Research Highlights
ARTICLES | IIA

In the last decade, there have been several discoveries of active galactic nuclei (AGN) in dwarf galaxies including an AGN in an ultracompact dwarf galaxy with a black hole mass >106 Me. However, finding a supermassive black hole (SMBH) in a dwarf low surface brightness (LSB) galaxy is rare. We report the discovery of a Seyfert type 2 class AGN that is associated with a nuclear SMBH of mass ∼6.5 × 106 Me in a dwarf LSB galaxy (μ0,r > 23.8 mag arcsec−2) that we denote by MJ0818+2257. The galaxy was previously thought to be an outlying emission blob around the large spiral galaxy LEDA 1678924. In our current analysis, which includes the detection of the optical counterpart of MJ0818+2257, we study its ionized gas kinematics and find that the dynamical mass within the ionized gas disk is ∼5.3 × 109 Me. This is comparable to its stellar mass, which is ∼3 × 109 Me, and suggests that MJ0818+2257 is moderately dark matter dominated within the stellar disk. The SMBH-mass-togalaxy-stellar-mass ratio is MBH/M(*) > 0.022, which is high compared to disk galaxies. Our detection of an SMBH in a bulgeless LSB dwarf galaxy raises questions about the growth of SMBHs in low-luminosity galaxies and suggests the possibility of detecting heavy seed black holes from early epochs in LSB dwarf galaxies in the low-redshift Universe.

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Fast X-ray transients (FXTs) are X-ray flares that last from minutes to hours. Multi-wavelength counterparts to these FXTs have proven hard to find. As a result, distance measurements are made through indirect methods such as a host galaxy identification. Of the three main models proposed for FXTs, that is, supernova shock breakout emission (SN SBO), binary neutron star (BNS) mergers, and tidal dirsuption events (TDEs) of an intermediate-mass black hole (IMBH) disrupting a white dwarf (WD), the SN SBO predicts a much lower maximum peak X-ray luminosity (LX,peak). If the distance to FXTs were to be obtained, it would be a powerful probe for investigating the nature of these FXTs. Aims. We aim to obtain distance measurements to four FXTs by identifying candidate host galaxies. Through a redshift measurement of the candidate host galaxies, we derive LX,peak and the projected offset between the candidate host galaxy and the FXT. Methods. We obtained Very Large Telescope (VLT)/Multi Unit Spectroscopic Explorer (MUSE) observations of a sample of FXTs. We report the redshift of between 13 and 22 galaxies per FXT. We used these redshifts to calculate the distance, LX,peak and the projected offsets between the FXT position and the position of the sources. Additionally, we computed the chance alignment probabilities for these sources with the FXT postitions. Results. We find LX,peak > 1044 erg s−1 when we assume that any of the sources with a redshift measurement is the true host galaxy of the corresponding FXT. For XRT 100831, we find a very faint galaxy (mR,AB = 26.5 ± 0.3, z ∼ 1.22, LX,peak ∼ 8 × 1045 erg s−1 if the FXT is at this distance) within the 1σ uncertainty region with a chance alignment probability of 0.04. For XRT 060207, we find a candidate host galaxy at z = 0.939 with a low chance alignment probability within the 1σ uncertainty region. However, we also report the detection of a late-type star within the 3σ uncertainty region with a similar chance alignment probability. For the remaining FXTs (XRT 030511 and XRT 070618), we find no sources within their 3σ uncertainty regions. The projected offsets between the galaxies and the FXT positions is >33 kpc at 1σ uncertainty. Therefore, if one of these candidate host galaxies turns out to be the true host galaxy, it would imply that the FXT progenitor originated from a system that received a significant kick velocity at formation. Conclusions. We rule out an SN SBO nature for all FXTs based on LX,peak and the projected offsets between the FXT position and the sources, assuming any of the candidate host galaxies with a redshift determination is the true host galaxy to the FXT. For XRT 100831, we conclude that the detected galaxy within the 1σ uncertainty position is likely to be the host galaxy of this FXT based on the chance alignment probability. From the available information, we are not able to determine whether XRT 060207 originated from the galaxy found within 1σ of the FXT position or was due to a flare from the late-type star detected within the 3σ uncertainty region. Based on LX,peak and the offsets within our sample, we are not able to distinguish between the BNS merger and the IMBD-WD TDE progenitor model. However, for the candidate host galaxies with an offset &30 kpc, we can conclude that the IMBH-WD TDE is unlikely due to the large offset.

