenarios for the high-energy neutrino production. Here we report the detection of a high-energy neutrino flare at a 3.9σ significance from SN 2017hcd, made by our analysis of the public track-like neutrino data taken by the IceCube Neutrino Observatory . A Type IIn SN with optical emissions arising from the ejecta–CSM interaction, SN 2017hcd's neutrino flare lasted ∼1–2 month, with its central time ∼14-day prior to the SN's optical discovery time. Its estimated isotropic neutrino energy (all flavors) is approximately two orders of magnitude higher than the energy (∼ 10^50 erg) carried in the SN's ejecta, too high to be explained with the ejecta–CSM scenario. Thus, a choked jet may be the source of the neutrino flare.

Context. Changing-look active galactic nuclei (CL-AGNs) challenge the unified model of AGNs and offer key insights into the physics of the accretion processes of super-massive black holes. While systematic spectroscopic comparisons have successfully identified large samples of CL-AGNs, photometric selection based on variability features provides an efficient alternative. Aims. We used the color–magnitude (CM) variability pattern method to continue our identification of the CL transition in AGNs. The resulting CL-AGN sample can help improve understanding of this phenomenon. Methods. The CM variability pattern method utilizes the slope ( k ) of the CM variations to identify strong “bluer-when-brighter” behavior, while the variation amplitudes in optical and mid-infrared (MIR) bands are also considered. The candidates selected from the Type-2 AGNs given in the Sloan Digital Sky Survey catalog were spectroscopically observed by us using the 3.6-m Devasthal Optical Telescope and the 2-m Himalayan Chandra Telescope. Results. We successfully confirm seven turn-on CL-AGNs among 12 candidates in this work. Compared with both the general AGN populations and the spectroscopically identified CL-AGN sample, these CL-AGNs show larger optical and MIR variations and k values. The extreme CM variabilities of these sources (with optical magnitude changes > 0.9) occurred recently. For four sources, flare-like brightening episodes were temporally associated with turn-on transitions within 3–7 years, suggesting that these flares may trace short-timescale accretion enhancement, central brightening, and broad-line region re-illumination. In addition, the estimated Eddington ratios for the confirmed CL-AGNs appear to cluster around a critical value of λ Edd ∼ 0.01. Conclusions. The extreme CM variability serves as a highly efficient criterion for finding CL-AGNs. The properties of the CL-AGNs thus found suggest that they may represent AGNs at a pivotal state, which likely occurs in CL transitions due to enhanced accretion activity, while the cause of the accretion activity, which was determined to have a timescale of several years in this work, remains to be investigated further.

Changing-look active galactic nuclei (CL AGNs) show the appearance or disappearance of broad emission lines on timescales of years. Among them, the repeating CL (RCL) AGNs may provide a clear clue for our understanding of the CL transitions because the same nucleus crosses some physical boundaries more than once. We search for RCL AGNs in known CL-AGN samples using long-term multi-band light curves, and selected 34 candidates for spectroscopic follow-up. We confirm 25 RCL AGNs, including 22 newly identified cases. Properties of these RCL AGNs are analyzed. The observed rest-frame intervals of the second transitions are mostly 3–4 yr, while the variations of the optical light curves suggest that some transitions might occur on timescales of several months. The latest spectra show that the on/off states correspond to higher/lower Eddington-ratio, in the expected direction relative to the parent CL-AGN samples. As seven RCL AGNs are well covered by nearly continuous single-band light curves, their on/off states can be found to follow multi-year optical excursions, and their Eddington ratios vary consistently with the photometric changes. We also find that the Hβ-only transitions occur at higher Eddington ratios than the transitions involving both Hα and Hβ, suggesting a line-dependent Broad-Line-Region (BLR) visibility threshold. These results support a picture in which different accretion-flow processes drive reversible changes in the central ionizing emissions, while the observed RCL transitions are produced by the BLR breathing across line-dependent visibility thresholds.

