Research Highlights
ARTICLES | IIA

Quiet-Sun Ellerman bombs (QSEBs) are small-scale magnetic reconnection events in the lower solar atmosphere. Sometimes, they exhibit transition region counterparts, known as ultraviolet (UV) brightenings. Magnetic field extrapolations suggest that QSEBs can occur at various locations of a fan-spine topology, with UV brightening occurring at the magnetic null point through a common reconnection process. Aims. We aim to understand how more complex magnetic field configurations such as interacting fan-spine topologies can cause small-scale dynamic phenomena in the lower atmosphere. Methods. QSEBs were detected using k-means clustering on Hβ observations from the Swedish 1-m Solar Telescope (SST). Further, chromospheric inverted-Y-shaped jets were identified in the Hβ blue wing. Magnetic field topologies were determined through potential field extrapolations from photospheric magnetograms derived from spectro-polarimetric observations in the Fe i 6173 Å line. UV brightenings were detected in IRIS 1400 Å slit-jaw images. Results. We identify two distinct magnetic configurations associated with QSEBs, UV brightenings, and chromospheric inverted-Yshaped jets. The first involves a nested fan-spine structure where, due to flux emergence, an inner 3D null forms inside the fan surface of an outer 3D null with some overlap. The QSEBs occur at two footpoints along the shared fan surface, with the UV brightening located near the outer 3D null point. The jet originates close to the two QSEBs and follows the path of high squashing factor, Q. We discuss a comparable scenario using a 2D numerical experiment with the Bifrost code. In the second case, two adjacent fan-spine topologies share fan footpoints at a common positive polarity patch, with the QSEB, along with a chromospheric inverted-Y-shaped jet, occurring at the intersection having high Q values. The width of the jets in our examples is about 0.003, and the height varies between 100–200. The width of the cusp measures between 100–200. Conclusions. This study demonstrates through observational and modelling support that small-scale dynamic phenomena, such as associated QSEBs, UV brightenings, and chromospheric inverted-Y-shaped jets, share a common origin driven by magnetic reconnection between interacting fan-spine topologies

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The driving force behind outflows, often invoked to understand the correlation between the supermassive black holes powering active galactic nuclei (AGN) and their host galaxy properties, remains uncertain. We provide new insights into the mechanisms that trigger warm ionized outflows in AGN, based on findings from the MaNGA survey. Our sample comprises 538 AGN with strong [O iii] λ5007 emission lines, of which 197 are detected in radio and 341 are radio-undetected. We analyzed the [O iii] λ5007 line in summed spectra, extracted over their central 500 × 500 pc2 region. The calculated Balmer 4000 Å break, Dn4000, is larger than 1.45 for ∼95% of the sources, indicating that the specific star formation rate in their central regions is less than 10−11.5 yr−1, which points to evidence of negative AGN feedback suppressing star formation. Considering the whole sample, radio-detected sources show a greater outflow detection rate (56% ± 7%) than radio-undetected sources (25% ± 3%). They also show higher velocity, mass outflow rate, outflow power, and outflow momentum rate. We noticed a strong correlation between outflow characteristics and bolometric luminosity in both samples, except that the correlation is steeper for the radio-detected sample. Our findings suggest that (a) warm ionized outflows are prevalent in all types of AGN, (b) radiation from AGN is the primary driver of these outflows, (c) radio jets are likely to play a secondary role in enhancing the gas kinematics over and above that caused by radiation, and (d) there is very little star formation in the central regions of the galaxies, possibly due to negative feedback from AGN activity.

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During 2023 September the Alice ultraviolet spectrograph on the New Horizons (NH) spacecraft was used to map diffuse Lyα emission over most of the sky, at a range of ∼56.9 au from the Sun. At that distance, models predict that the interplanetary medium Lyα emissions result from comparable amounts of resonant backscattering of the solar Lyα line by interstellar hydrogen atoms (H i) passing through the solar system, in addition to an approximately isotropic background of ∼50 ± 20 R from the local interstellar medium (LISM). The NH observations show no strong correlations with nearby cloud structures of the LISM or with expected structures of the heliosphere, such as a hydrogen wall associated with the heliopause. To explain the relatively bright and uniform Lyα of the LISM, we propose that hot, young stars within the Local Hot Bubble shine on its interior walls, photoionizing H i atoms there. Recombination of these ions can account for the observed ∼50 R Lyα background, after amplification of the diffuse Lyα by resonant scattering, although sophisticated (i.e., 3D) radiative transfer models should be used to confirm this conjecture. Future observations of the diffuse Lyα, with instruments capable of resolving the line profile, could provide a new window on H i populations in the LISM and heliosphere. The NH Alice all-sky Lyα observations presented here may be repeated at some point in the future, if resources allow, and the two maps could be combined to provide a significant increase in angular resolution.

