We report the discovery of a radio-quiet to radio-loud transition in the narrow-line Seyfert 1 galaxy J1105+1452. The source has undergone a long-term evolution from a radio-quiet state in the 1990s to a persistently radio-bright state after 2017. Post-2017 flux densities in the 0.8-7 GHz range cluster between 32 and 43 mJy, whereas the 144 MHz flux density is only 1.94 +/- 0.23 mJy. This indicates strong low-frequency suppression from a compact, absorbed component. Modeling the radio spectral energy distribution with a synchrotron self-absorption model yields a turnover frequency nu p = 0.48 +/- 0.03 GHz and a peak flux density Sp = 38.9 +/- 4.7 mJy. These parameters classify J1105+1452 as a megahertz peaked-spectrum source, consistent with the new episode of an early-stage compact jet. Under the assumption of equipartition, we derive an intrinsic physical radius R similar to 0.68 pc and an average apparent expansion velocity beta app approximate to 0.64. The observed brightness temperature Tb approximate to 6.0 & times; 1011 K necessitates a Doppler factor delta approximate to 12, implying a relativistic jet viewed at theta less than or similar to 5 degrees. Despite the dramatic radio evolution, the X-ray spectrum remains stable and steep (Gamma similar or equal to 3.0), suggesting that the X-ray emission remains dominated by the disk/corona, while the radio band has become jet dominated. Our results identify J1105+1452 as a rare radio changing-state NLSy1, providing a unique laboratory for studying the birth and early evolution of relativistic jets at high Eddington ratios.

SDSS J1548+2208 is a unique partially obscured nuclear transient that exhibits multiwavelength outbursts in mid-infrared, X-rays, and radio. We present the results from multiwavelength photometric and spectroscopic follow-up observations with a time span of similar to 2500 days since its discovery. We find that the mid-infrared and X-ray emission (with a hard X-ray spectrum) are still at a high flux level relative to the pre-flare state, suggesting a sudden increase, and possibly long-sustained accreting activity from the central black hole. This is supported by the slowly evolving high-ionization coronal lines. The mid-infrared color turns blue slowly in the rising phase, which is distinct from stellar tidal disruption events (TDEs). All these properties point to the origin of outbursts from an extreme changing-look active galactic nucleus (AGN), and the scenario with a normal TDE seems disfavored. The radio spectral energy distribution (SED) in similar to 0.65-15 GHz is unusual, displaying a double-peak feature with distinct variability characteristics. In addition, we find evidence for the late-time radio rebrightening more than six years after the initial outburst, as well as a possibly new X-ray flare, though the significance for the latter is not high. The peculiar radio flux and SED evolution could be explained by a nascent outflow expanding into and shocking the circumnuclear diffuse medium filled by denser clouds. In this case, SDSS J1548+2208 represents a rare changing-look AGN that can launch radio outflows. Continued multiwavelength observations are required to map the dust and gas distribution on pc-scales, providing new insights into the environmental properties that could regulate AGN changing-look phenomenon.

Despite the growing number of high-energy neutrinos (TeV-PeV) detected by IceCube, their astrophysical origins remain largely unidentified. Recent observations have linked a few tidal disruption events (TDEs) to the production of high-energy neutrino emission, all of which display dust-reprocessed infrared flares, indicating a dust- and gas-rich environment. By cross-matching the neutrino events and a sample of mid-infrared outbursts in nearby galaxies with transient radio flares, we uncover an optically obscured TDE candidate, SDSS J151345.75 + 311125.2, which shows both spatial and temporal coincidence with the sub-PeV neutrino event IC170514B. Using a standard equipartition analysis of the synchrotron spectral evolution spanning 605 days post mid-infrared discovery, we find a little evolution in the radio-emitting region, with a kinetic energy up to 10(51) erg, depending on the outflow geometry and shock acceleration efficiency assumed. High-resolution European VLBI Network imaging reveals a compact radio emission that is unresolved at a scale of < 2.1 pc, with a brightness temperature of T-b > 5 & times; 10(6) K, suggesting that the observed late-time radio emission might originate from the interaction between a decelerating outflow and a dense circumnuclear medium. If the association is genuine, the neutrino production is possibly related to the acceleration of protons through pp collisions during the outflow expanding process, implying that the outflow-cloud interaction could provide a physical site with a high-density environment for producing the sub-PeV neutrinos. Such a scenario can be tested with future identifications of radio transients coincident with high-energy neutrinos.