We report the first search for high-energy neutrino emissions from dark matter (DM) annihilation in stellar-stream cores. Motivated by a recent gamma-ray study that proposed these cores as a new class of indirect DM targets, we analyze three stream cores in the Northern Hemisphere using the public ten-year track-like neutrino data released by IceCube. Under the χχ annihilation hypothesis, the most significant excess among the three targets is found at the position of the nearby dwarf galaxy Boötes III, the core of the Styx stream, with a best-fit DM mass of 26.5 TeV. The excess has a post-trial significance of 3.1σ. Considering the existing IceCube dwarf-galaxy limit for the same channel, we obtain a limit on the J-factor J_ ann, log_10(J_ ann/ GeV^2 cm^-5)≳ 19.1^+0.3_-0.5. This limit is broadly consistent with empirical estimates of J_ ann for Boötes III. The results provide the first candidate target with a possible HE neutrino signal associated with DM annihilation. This neutrino excess and the general existence of DM-induced neutrino signals from other similar sources will be confirmed with the near-future large high-energy neutrino detectors, thus enabling us to probe the nature of DM particles.

Context. Changing-look active galactic nuclei (CL-AGNs) challenge the unified model of AGNs and offer key insights into the physics of the accretion processes of super-massive black holes. While systematic spectroscopic comparisons have successfully identified large samples of CL-AGNs, photometric selection based on variability features provides an efficient alternative. Methods. We use the colour–magnitude (CM) variability method to continue our identification of the CL transition in AGNs, which utilizes the slope (k) of the CM variations to identify strong bluer-when-brighter behavior, while the variation amplitudes in optical and mid-infrared bands are also considered. The candidates thus selected from the Type-2 AGNs given in the Sloan Digital Sky Survey catalog are spectroscopically observed using the 3.6-m DOT and the 2-m HCT. Results. We confirm seven turn-on CL-AGNs among 12 candidates. Comparing them with both the general AGN populations and the spectroscopically identified CL-AGN sample, the CL-AGNs showed larger optical and MIR variations and k values. The extreme CM variabilities of these sources (with optical magnitude changes > 0.9) occurred recently. For four sources, flare-like brightening episodes were temporally associated with the turn-on transitions within 3–7 years, suggesting that these flares may trace short-timescale accretion enhancement, central brightening, and BLR re-illumination. Conclusions. The extreme CM variability serves as a highly efficient criterion for finding CL-AGNs. The properties of the CL-AGNs thus found suggest that they may represent AGNs at a pivotal state, which likely occur CL transitions due to enhanced accretion activity, while the cause of the accretion activity, determined to have a time scale of several years, remains to be investigated.

We present a search for pulsars associated with 14 unidentified very-high-energy (≥100 GeV) γ -ray sources detected by the High Energy Spectroscopy System (HESS) and the High-Altitude Water Cherenkov Observatory along the Galactic plane. Using the Five-hundred-meter Aperture Spherical radio Telescope (FAST), we observed four TeV targets (HESS J1828−099, 3HWC J0633+191, 3HWC J1743​​​​​​+149, and 3HWC J1847−017) with a sensitivity of S 1250 ≃ 15 μ Jy for a τ obs ≈ 720 s at a signal-to-noise ratio S/N = 10. Additionally, we analyzed archived Parkes observations covering 13 TeV targets. No convincing new pulsation was detected in the radio searches, aside from seven known pulsars. A tentative 17 ms signal was found in beam M15 of FAST for 3HWC J1847-017, which also overlaps with the position of 4FGL J1847.2−0141. Furthermore, we derived the 0.3–500 GeV test-statistic maps using data from the Fermi Large Area Telescope. Several GeV counterparts exhibited spectra potentially linked to TeV emission, with 4FGL J1847.2−0141 showing a pulsar-like spectral signature.

We report on our finding of an excess of 54-15+16 neutrinos at the location of the pulsar wind nebula (PWN) G63.7+1.1. By analyzing the IceCube track-like neutrino data for a group of 14 PWNe, which are selected as targets because of their reported association with molecular clouds, G63.7+1.1 is found to be the only one detected with neutrino emission, and the post-trail significance for the detection is 3.2 sigma. Previously, this PWN was estimated to have an age of greater than or similar to 8 kyr, contain a candidate pulsar detected in X-rays, and have a distance of similar to 6 kpc. More importantly, and related to the PWN's possible neutrino emission, surrounding molecular materials are seen to interact with the PWN. On the basis of these properties, we examine the proton-proton interactions as the process for the neutrino production. The PWN (or the pulsar) can provide sufficient energy to power the required high-energy (HE) protons. This possibly first neutrino-emitting case in our Galaxy, with problems or other possibilities to be solved or examined, may reveal to us that PWNe are the significant Galactic HE neutrino sources.