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Compact Symmetric Objects (CSOs) are a distinct category of jetted active galactic nuclei (AGN) whose optical variability characteristics have not been well investigated. We present here the results of our investigation on the optical flux and colour variability properties of a bona fide sample of 38 CSOs. We used the g-, r- and i-bands data from the Zwicky Transient Facility survey that spans a duration of about 5 years. We also considered a comparison sub-sample of blazars that includes 5 flat spectrum radio quasars and 12 BL Lac objects with redshifts and g-band magnitudes similar to the limited sub-sample of 9 CSOs. These two sub-samples of AGN, chosen for this comparative study of their long-term optical variability, represent different orientations of their relativistic jets with respect to the observer. We found that both CSOs and blazars exhibit optical flux variations, although variability of CSOs is lower than that of blazars. The observed variability in both CSOs and blazars is attributed to the relativistic jets and the increased optical variations in blazars relative to CSOs are likely due to beaming effects. CSOs and blazars exhibit similar colour variations, with both of them showing a bluer when brighter trend. Such a colour variability pattern is expected due to processes associated with their relativistic jets.

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Using solar cycle–long helioseismic measurements of meridional and zonal flows in the near-surface shear layer (NSSL) of the Sun, we study their spatiotemporal variations and connections to active regions. We find that near-surface inflows toward active latitudes are part of a local circulation with an outflow away from them at depths around 0.97 R⊙, which is also the location where the deviations in the radial gradient of rotation change sign. These results, together with opposite signed changes, over latitude and depth, in the above quantities observed during the solar minimum period, point to the action of the Coriolis force on large-scale flows as the primary cause of changes in rotation gradient within the NSSL. We also find that such Coriolis force mediated changes in near-surface flows toward active latitudes only marginally change the amplitude of zonal flow and hence are not likely to be its driving force. Our measurements typically achieve a high signal-to-noise ratio (>5σ) for near-surface flows but can drop to 3σ near the base (0.95 R⊙) of the NSSL. Close agreements between the depth profiles of changes in rotation gradient and in meridional flows measured from quite different global and local helioseismic techniques, respectively, show that the results are not dependent on the analysis techniques.

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Aditya-L1, India’s first dedicated mission to study the Sun and its atmosphere from the Sun-Earth Lagrangian L1 location was successfully launched on September 2, 2023. It carries seven payloads. The Visible Emission Line Coronagraph (VELC) is a major payload on Aditya-L1. VELC is designed to carry out imaging and spectroscopic observations (the latter in three emission lines of the corona), simultaneously. Images of the solar corona in the continuum at 5000 Å, with a field of view (FoV) from 1.05 R to 3 R can be obtained at variable intervals depending on the data volume that can be downloaded. Spectroscopic observations of the solar corona in three emission lines, namely 5303 Å FeXIV, 7892 Å FeXI, and 10,747 Å FeXIII are possible simultaneously, with different exposure times and cadence. Four slits, each of width 50 μm, separated by 3.75 mm help to simultaneously obtain spectra at four positions in the solar corona in all the aforementioned lines. A Linear Scan Mechanism (LSM) makes it possible to scan the solar corona up to ± 1.5 R. The instrument has the facility to carry out spectropolarimetric observations at 10,747 Å also in the FoV range 1.05 – 1.5 R. Various components of the instrument were tested interferometrically on the optical bench before installation. The individual components were aligned and performance of the payload was checked in the laboratory using a laser source and tungsten lamp. Wavelength calibration of the instrument was verified using the Sun as a light source. All the detectors were calibrated for different parameters such as dark current and its variation with exposure time. Here, we discuss the various features of the VELC, alignment, calibration, performance, possible observations, initial data analysis, and results of initial tests conducted in-orbit.

