arXiv subjects

Roberto Soria

Publications and source records attributed to Roberto Soria.

At least 19 recordsLinked to original sources

arXiv2026

Extreme Multiwavelength Variability in the Recurrent Intermediate-type Changing-look Seyfert Galaxy SDSS J234728.73+242745.7

We report the discovery of the Seyfert galaxy J234728.73+242745.7 as a recurrent intermediate-type CLAGN with extreme optical/UV variability. Optical spectra spanning nearly three decades reveal Type 1.0$\rightarrow$1.5$\rightarrow$1.2 transitions, accompanied by the disappearance and reappearance of broad He II $\lambda4686$ and a broad-line Balmer decrement evolving from 4.5 to 7.2 and then 2.4. Monitoring reveals coordinated variability from X-rays to the MIR. SED decomposition yields a host-corrected g-band high-to-low-state flux ratio of $5.4\pm0.4$, exceeding the Swift/XRT ratio of $2.7\pm0.8$ across the changing-look transition. This discrepancy may arise from reduced dust extinction, as correcting for the low-state continuum reddening ($A_V^{\rm cont}=0.66\pm0.23$ mag) brings the optical/UV ratios close to the X-ray value, although the correction is model-dependent and intrinsically weaker X-ray variability may also contribute. However, X-ray spectra show no compelling evidence for intrinsic gas absorption or its variability ($N_{\rm H}^{\rm intr}\lesssim5\times10^{20}$ cm$^{-2}$). In contrast, EP/WXT monitoring during the recent high state reveals order-of-magnitude X-ray fluctuations on monthly-to-sub-year timescales, exceeding contemporaneous optical variations. Meanwhile, the radio flux density remains stable at $\sim2$ mJy across all ASKAP observations since mid-2020. Together with optical/UV and MIR variability and the broad He II transition, these results favor intrinsic accretion-state changes as the primary driver, while allowing a secondary contribution from variable obscuration. The source displays a loop-like MIR color--magnitude evolution, likely driven by the delayed response of an extended torus. This recurrent CLAGN offers a laboratory for probing the dynamic response of the broad-line region and circumnuclear dust to accretion-state transitions.

astro-ph.HE↗

arXiv2026

Jets and winds from the most powerful compact object in NGC 5408

Super-critical accretion onto stellar-mass compact objects is sometimes revealed by ultraluminous X-ray sources (ULXs). In other cases, X-rays are heavily obscured along our line of sight, but a super-Eddington accretion power can be indirectly revealed by multiband studies. We have identified a candidate super-critical source in NGC 5408 (labelled X-2). We argue that: it is powered by accretion; it is among the most powerful in the local universe, despite its apparent X-ray faintness; it has exceptional optical properties (both in the continuum and in the lines). We carried out a multiband imaging and spectral study with the ATCA, the VLA, HST, the VLT and Chandra. We found an unresolved source with: variable radio emission (5-GHz luminosity of a few x 10^{35} erg/s); red optical continuum with absolute mag M_{V,0} = -9.1, M_{I,0} = -10.6; exceptionally broad low-ionization emission lines (FWHM ~ 1700 km/s) with P-Cygni profiles; narrow, photoionized emission lines including strong He II 4686; point-like hard X-rays; marginally resolved soft X-rays. We also found an ~20-pc He^{++} nebula near X-2, associated with candidate Wolf-Rayet stars. The optical continuum declined by 2 mag between 2010 and 2024, as did the intensity of broad and narrow lines. We argue that the system is powered by the radiative and mechanical output of a super-Eddington stellar mass BH, with an accretion power >~10^{40} erg/s. We propose that X-2 is a Compton-thick ULX seen at high inclination, with a flaring jet and X-ray emission emerging from the polar funnel. We argue that the optical continuum and the broad emission lines are signatures of an optically thick outflow, launched either from the accretion disk, or from an LBV-like donor star, or, more speculatively, caused by the elusive phase of common envelope ejection. We suggest that we are looking at a short-lived phase of hyper-Eddington accretion.

astro-ph.HE↗

arXiv2026

Infrared spectroscopy of the longest-period Galactic Mira variables from Palomar Gattini-IR: Identification of a candidate thermally pulsing Super-Asymptotic Giant Branch star