On May 5 2022, a type Ic supernova (SN) explosion SN 2022jli was discovered. This SN showed additional optical emissions, which were found to exhibit 12.4-day periodic undulations and concordant periodic velocity shifts. These key features likely indicate a compact object in a binary system was formed. A faint γ-ray source was also detected at the position of the SN and upon checking the γ-ray photons' arrival times, it was revealed that the same 12.4-day periodicity was likely present. Here we report our detailed analysis results for the γ-ray source. Not only was the γ-ray emission detectable for ∼1.5 years since the discovery time, but a strong modulation at period 12.5 day was also clearly determined. Considering the newly formed compact object to be a neutron star or a stellar-mass black hole, the putative binary, having an orbital period of 12.5 day, is likely the first extragalactic high-energy system detected. The system may serve as a valuable example for the formation of many such binaries observed in the Milky Way and nearby galaxies.

The Seyfert 1 galaxy J1626+5120 is estimated to host a 108M circle dot black hole (BH) accreting at Eddington ratio mEdd approximate to 0.043 . Its long-term multiband light-curve data show flicker-like variations, but in a well-sampled g-band light curve, we are able to determine a similar or equal to 329 day quasiperiodic oscillation (QPO) at a similar to 4.53 sigma significance. Six optical spectra were obtained for the source, three of which were taken by us. The spectra show that the variations were mainly because of flux changes blueward of 4000 & Aring;. We also analyze X-ray and ultraviolet (UV) data obtained with the Neil Gehrels Swift Observatory (Swift), which targeted the source in the past 2 yr. X-ray and UV emissions of the source show variations correlated with optical. Time lags of four UV bands and four optical bands are determined with respect to the X-ray emission, which are consistent with a continuum reprocessing disk model. These properties point out a disk origin for the QPO, likely due to Lense-Thirring (LT) precession of the accretion flow at similar to 20 gravitational radii of the BH. This QPO could be a key case linking subyear-long QPOs in jets, which have more cases reported, to LT precession.

Polarimetric features during the prompt phase of Gamma-ray Bursts (GRBs) have been essential for elucidating the debated emission mechanisms and gaining insight into the inner structure of GRBs. However, the potential impact of photon-Axion-Like-Particle (ALP) mixing in extragalactic magnetic fields, leading to significant modifications to the initial polarization state, has been overlooked in discussions concerning prompt phase constraints. In this work, we first examine the statistical characteristics of linear polarization degree ($\Pi_{L}$) in GRBs, by utilizing data from polarimetric missions focusing on sub-MeV emissions. Our analysis, conducted with a restricted sample of GRBs spanning various redshifts, reveals a diverse distribution of $\Pi_{L}$, which currently shows no correlation with the GRBs' spectral parameters or properties of candidate host galaxies. We then explore alternations to the initial $\Pi_{L}$ due to photon-ALP mixing within a domain-like structure of the intergalactic magnetic field (${\bf B}_{\rm IGM} $). With the existence of ALPs with $m_{a}$$~$$\lesssim$$~$$10^{-14}$$~$eV and $g_{a\gamma}~$$\simeq$$~0.5\times10^{-11}$, the mixing leads to a decrease in the polarization degree of initially fully linearly polarized photons, while it induces a certain degree of polarization to initially unpolarized photons. To ensure that the effect of mixing is small enough to be negligible, the mixing term $\Delta_{a\gamma} \equiv 1/2\ g_{a\gamma} {\bf B}_{\rm IGM}$ should be less than $1.5\times 10^{-4}$ Mpc$^{-1}$. Currently, the number of GRBs with both sub-MeV polarization measurement and redshift confirmation remains very limited. Certification of redshift for GRBs with low $\Pi_{L}$ would further constrain the parameter space of ALPs or provide an independent means to determine the upper limit on ${\bf B}_{\rm IGM}$.

We report our counterpart identification study for two high-energy neutrino events IC-130127A and IC-131204A listed in the IceCube Event Catalog of Alert Tracks. These two events belong to Gold alerts, which have a significant probability of being of astrophysical origin. Within the events’ 90% positional uncertainty regions, we, respectively, find PKS 2332–017 and PMN J1916–1519. The first source is a flat-spectrum radio quasar at redshift z = 1.18, and the second is a blazar of an uncertain type with photometric z = 0.968. As they correspondingly had a γ -ray flare temporally coincident with the arrival times of IC-130127A and IC-131204A, we identify them as the respective neutrino emitters. Detailed analysis of the γ -ray data for the two blazars, obtained with the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope, is conducted. The two flares, respectively, from PKS 2332–017 and PMN J1916–1519 lasted ~4 yr and ~4 months and showed possible emission hardening by containing high-energy ~2–10 GeV photons in the emissions. Accompanying the flare of PKS 2332–017, optical and mid-infrared brightening variations were also observed. We discuss the properties of the two sources and compare the properties with those of the previously reported (candidate) neutrino-emitting blazars.