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This study examines the flux and photon index distributions of 11 Very High Energy (VHE) Flat Spectrum Radio Quasars (FSRQs) using over 16 yr of Fermi-LAT γ -ray data. The distributions reveal double lognormal profiles in both flux and index, primarily in the 3-d and 7-d binnings, supporting the ‘two-flux-state hypothesis’ for blazars. These profiles, which become insignificant at 30-d binning, suggest that shorter time-scales are better at capturing distinct states, while longer time-scales smooth out shorter variations. Most VHE FSRQs exhibit a ‘harder-when-brighter’ trend, where the photon index decreases during high-flux states, suggesting efficient particle acceleration and possibly reduced radiative cooling. In contrast, two sources display a ‘softer-when-brighter’ behaviour, likely due to enhanced radiative cooling in high photon density environments. Additionally, we observe that the Spearman rank correlation between flux and photon index strengthens with increasing time bin sizes, indicating more pronounced correlations over longer time-scales. This possibly indicates that, on shorter time-scales, flux variations are driven by a combination of photon index changes and normalization effects. Averaging flux over longer durations minimizes the effect of normalization variation, thereby enhancing the observed correlation. We also compare the flux and index distributions of VHE and non-VHE FSRQs, emphasizing the differences in their variability and emission patterns.

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Dust is expected to form on a year timescale in core-collapse supernova (SN) ejecta. Its existence is revealed through an infrared brightening, an optical dimming, or a blue-red emission-line profile asymmetry. To investigate how the dust location and amount impact observations, we computed ultraviolet-to-optical spectra of interacting and standard, noninteracting Type II SNe using state-of-the-art models – for simplicity we adopted 0.1 μm silicate grains. These models account for the full ejecta and treat both radioactive decay and shock power that arises from interaction of the ejecta with circumstellar material. In a Type IIn SN such as 1998S at one year, approximately 3×10‑4 M⊙ of dust within the dense shell reproduces the broad, asymmetric Hα profile. It causes an optical dimming of ∼2 mag (which obscures any emission from the inner, metal-rich ejecta) but, paradoxically, a more modest dimming of the ultraviolet, which originates from the outer parts of the dense shell. In Type II SNe with late-time interaction, such as SN 2017eaw, dust in the low-mass, fast outer ejecta dense shell tends to be optically thin, impacting little the optical spectrum for dust masses of order 10‑4 M⊙. In such SNe II with interaction, dust in the inner metal-rich ejecta has negligible effect on observed spectra in the ultraviolet and optical. In noninteracting SNe II, dust within the metal-rich ejecta preferentially quenches the [O I] λλ 6300, 6364 and [Ca II] λλ 7291, 7323 metal lines, biasing the emission in favor of the H-rich material which generates the Hα and Fe II emission below 5500 Å. Our model with 5×10‑4 M⊙ of dust below 2000 km s‑1 closely matches the optical spectrum of SN 1987A at 714 d. Modeling historical SNe requires one to treat both the ejecta material and the dust, as well as multiple power sources, although interaction power will generally dominate.

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We present the results of a detailed high-resolution spectroscopic analysis (SUBARU/HDS spectra, R ∼50,000) of three faint high-latitude carbon stars HE 1104 − 0957, HE 1205 − 0521, and HE 1244 − 3036. Our estimated metallicity for these objects is − 2.96, − 2.63, and − 2.49, respectively. The surface chemical compositions of the objects are found to be characterised by enhanced carbon and heavy elements, such as Y, Ba, La, and Ce. Using the classification criteria for carbon-enhanced metal-poor (CEMP) stars the objects HE 1104 − 0957 and HE 1205 − 0521 could not be classified into any known CEMP sub-classes, whereas the object HE 1244 − 3036 is found to be likely a CEMP-s star. The observed abundance patterns in HE 1244 − 3036 are also found to match well with the yields of a 2 M ⊙ AGB star with [Fe/H] = − 2.50. Although our kinematic analysis indicates that the objects belong to the halo population, the elemental abundance ratios of HE 1104 − 0957 and HE 1205 − 0521 do not match well with those of typical halo objects. Estimated elemental abundances are presented, and kinematic properties of the stars are discussed.

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Star sensors are an essential instrument used to determine the attitude of satellites by identifying the stars in the field of view. The high cost and large sizes of commercially available star sensors pose challenges for small satellite missions. We at the Indian Institute of Astrophysics have developed a low-cost star sensor, StarberrySense, based on the Raspberry Pi as the main controller and built from commercial off-the-shelf components. The StarberrySense was flown on the PS4 experimental orbital platform module of the Polar Satellite Launch Vehicle C-55 by the Indian Space Research Organization. This work describes the flight hardware, environmental tests in preparation for the flight, and in-orbit performance of our StarberrySense.

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