Context . SDSS J143016.05+230344.4 ( z = 0.08105) has been proposed as a candidate pre-coalescence supermassive black hole binary and shows remarkable multiwavelength variability. Its radio evolution provides a direct probe of the compact emitting region and of the physical origin of the late-time activity. Aims . We aim to localize the variable radio emission, characterize its spectral evolution, and constrain whether the radio brightening is produced by a newly emerging compact component, external absorption, or dissipation in a structured circumnuclear environment. Methods . We analyzed 4.7–22.2 GHz very long baseline interferometry (VLBI) observations obtained between 2022 February and 2024 February, together with quasi-simultaneous connected-array spectra covering 0.7–16.5 GHz. We combined the VLBI brightnesstemperature and compactness constraints with spectral decomposition and equipartition-based estimates of source size and ambient density structure. Results . At all epochs, the radio emission is dominated by a single unresolved milliarcsecond core with T B ≳ 10 7 K, constraining the variable emission to ≲0.3 pc. The broadband spectra require two synchrotron self-absorbed components: a persistent low-frequency component with ν p,steady ≈ 0.74 GHz and S p,steady ≈ 1.22 mJy, and a flare component whose turnover evolves from (6.35 GHz, 0.18mJy) in February–May 2022 to (8.61 GHz, 0.38 mJy) in December 2022, and then to (5.83 GHz, 0.25 mJy) in March–April 2023. The flare contribution at 15 GHz reaches ~80% and matches the near-epoch VLBI recovery fraction, showing that the high-frequency brightening arises from a newly formed compact synchrotron component. A second brightening of the 15.2 GHz VLBI core is detected between September 2023 and February 2024, while the source remains unresolved. Equipartition scalings imply characteristic radii of ~5 × 10 −4 pc for the flare and ~9 × 10 −3 pc for the steady component, and indicate a steep inner circumnuclear density profile, n ∝ R −1.7 . Conclusions . The delayed radio flare is best explained by dissipation in an outflow or jet-base disturbance propagating through a structured circumnuclear medium.

SDSS J1548+2208 is a unique partially-obscured nuclear transient that exhibits multiwavelength outbursts in mid-infrared, X-ray and radio. We present the results from multiwavelength photometric and spectroscopic follow-up observations with a time span of 2500 days since its discovery. We find that the mid-infrared and X-ray emission (with a hard X-ray spectrum) are still in a high flux level relative to the pre-flare state, suggesting a sudden increased, and possibly long-sustained accreting activity from central black hole. This is supported by the slowly-evolving high-ionization coronal lines. The mid-infrared color turns blue slowly in the rising phase, which is distinct from stellar tidal disruption events (TDEs). All these properties point to the origin of outbursts from an extreme changing-look AGN and the scenario with a normal TDE seems disfavored. The radio spectral energy distribution (SED) in 0.65-15 GHz is unusual, displaying a double-peak feature with distinct variability characteristics. In addition, we find evidence for the late-time radio rebrightening more than six years since the initial outburst, as well as a possibly new X-ray flare, though the significance for the latter is not high. The peculiar radio flux and SED evolution could be explained by a nascent outflow expanding into and shocking circumnuclear diffuse medium filled by denser clouds. In this case, SDSS J1548+2208 represents a rare changing-look AGN which can launch radio outflows. Continued multiwavelength observations are required to map the dust and gas distribution on pc-scales, providing new insights into the environmental properties that could regulate AGN changing-look phenomenon.

SDSS J1115+0544 is a unique low-ionization nuclear emission-line region galaxy with energetic ultraviolet (UV), optical, and mid-infrared outbursts occurring in its nucleus. We present the results from an analysis of multiwavelength photometric and radio follow-up observations with a time span of ≈9 yr since its discovery. We find that following a luminosity plateau of ≈500 days, the UV/optical emission has decayed back to the preoutburst level, suggesting that the nuclear outburst might be caused by a stellar tidal disruption event (TDE). In this case, J1115+0544 could be an unusually slowly evolving optical TDE with longest rise and decline time scales ever found. Three years later than the optical peak, a delayed radio brightening was found with a luminosity as high as νL _ν (5.5 GHz) ∼ 1.9 × 10 ^39 erg s ^−1 . Using a standard equipartition analysis, we find that the outflow powering the radio emission was launched at t ≳ 1150 days with a velocity of v ≲ 0.1 c and a minimal kinetic energy of E _K ≳ 3 × 10 ^49 erg. The delayed radio brightening coupled with the disappearing plateau in the UV/optical light curves is consistent with the scenario involving delayed ejection of an outflow from a state transition in the disk. SDSS J1115+0544 is the first TDE candidate displaying both a short-lived UV/optical plateau emission and a late-time radio brightening. Future radio observations of these TDEs in the postplateau decay phase will help to establish the connection between outflow launching and changes in accretion rate.