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We present the analysis of optical photometric and spectroscopic observations of two non-magnetic cataclysmic variables, namely CRTS J080846.2+313106 and V416 Dra. We find CRTS J080846.2+313106 to vary with a period of 4.9116 ± 0.0003 h, which was not found in earlier studies and which we provisionally suggest is the orbital period of the system. In both long-period systems, the observed dominant signal at the second harmonic of the orbital frequency and the orbital modulation during quiescence are suggestive of ellipsoidal variation from changing aspects of the secondary, with an additional contribution from the accretion stream or hotspot. However, during the outburst, the hotspot itself is overwhelmed by the increased brightness, which is possibly associated with the accretion disc. The mid-eclipse phase for V416 Dra occurs earlier and the width of the eclipse is greater during outbursts compared to quiescence, suggesting an increased accretion disc radius during outbursts. Furthermore, from our investigation of the accretion disc eclipse in V416 Dra, we find that a total disc eclipse is possible during quiescence, whereas the disc seems to be partially obscured during outbursts, which further signifies that the disc may grow in size as the outburst progresses. The optical spectra of CRTS J080846.2+313106 and V416 Dra are typical of dwarf novae during quiescence, and they both show a significant contribution from the M2-4V secondary. The light curve patterns, orbital periods, and spectra observed in the two systems look remarkably similar, and seem to resemble the characteristics of U Gem-type dwarf novae.

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Decayless kink oscillations are ubiquitously observed in active region coronal loops with an almost constant amplitude for several cycles. Decayless kink oscillations of coronal loops triggered by coronal rain have been analyzed, but the impact of coronal rain formation in an already oscillating loop is unclear. As kink oscillations can help diagnose the local plasma conditions, it is important to understand how these are affected by coronal rain phenomena. In this study, we present the analysis of an event of coronal rain that occurred on 25 April 2014 and was simultaneously observed by Slit-Jaw Imager (SJI) on board Interface Region Imaging Spectrograph (IRIS) and Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory (SDO). We investigated the oscillation properties of the coronal loop in AIA images before and after the appearance of coronal rain as observed by SJI. We find signatures of decayless oscillations before and after coronal rain at similar positions to those found during coronal rain. The individual cases show a greater amplitude and period during coronal rain. The mean period is increased by 1.3 times during coronal rain, while the average amplitude is increased by 2 times during rain, in agreement with the expected density increase from coronal rain. The existence of the oscillations in the same loop at the time of no coronal rain indicates the presence of a footpoint driver. The properties of the observed oscillations during coronal rain can result from the combined contribution of coronal rain and a footpoint driver. The oscillation amplitude associated with coronal rain is approximated to be 0.14 Mm. The properties of decayless oscillations are considerably affected by coronal rain, and without prior knowledge of coronal rain in the loop, a significant discrepancy can arise from coronal seismology with respect to the true values.

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Galaxy interactions in groups can lead to intense starbursts and the activation of active galactic nuclei (AGNs). The stripped gas from the outer disk can lead to star-forming clumps along the tidal tails or sometimes tidal dwarf galaxies. We investigate the impact of interaction on various galaxy properties, including morphology, star formation rates, and chemical composition in the galaxy group AM 1054-325 using Multi Unit Spectroscopic Explorer (MUSE) data. We conduct a comprehensive spatially and spectrally resolved investigation of the star formation rate, star formation histories, metallicity, and AGN activity. The galaxy subgroup AM 1054-325A shows multiple star-forming clumps in Hα emission along the western tidal tail, which are formed due to tidal stripping. These clumps also have higher metallicities. AM 1054-325B is quenched and shows disturbed gas kinematics and the signature of gas accretion in the Hα map. The specific star formation along the tidal tail is higher, contributing to the galaxy’s overall stellar mass growth.