We report a candidate repeating tidal disruption event (TDE), AT2022sxl, found from large-field optical survey data. Two flares with a separation time of ∼7.2 yr between the two optical peaks are observed. Related mid-infrared (MIR) flares, with delay times of ∼200 days are also seen. We analyze two optical spectra of the TDE source, one near the optical peak of the second flare from the Transient Name Server and one at the quiescent flux level after the second flare. The latter was taken by us with the 10.4 m Gran Telescopio Canarias. Comparing the features of the two spectra, we identify that the host is likely a composite galaxy at redshift 0.23, and the TDE event, probably an H+He type, mainly powered broad components in the emission lines of H α , H β , and He I λ 5876. More interestingly, we find that two Bronze-type neutrino events, detected by the IceCube neutrino observatory, match the TDE in position and the second flare, especially the delayed MIR flare, in time. We discuss the MIR luminosity properties of the currently reported (candidate) neutrino-emitting TDEs and suggest that luminous MIR emission is a prerequisite for neutrino production in TDEs.

Ultra-high-energy (UHE), exceeding 100 TeV (10^12 electronvolts), γ-rays manifests extreme particle acceleration in astrophysical sources. Recent observations by γ-ray telescopes, particularly by the Large High Altitude Air Shower Observatory (LHAASO), have revealed a few tens of UHE sources, indicating numerous Galactic sources capable of accelerating particles to PeV (10^15 electronvolts) energies. However, discerning the dominant acceleration mechanisms (leptonic versus hadronic), the relative contributions of specific source classes, and the role of particle transport in shaping their observed emission are central goals of modern UHE astrophysics. Here we report the discovery of a giant UHE γ-ray emitter at -17.5° off the Galactic plane - a region where UHE γ-ray sources are rarely found. The emitter exhibits a distinctive asymmetric shape, resembling a giant "Peanut" spanning 0.45° \times 4.6°, indicative of anisotropic particle distribution over a large area. A highly aged millisecond pulsar (MSP) J0218+4232 is the sole candidate accelerator positionally coincident with the Peanut region. Its association with UHE γ-rays extending to 0.7 PeV, if confirmed, would provide the first evidence of a millisecond pulsar powering PeV particles. Such a finding challenges prevailing models, which posit that millisecond pulsars cannot sustain acceleration to PeV energies. The detection reveals fundamental gaps in understanding particle acceleration, cosmic-ray transport, and interstellar magnetic field effects, potentially revealing new PeV accelerator (PeVatron) classes.

Black holes (BHs), one of the most intriguing objects in the universe, can manifest themselves through electromagnetic radiation initiated by the accretion flow. Some stellar-mass BHs drive relativistic jets when accreting matter from their companion stars, forming microquasars. Non-thermal emission from the radio to teraelectronvolt gamma-ray band has been observed from microquasars, indicating the acceleration of relativistic particles. Here we report detection of four microquasars (SS 433, V4641 Sgr, GRS 1915+105, MAXI J1820+070) of spectra extending to the ultrahigh-energy (UHE; photon energy [Formula: see text] TeV) band, and one microquasar (Cygnus X-1) with a spectrum approaching 100 TeV, using the Large High Altitude Air Shower Observatory. Notably, the total emission associated with SS 433 cannot be interpreted with a single leptonic component. In the UHE band, its emission is in spatial coincidence with a giant atomic cloud, which is consistent with a hadronic origin. An elongated source is discovered from V4641 Sgr with the spectrum continuing up to 800 TeV. The detection of UHE gamma rays demonstrates that accreting BHs and their environments can operate as extremely efficient accelerators of particles up to 1 PeV, suggesting that microquasars are important contributors to Galactic cosmic rays, especially around the 'knee' region.