We present 4.7–22.2 GHz Very Long Baseline Interferometry (VLBI) monitoring of the candidate pre-coalescence supermassive black hole binary SDSS J143016.05+230344.4 (z=0.08105) from 2022 February to 2024 February, together with quasi-simultaneous 0.7–16.5 GHz connected-array spectra. At all epochs, the radio emission is dominated by a single unresolved milliarcsecond core with T_ B≳10^7 K, confining the variable emission to ≲0.3 pc. The spectra require two self-absorbed synchrotron components: a persistent low-frequency component with ν_ p,steady≈0.74 GHz and S_ p,steady≈1.22 mJy, and a flare component whose turnover evolves from (6.35 GHz,0.18 mJy) in 2022 February–May to (8.61 GHz,0.38 mJy) in 2022 December and then to (5.83 GHz,0.25 mJy) in 2023 March–April. The 15 GHz flare fraction peaks at ≃80% and matches the near-epoch VLBI recovery fraction, showing that the high-frequency brightening arises from a new compact synchrotron component. A second 15.2 GHz VLBI-core brightening is detected from 2023 September to 2024 February while the source remains unresolved. Equipartition scalings imply characteristic radii of R_ eq∼5×10^-4 pc for the flare and ∼9×10^-3 pc for the steady component, and a steep inner circumnuclear density profile, n∝ R^-1.7. The delayed radio peak is consistent with dissipation of an outflow or jet-base disturbance in a structured circumnuclear medium, while a uniform free–free absorber is disfavored.

We present a comprehensive study of an infrared (IR) flare in the star-forming galaxy SDSS J010320.39+140152.5, which is selected from the sample of mid-IR (MIR) outbursts in nearby galaxies. Its MIR luminosity rose rapidly to a peak of similar to(5-6) & times; 1043 erg s-1, remained in the high state for about a year, and decreased continuously afterward. No optical variability was detected throughout the IR flare. Near-IR follow-up observations around the peak pinpointed the flare's location to spatially coincide with the galactic nucleus, with a 3 sigma upper limit of the offset of less than or similar to 100 pc. The IR spectral energy distribution of the flare is consistent with thermal emission of dust with temperatures of similar to 900 K. Using a dust radiative transfer model, we inferred a peak UV luminosity of similar to(4-10) & times; 1044 erg s-1 and a total energy of similar to(0.9-2) & times; 1052 erg released. We ruled out the possibility of a supernova, and prefer that the IR flare originated from an obscured tidal disruption event (TDE) rather than a changing-look active galactic nucleus. This flare stands as one of the most compelling cases to date for the class of dust-obscured TDEs that has emerged in recent years. They are missed by optical surveys, which partly accounts for the observed bias in TDE host galaxies, and they represent a crucial, yet often overlooked, component for a complete understanding of the TDE population.

We present a systematic investigation of long-term mid-infrared (MIR) color variability in 1718 narrow-line Seyfert 1 galaxies (NLSy1s) using 14 yr of Wide-field Infrared Survey Explorer/NEOWISE monitoring data. Through Pearson correlation analysis between photometric magnitude and color, we identify (1) a radio-quiet NLSy1 (RQ-NLSy1) population comprising 230 bluer-when-brighter (BWB) sources, 131 redder-when-brighter (RWB) sources, and 1323 objects showing weak or statistically insignificant color variations; and (2) a radio-loud NLSy1 (RL-NLSy1) population containing 5 BWBs, 2 RWBs, and 27 sources with weak/no color variations. Our analysis reveals that the BWB tendency strengthens significantly in galaxies with redder mean MIR colors W1 -W2 and lower starlight contamination. Furthermore, this color-change pattern demonstrates that the most bolometric luminous sources exhibit the most pronounced BWB behavior. While similar trends exist for black hole mass and Eddington ratio, bolometric luminosity appears to be the primary physical driver. Potential origins of these variations (e.g., host galaxy contribution, accretion disk variability, and dust reprocessing) are discussed. We conclude that temperature-dependent dust reprocessing dominates the observed BWB, RWB, and no/weak variation patterns. This interpretation may also apply to similar MIR color variations (MCVs) observed in other extragalactic MIR transients, such as tidal disruption events, ambiguous nuclear transients, and changing-look AGNs. In addition, we find no significant difference in long-term MCVs between RL-NLSy1s and RQ-NLSy1s; however, RL-NLSy1s show significantly greater dispersion in intrinsic variability amplitude compared to RQ-NLSy1s due to jet-induced complexity, where nonthermal synchrotron emission from relativistic jets obscures thermal dust signatures.