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We present optical and near-infrared photometry and spectroscopy of SN 2023aew and our findings on its remarkable properties. This event, initially resembling a Type IIb supernova (SN), rebrightens dramatically ∼90 d after the first peak, at which time its spectrum transforms into that of a SN Ic. The slowly evolving spectrum specifically resembles a post-peak SN Ic with relatively low line velocities even during the second rise. The second peak, reached 119 d after the first peak, is both more luminous (Mr = −18.75 ± 0.04 mag) and much broader than those of typical SNe Ic. Blackbody fits to SN 2023aew indicate that the photosphere shrinks almost throughout its observed evolution, and the second peak is caused by an increasing temperature. Bumps in the light curve after the second peak suggest interaction with circumstellar matter (CSM) or possibly accretion. We consider several scenarios for producing the unprecedented behavior of SN 2023aew. Two separate SNe, either unrelated or from the same binary system, require either an incredible coincidence or extreme fine-tuning. A pre-SN eruption followed by a SN requires an extremely powerful, SNlike eruption (consistent with ∼1051 erg) and is also disfavored. We therefore consider only the first peak a true stellar explosion. The observed evolution is difficult to reproduce if the second peak is dominated by interaction with a distant CSM shell. A delayed internal heating mechanism is more likely, but emerging embedded interaction with a CSM disk should be accompanied by CSM lines in the spectrum, which are not observed, and is difficult to hide long enough. A magnetar central engine requires a delayed onset to explain the long time between the peaks. Delayed fallback accretion onto a black hole may present the most promising scenario, but we cannot definitively establish the power source.

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Propagating (intensity) disturbances (PDs) have been extensively reported in observations of coronal loops and polar plumes, along with more recent links to co-temporal spicule activity in the solar atmosphere. However, despite their appearance in observations, PDs have yet to be studied or modelled in depth. Methods. In this work, we present results from a three-dimensional magnetohydrodynamic (3D MHD) numerical model. It features a stratified solar atmosphere perturbed by a p-mode wave driver at the photosphere, subsequently forming spicules described by the rebound shock model. Results. We find the features of the detected PDs to be consistent with the co-temporal transition region dynamics and spicular activity resulting from non-linear wave steepening and shock formation. Furthermore, the PDs could be interpreted as slow magnetoacoustic pulses propagating along the magnetic field, rather than high-speed plasma upflows carrying sufficient energy flux to (at least partially) heat the lower coronal plasma. Using forward modelling, we demonstrate the similarities between the PDs in the simulations and those reported from observations with IRIS and SDO/AIA. Conclusions. Our results suggest that in the model presented here, the dynamical movement of the transition region is a result of wave dynamics and shock formation in the lower solar atmosphere. We find that PDs are launched co-temporally with the rising of the transition region, regardless of the wave-generating physical mechanisms occurring in the underlying lower solar atmosphere. However, it is clear that signatures of PDs appear much clearer when a photospheric wave driver is included. Finally, we present the importance of PDs in the context of providing a source for powering the (fast) solar wind.