We analyze the optical light curve data, obtained with the Zwicky Transient Facility (ZTF) survey, for 47 gamma-ray blazars monitored by the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope (Fermi). These 47 sources are selected because they are among the Fermi blazars with the largest optical variations in the ZTF data. Two color-magnitude variation patterns are seen in them, with one being redder-to-stable-when-brighter (RSWB; in 31 sources) and the other being stable when brighter (in 16 sources). The patterns fit with the results recently reported in several similar studies with different data. Moreover, we find that the colors in the stable state of the sources share similar values, for which (after being corrected for the Galactic extinction) most sources are in a range of 0.4-0.55. This feature could be intrinsic and may be applied in, for example, study of the intragalactic medium. We also determine the turning points for the sources showing the RSWB pattern, after which the color changes saturate and become stable. We find a correlation between optical fluxes and gamma-ray fluxes at the turning points. The physical implications of the correlation remain to be investigated, probably better with a sample of high-quality gamma-ray flux measurements.

We report our identification of three gigaelectronvolt γ-ray sources, 4FGL J0502.6+0036, 4FGL J1055.9+6507,and 4FGL J1708.2+5519, as Active Galactic Nuclei(AGNs). They are listed in the latest Fermi-Large Area Telescope source catalog as unidentified ones. We find that the sources all showed γ-ray flux variations in recent years. Using different survey catalogs, we are able to find a radio source within the error circle of each source's position. Further analysis of optical sources in the fields allows us to determine the optical counterparts, which showed similar variation patterns to those seen in γ-rays. The optical counterparts have reported redshifts of 0.6,1.5, and 2.3, respectively, estimated from photometric measurements. In addition, we also obtain an X-ray spectrum of 4FGL J0502.6+0036 and a flux upper limit on the X-ray emission of 4FGL J1055.9+6507 by analyzing the archival data. The broadband spectral energy distributions of the three sources from radio to γ-rays are constructed. Comparing mainly the γ-ray properties of the three sources with those of different sub-classes of AGNs, we tentatively identify them as blazars. Followup optical spectroscopy is highly warranted for obtaining their spectral features and thus verifying the identification.

We report our study of the field of the similar or equal to 0.2 PeV neutrino event IC-190619A. This neutrino belongs to Gold events, which more likely have an astrophysical origin. Among the two gamma-ray sources within the neutrino's positional uncertainty region, we find that one of them, the BL Lac-type blazar PKS 2254+074, had a gamma-ray flare at the arrival time of the neutrino. The flare is determined to have lasted similar to 2.5 yr in a 180 day binned light curve, constructed from the data collected with the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. Accompanying the flare, optical and mid-infrared brightening is also seen. In addition, >= 10 GeV high-energy photons from the source have been detected, suggesting a hardening of the emission during the flare. Given both the positional and temporal coincidence of PKS 2254+074 with IC-190619A, we suggest that this blazar is likely another member of a few recently identified (candidate) neutrino-emitting blazars.

Since the science white paper of the Large High Altitude Air Shower Observatory (LHAASO) published on arXiv in 2019 [e-Print: 1905.02773 (astro-ph.HE)], LHAASO has completed the transition from a project to an operational gamma-ray astronomical observatory LHAASO is a new generation multi-component facility located in Daocheng, Sichuan province of China, at an altitude of 4410 meters. It aims at measuring with unprecedented sensitivity the spectrum, composition, and anisotropy of cosmic rays in the energy range between 10$^{12}$ and 10$^{18}$~eV, and acting simultaneously as a wide aperture (one stereoradiant) continuously operating gamma-ray telescope in the energy range between 10$^{11}$ and $10^{15}$~eV with the designed sensitivity of 1.3\% of the Crab Unit (CU) above 100 TeV. LHAASO's capability of measuring simultaneously different shower components (electrons, muons, and Cherenkov/fluorescence light), will allow it to investigate the origin, acceleration, and propagation of CR through measurement of the energy spectrum, elemental composition, and anisotropy with unprecedented resolution. The remarkable sensitivity of LHAASO will play a key role in CR physics and gamma-ray astronomy for a general and comprehensive exploration of the high energy universe and will allow important studies of fundamental physics (such as indirect dark matter search, Lorentz invariance violation, quantum gravity) and solar and heliospheric physics. The LHAASO Collaboration organized an editorial working group and finished all editorial work of this science book, to summarize the instrumental features and outline the prospects of scientific researches with the LHAASO experiment.