We present a multiepoch study of the extreme X-ray variability of the type 1 quasar SDSS J000532.84+200717.4 using archival observations from XMM-Newton, Swift/XRT, EP-FXT, and ROSAT, together with new optical spectroscopy and multiwavelength photometry. The 0.2-10 keV X-ray flux exhibits a transition from a high state to a subsequent low state, declining by more than an order of magnitude and placing the source in the X-ray-weak regime (Delta alpha ox less than or similar to -0.3). Significant variability on timescales of days to weeks persists within the low state. In contrast, the optical and mid-infrared emission remain stable over decade-long timescales, while the ultraviolet continuum varies only mildly and broadly tracks the X-ray evolution. Multiepoch optical spectroscopy shows no significant long-term changes in either the continuum shape or the broad emission-line profiles. The Mg ii emission is relatively weak compared with typical quasars, suggesting similarities to weak-line quasars. The pronounced wavelength-dependent variability indicates that the accretion disk remains largely intact while the X-ray emission undergoes dramatic changes. The spectral hardening in the low state and the viability of ionized partial-covering models are consistent with variable, largely dust-free absorbing gas, possibly associated with clumpy inner disk winds, although intrinsic coronal variations cannot be excluded. SDSS J0005+200717.4 therefore provides evidence that extreme X-ray weakness can arise as a transient phase in otherwise normal quasars.

We report recurrent X-ray state transitions in the nearby narrow-line Seyfert~1 galaxy Mrk~382 using multi-epoch observations from \textit{Swift}, \textit{Chandra}, \textit{XMM-Newton}, and eROSITA, together with archival ultraviolet, optical, and infrared data. The 0.3--2 keV flux varies by nearly an order of magnitude over the past $\sim15$ yr, with multiple transitions between bright and faint states. The source brightened by a factor of $\sim10$ between the 2010 \textit{Chandra} observation and the 2011 \textit{XMM-Newton} high state, then declined by $\sim6$--7 to a low state in 2019, followed by renewed brightening in recent \textit{Swift} monitoring. The X-ray spectrum shows strong state-dependent evolution, changing from a steep high-state continuum ($Γ=2.32\pm0.04$) to a much harder low-state spectrum ($Γ=1.39\pm0.06$). The low-state spectrum also exhibits a narrow Fe K$α$ line with an equivalent width of $\sim330$ eV. Reflection modeling indicates that the low-flux state is strongly reflection dominated, with the reflection fraction increasing from $R_{\rm refl}\sim4$ to $\sim34$, consistent with a compact corona subject to strong light-bending effects. The ultraviolet emission broadly follows the long-term X-ray variability but with smaller amplitude, while the optical and mid-infrared bands vary more mildly. Despite the dramatic X-ray variability, Mrk~382 does not enter an extreme X-ray-weak state, and we did not detect clear optical spectral-type changes based on the currently available observations. Mrk~382 is therefore a rare nearby Seyfert galaxy undergoing recurrent X-ray state transitions, providing a valuable laboratory for studying changing coronal geometry and multiwavelength AGN variability.