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Quiet-Sun Ellerman bombs (QSEBs) are key indicators of small-scale photospheric magnetic reconnection events. Recent high-resolution observations have shown that they are ubiquitous and that large numbers of QSEBs can be found in the quiet Sun. Aims. We aim to understand the impact of QSEBs on the upper solar atmosphere by analyzing their spatial and temporal relationship with the UV brightenings observed in transition region diagnostics. Methods. We analyzed high-resolution Hβ observations from the Swedish 1-m Solar Telescope and utilized k-means clustering to detect 1423 QSEBs in a 51 min time series. We used coordinated and co-aligned observations from the Interface Region Imaging Spectrograph (IRIS) to search for corresponding signatures in the 1400 Å slit-jaw image (SJI) channel and in the Si IV 1394 Å and Mg II 2798.8 Å triplet spectral lines. We identified UV brightenings from SJI 1400 using a threshold of 5σ above the median background. Results. We focused on 453 long-lived QSEBs (> 1 min) and found 67 cases of UV brightenings from SJI 1400 occurring near the QSEBs, both temporally and spatially. Temporal analysis of these events indicates that QSEBs start before UV brightenings in 57% of cases, while UV brightenings lead in 36% of instances. The majority of the UV brightenings occur within 1000 km of the QSEBs in the direction of the solar limb. We also identify 21 QSEBs covered by the IRIS slit, four of which show emissions in the Si IV 1394 Å and/or Mg II 2798.8 Å triplet lines, at distances within 500 km of the QSEBs in the limb direction. Conclusions. We conclude that a small fraction (15%) of the long-lived QSEBs contribute to the localized heating observable in transition region diagnostics, indicating they play a minimal role in the global heating of the upper solar atmosphere.

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The discovery of NGC 1052−DF2 and subsequent modeling have shown that NGC 1052−DF2 is deficient in dark matter and is in conflict with the standard stellar-to-halo mass ratio. In this work, we aim to resolve the degeneracy between the dynamical models on the mass estimate of the NGC 1052−DF2. Methods. We constructed mass models of NGC 1052−DF2 using an anisotropic distribution function with a radially varying anisotropy parameter and studied the effect of the various model parameters on the dark matter estimates. We used the observed stellar photometry as an input parameter to construct the distribution function and employed a Markov chain Monte Carlo (MCMC) method to estimate the dark matter model parameters. Results. We find that mass models with a cuspy dark matter halo have comparable χ2 to models with zero dark matter. Moreover, the cuspy dark matter halo fails to consistently account for the observed velocity dispersion in the inner and outer regions of the galaxy. Consequently, we rule out the possibility of a cuspy dark matter halo for describing the mass models of NGC 1052−DF2. Our study shows that the cored dark matter halo model with a total mass of log(MDM/M⊙) = 10.5 explains the observed kinematics but requires an extraordinarily large scale length (20 kpc) and an outer cutoff radius (26 kpc). While the cored mass model provides a comparatively better fit, our findings emphasize that the mass models are largely unconstrained by the available kinematic data. Our results suggest that NGC 1052−DF2 may not only have an ultra-diffuse stellar distribution but that it can, within uncertainties in the available kinematic data, potentially host an ultra-diffuse dark matter distribution compatible with the standard stellar-to-halo mass relation (SHMR) predicted by galaxy formation and evolution models

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We present results from simultaneous far-ultraviolet (FUV) and near-ultraviolet (NUV) observations of T Tauri stars (TTSs) in the Taurus molecular cloud with UVIT/AstroSat. This is the very first UVIT study of TTSs. From the spectral energy distribution of TTSs from FUV to IR, we show that classical TTSs (CTTSs) emit significantly higher UV excess compared to weak-line TTSs (WTTSs). The equivalent blackbody temperatures corresponding to the UV excess in CTTSs (>104 K) are also found to be relatively higher than those in WTTSs (<9250 K). From the UV excess, we have reclassified two WTTSs (BS Tau and V836 Tau) as CTTSs, which has been supported by the follow-up optical spectroscopic study using the Himalayan Chandra Telescope, showing strong Hα line emission. We find that CTTSs show strong excess emission in both the FUV (>107) and NUV (>103) bands, while WTTSs show strong excess only in the FUV (105), suggesting that excess emission in the NUV can be used as a tool to classify the TTSs. We also find a linear correlation between UV luminosity (a primary indicator of mass accretion) and Hα luminosity (a secondary indicator of mass accretion) with a slope of 1.20 ± 0.22 and intercept of 2.16 ± 0.70

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