AT 2019qiz is the first standard optical tidal disruption event (TDE) with detection of X-ray quasiperiodic eruptions (QPEs), providing strong evidence for TDE–QPE association. Moreover, it belongs to the rare subset of optical TDEs with prominent infrared (IR) echoes revealed by the multiepoch photometry from the Wide-field Infrared Survey Explorer (WISE). The IR light curve shows an early bump, followed by a steady rise until the second-to-last epoch, after which it appears to enter a plateau phase. The dust temperature decreased until the fourth epoch and remains approximately constant for the subsequent five epochs. We have fitted the last five epochs using a convex dust ring model, resulting in an inner radius >1.2 pc. Such a large radius greatly exceeds the inner radius of the active galactic nucleus (AGN) torus for a 10 6 M ⊙ black hole and thus could be a torus remnant with the inner part having vanished, further supporting the unified scenario of recently faded AGNs, TDEs, and QPEs. Consequently, a connection between QPEs and IR-bright TDEs is naturally expected. Moreover, the echo requires at least a peak bolometric luminosity of (6.6, 9.5, 1.0) × 10 44 erg s −1 assuming silicate, silicon carbide, and graphite dust grains, respectively, all of which are significantly higher than the peak optical blackbody luminosity. It adds to the accumulating evidence that the missing energy of TDEs may lie in the unobservable extreme UV. This work highlights the unique value of IR echoes in the study of TDEs and QPEs and a promising prospect in the era of the Near-Earth Object Surveyor, the successor to WISE.

We study the secular periodic evolution of quasiperiodic eruptions (QPEs) for GSN069 and eRO-QPE2 assuming that they are driven by star−disk collisions. We set up numerical simulations and compared them with the observed periodic decay of ∼−3160 ± 720 s yr ^−1 in GSN069 and ∼−370 ± 40 s yr ^−1 in eRO-QPE2. We find the following: (1) Stellar-mass black holes are unlikely to be the orbiters in these two sources, as their periodic decays are on the order of <10 s yr ^−1 . (2) A naked degenerate core (including a white dwarf) is unlikely to be the orbiter in GSN069, as the decay is on the order of <200 s yr ^−1 . However, it is possible in eRO-QPE2, although the required surface density of the accretion disk is relatively high (e.g., Σ ≳ 10 ^7 –10 ^8 g cm ^−2 ). (3) Both the orbiters in GSN069 and eRO-QPE2 can be solar-like main-sequence (MS) stars. However, each collision can lead to gradual ablation of the stellar envelope on the order of 10 ^−5 to 10 ^−3 M _⊙ . To reproduce the observed decay while surviving for ≳3 yr, the surface density of the disk needs to be within a certain range. For example, given a 1 M _⊙ MS orbiter, the surface density of the disk gas should be in the range of 3 × 10 ^5 g cm ^−2 to 2 × 10 ^6 g cm ^−2 for GSN069 or 5 × 10 ^4 g cm ^−2 to 10 ^6 g cm ^−2 for eRO-QPE2. In both of these sources, the MS star cannot survive for more than ∼12 yr. We expect that future observations of these two sources can help to distinguish whether the orbiters are degenerated compact objects or gaseous stars.

It is still in dispute the existence of intermediate-mass black holes (IMBHs) with a mass of ~10^3-10^5 solar masses (Msun), which are the missing link between stellar-mass black holes (5-50 Msun) and supermassive black holes (10^6-10^10 Msun). The bright flares from tidal disruption events (TDEs) provide a new and direct way to probe IMBHs. 3XMM J215022.4-055108 is a unique off-nuclear X-ray transient which can be best explained as the TDE by an IMBH in a massive star cluster, though its mass is not well determined. Here, we report the discovery of a transient X-ray quasi-periodicity signal from 3XMM J215022.4-055108 with a period of ~85 seconds (at a significance of >3.51sigma) and fractional root-mean-squared amplitude of ~10%. Furthermore, the signal is coherent with a quality factor ~16. The significance drops to >3.13sigma if considering all light curves with sufficient quality for QPO search. Combining with the results from X-ray continuum fittings, the detection of QPO allows for joint constraints on the black hole mass and dimensionless spin in the range [9.9*10^3-1.6*10^4 Msun]$ and [0.26-0.36], respectively. This result supports the presence of an IMBH in an off-nuclear massive star cluster and may open up the possibility of studying IMBHs through X-ray timing of TDEs.

We present a sample of 12 quasar candidates with highly variable soft X-ray emission, selected from the fourth XMM-Newton Serendipitous Source Catalog (4XMM-DR13), using random forest (RF). Optical to mid-IR photometric data for the 4XMM-DR13 sources were obtained by correlating the sample with the Sloan Digital Sky Survey (SDSS) DR18 photometric catalog and the AllWISE database. By further cross matching with known spectral catalogs from the SDSS and LAMOST surveys, we compiled a training data set containing stars, galaxies, and quasars. The RF algorithm was trained to classify the XMM–Wide-field Infrared Survey Explorer–SDSS sample. We then refined the quasar candidate selection by applying Gaia proper motion data to eliminate stellar contaminants. As a result, 52,486 quasar candidates were classified, with 8410 of them matching known quasars in SIMBAD. The quasar candidates exhibit systematically lower X-ray fluxes compared to quasars in the training set, suggesting that the classifier is effective in identifying fainter quasars. From this quasar candidate sample, we constructed a subset of 12 sources that have shown variations in their soft X-ray flux by a factor of 10 over ∼20 yr in the XMM-Newton survey. These highly variable quasar candidates extend the quasar sample characterized by extreme soft X-ray variability to the optically faint end, with magnitudes around r ∼ 22. Notably, none of these 12 sources were detected in ROSAT observations. Given the flux sensitivity of ROSAT, the result indicates that quasars exhibiting more than 2 orders of magnitude of variation are extremely rare.

We present a Chandra/ACIS-I study of X-ray sources in BOSS 1441, a protocluster at z=2.32±0.02 that exhibits a prominent overdensity of Lyα emitters (LAEs). Using a 45 ks observation, we identify seven X-ray sources spatially coincident with LAE density peaks. The average X-ray photon index for the seven sources, derived from an absorbed power-law model with Galactic absorption fixed, is 1.49 (ranging from -0.68 to 2.51), corresponding to an average luminosity of 6.85× 10^44 erg s^-1 in the rest-frame 2-33 keV band, with individual luminosities spanning (3.57 - 13.96)× 10^44 erg s^-1. Three sources exhibit relatively flat spectral slopes. Two are associated with the MAMMOTH-1 nebula, while the third, located at the edge of BOSS 1441 with a > 5' offset from the LAE density peak, resides in a region with a high submillimeter-band density. We estimate the fraction of X-ray detected AGNs among the LAEs to be 11.5^+3.8_-4.6%, approximately double that of previously studied LAEs. This elevated fraction suggests BOSS 1441 is in a mature evolutionary stage, with even higher AGN fractions expected in massive LAEs such as PKS 1138-262. In contrast, the submillimeter galaxy population shows a lower AGN fraction (6.9^+6.9_-4.5%), consistent with their typically obscured nature. These results indicate that the protocluster's massive galaxies are evolving into the bright red sequence galaxies observed in local clusters, where AGNs likely play a critical role in quenching their star formation.

We present a Chandra/ACIS-I study of X-ray sources in BOSS 1441, a protocluster at z = 2.32 +/- 0.02 that exhibits a prominent overdensity of Ly alpha emitters (LAEs). Using a 45 ks observation, we identify seven X-ray sources spatially coincident with LAE density peaks. The average X-ray photon index for the seven sources, derived from an absorbed power-law model with Galactic absorption fixed, is 1.49 (ranging from -0.68 to 2.51), corresponding to an average luminosity of 6.85 x 10(44)erg s(-1) in the rest-frame 2-33 keV band, with individual luminosities spanning (3.57-13.96) x 10(44) erg s(-1). Three sources exhibit relatively flat spectral slopes. Two are associated with the MAMMOTH(-1 )nebula, while the third, located at the edge of BOSS 1441 with a >5 ' offset from the LAE density peak, resides in a region with a high submillimeter-band density. We estimate the fraction of X-ray-detected active galactic nuclei (AGNs) among the LAEs to be 11.5(-4.6)(+3.8)% , approximately double that of previously studied LAEs. This elevated fraction suggests BOSS 1441 is in a mature evolutionary stage, with even higher AGN fractions expected in massive LAEs such as PKS 1138-262. In contrast, the submillimeter galaxy population shows a lower AGN fraction ( 6.9(-4.5)(+6.9)% ), consistent with their typically obscured nature. These results indicate that the protocluster's massive galaxies are evolving into the bright red sequence galaxies observed in local clusters, where AGNs likely play a critical role in quenching their star formation.

SDSS J1115+0544 is a unique low-ionization nuclear emission-line region (LINER) galaxy with energetic ultraviolet (UV), optical and mid-infrared outbursts occurring in its nucleus. We present the results from an analysis of multi-wavelength photometric and radio follow-up observations covering a period of 9 years since its discovery. We find that following a luminosity plateau of 500 days, the UV/optical emission has decayed back to the pre-outburst level, suggesting that the nuclear outburst might be caused by a stellar tidal disruption event (TDE). In this case, SDSS J1115+0544 could be an unusually slow-evolved optical TDE with longest rise and decline time-scales ever found. Three years later than the optical peak, a delayed radio brightening was found with a 5.5 GHz luminosity as high as 1.9x10^39 erg/s. Using a standard equipartition analysis, we find the outflow powering the radio emission was launched at t 1260 days with a velocity of beta< 0.1 and kinetic energy of E_K >10^50 erg. The delayed radio brightening coupled with the disappearing plateau in the UV/optical light curves is consistent with the scenario involving delayed ejection of an outflow from a state transition in the disk. SDSS J1115+0544 is the first TDE displaying both a short-lived UV/optical plateau emission and a late-time radio brightening. Future radio observations of these TDEs in the post-plateau decay phase will help to establish the connection between outflow launching and changes in accretion rate.

Context. In theory, recurring tidal disruption events (TDEs) may occur when a close stellar binary encounters a supermassive black hole, if one star is captured and undergoes repeating partial TDEs, or if both stars are tidally disrupted (double TDEs). In addition, independent TDEs may be observed over decades in some special galaxies where the TDE rate is extremely high. Exploring the diversity of recurring TDEs and probing their natures with rich observational data helps us to understand these mechanisms. Aims. We report the discovery of a second optical flare that occurred in September 2021 in IRAS F01004-2237, where a first flare that occurred in 2010 had already been reported. We also present a detailed analysis of multi-band data. We aim to understand the nature of the flare and explore the possible causes of the recurring flares. Methods. We describe our analysis of the position of the flare, the multi-band light curves (LCs), the optical and ultraviolet (UV) spectra, and the X-ray LC and spectra. Results. The position of the flare coincides with the galaxy centre with a precision of 650 pc. The flare peaks in similar to 50 days with an absolute magnitude of similar to - 21 and fades in two years, roughly following L proportional to t(-5/3). It maintains a nearly constant blackbody temperature of similar to 22 000 K in later stages. Its optical and UV spectra show hydrogen and helium broad emission lines with full width at half maxima of 7000-21 000 km s(-1) and a He II/H alpha ratio of 0.3-2.3. It shows weak X-ray emission relative to UV emission, with X-ray flares lasting for < 2 - 3 weeks, during which the spectrum is soft with a power-law index of Gamma = 4.4(-1.3)(+1.4). These characters are consistent with a TDE, ruling out the possibilities of a supernova or an active galactic nucleus flare. With a TDE model, we infer a peak UV luminosity of 3.3 +/- 0.2 x 10(44) erg s(-1) and an energy budget of 4.5 +/- 0.2 x 10(51) erg. Conclusions. A TDE caused the flare that occurred in 2021. The two optical flares separated by 10.3 +/- 0.3 years can be interpreted as repeating partial TDEs, double TDEs, or two independent TDEs. Although no definitive conclusion can be drawn, the partial TDEs interpretation predicts a third flare around 2033, and the independent TDEs interpretation predicts a high TDE rate of greater than or similar to 10(-2) yr(-1) in F01004-2237, both of which can be tested by future observations.

We report the discovery of a second optical flare that occurred in September 2021 in IRAS F01004-2237, where the first flare occurred in 2010 has been reported, and present a detailed analysis of multi-band data. The position of the flare coincides with the galaxy centre with a precision of 650 pc. The flare peaks in ∼50 days with an absolute magnitude of ∼-21 and fades in two years roughly following L∝ t^-5/3. It maintains a nearly constant blackbody temperature of ∼22,000 K in the late time. Its optical and UV spectra show hydrogen and helium broad emission lines with full width at half maxima of 7,000–21,000 km s^-1 and He II/Hα ratio of 0.3–2.3. It shows weak X-ray emission relative to UV emission, with X-ray flares lasting for <2-3 weeks, during which the spectrum is soft with a power-law index Γ=4.4^+1.4_-1.3. These characters are consistent with a tidal disruption event (TDE), ruling out the possibilities of a supernova or an active galactic nuclei flare. With a TDE model, we infer a peak UV luminosity of 3.3±0.2×10^44 erg s^-1 and an energy budget of 4.5±0.2×10^51 erg. The two optical flares separated by 10.3±0.3 years can be interpreted as repeating partial TDEs, double TDEs, or two independent TDEs. Although no definitive conclusion can be drawn, the partial TDEs interpretation predicts a third flare around 2033, and the independent TDEs interpretation predicts a high TDE rate of ≳10^-2 yr^-1 in F01004-2237, both of which can be tested by future observations.