The population of extremely long-period Mira variables in the Milky Way remains poorly characterized. Their rarity and heavy circumstellar obscuration have precluded systematic studies in optical surveys, yet they are expected to trace the most massive stars evolving through the Asymptotic Giant Branch (AGB). We present IRTF/SpeX spectroscopy of the longest-period ($P\gtrsim1000$ d) Galactic Mira variables identified with the Palomar Gattini-IR survey. Twelve of thirteen targets exhibit characteristics consistent with massive O-rich AGB stars. In contrast, one peculiar source IRAS 19552+3142 exhibits i) a large-amplitude ($ΔJ\gtrsim2$ mag) pulsations with $P\approx1060$ d, ii) strong C$_2$ and CO molecular bands indicating C/O$\gtrsim1$, and iii) SiC emission at $\approx11.3\,μ$m in archival mid-infrared spectra. Its spectroscopically confirmed chemistry makes it the longest period C-rich Galactic Mira variable known; together with the rarity of similar objects, this source is difficult to reconcile with standard massive AGB evolution where hot-bottom burning maintains an O-rich surface composition. Instead, its properties may be explained by super-AGB evolution where dredge-out episodes produce a C-rich surface and circumstellar material. Its high luminosity ($M_{[4.5]}\lesssim-10$ mag), red color ($[3.6] - [4.5]\gtrsim2.0$ mag), and high mass-loss rate ($\gtrsim 10^{-4} \mathrm{M_\odot yr^{-1}}$) resemble extreme AGB star progenitors of intermediate-luminosity red transients in nearby galaxies and a known chemically peculiar super-AGB star candidate in the Small Magellanic Cloud. IRAS 19552+3142 may thus provide a rare look at a Galactic super-AGB star in the thermally pulsing phase. Our results motivate improved theoretical predictions and long-baseline infrared searches for these short-lived, dust-enshrouded endpoints of stellar evolution.

astro-ph.SR↗

arXiv2026

A Real-Time Jet Laboratory in Swift J1727.8-1613

We present the results of our intensive VLBI campaign on the black-hole low-mass X-ray binary (LMXB) Swift J1727.8-1613 during its 2023-2024 outburst. We observed the repeated quenching and re establishment of the highly-extended continuous jet during several transitions between hard-intermediate and soft-intermediate states, and the repeated ejection of transient jets. Using time-dependent visibility model fitting, we tracked the motion of nine discrete jet knots, obtaining some of the most precise measurements of transient jet proper motions and ejection dates in an LMXB. These ejecta were only detectable for a short time with VLBI, and some showed rapid intra-observation flux density variability that was not captured in image reconstructions. For the first time, we use time-dependent visibility modelling to fit a piecewise model for the jet knot flux densities, allowing us to create complex, non-parametric light curves of their intra-observation variability. We observed the launching of multiple ejecta across several state transitions, however, we could not identify a consistent signature of jet ejection in the available X-ray intensity or hardness data. We constrained the intrinsic speeds and bulk Lorentz factors of the jet knots, finding that Swift J1727.8-1613 launched both mildly relativistic and highly relativistic ejecta throughout its outburst. We used their proper motions to constrain a posterior distribution for the maximum inclination angle of the jet axis, which had 50th, 84th, and 99th percentiles of 40°, 50°, and 66°, respectively. These unique observations of the repeated ejection of transient jets by a single LMXB reveal that fixed parameters such as black-hole mass, black-hole spin, and spin-orbit misalignment do not uniquely determine the varying properties of transient jets, particularly their speeds and Lorentz factors.

astro-ph.HE↗

arXiv2026

X-ray and optical decline of the intermediate mass black hole HLX-1

HLX-1 is a prominent intermediate-mass black hole (IMBH) candidate, historically exhibiting recurrent X-ray outbursts with spectral state transitions analogous to those observed in stellar-mass black holes. Here, we present new Hubble Space Telescope, Chandra, and Swift observations from 2018-2022 to characterise the late-time flux decline. HLX-1 has persisted in a low X-ray luminosity state (L_X ~ a few x 10^{39} erg/s) since the end of its last outburst in 2017. We observe a significant decoupling between the X-ray and optical/UV emission: while the X-rays have faded by at least two orders of magnitude from peak outburst luminosity (in 2010) to the current low state, the optical/UV flux has declined much more slowly over the same time. This results in an X-ray/optical luminosity ratio inconsistent with X-ray reprocessing in a standard accretion disk, as this would require an unphysical reprocessing fraction >100% at late times. Instead, we find that the optical/UV evolution is well-fitted by a cooling, expanding photosphere (T ~ 30,000 K), similar to the late-stage evolution seen in tidal disruption events (TDEs). The redder component of the optical emission is instead consistent with the old stellar population of a massive star cluster (IMBH host). The pre-2017 X-ray bursting phase is consistent with simulations of disk instabilities in TDE evolution: this strengthens the scenario of HLX-1 as an IMBH TDE. Furthermore, our observations resolve the morphology and flux of the mysterious far-UV emitter, seen in projection next to HLX-1, into a ring-like star-forming structure. We re-assess the possibility that HLX-1 and its host star cluster are physically associated with this starburst dwarf, perhaps via a high-speed collision.

astro-ph.HE↗

arXiv2026

Photoionization of the Composite Nebula Surrounding NGC 5408 X-1: Implications for Beamed Emission

NGC 5408 X-1 is one of the best studied ultraluminous X-ray sources (ULXs) and is surrounded by a photoionized nebula. Previous optical spectroscopy established the presence of strong Balmer, [O III], and He II $\lambda4686$ emission from the nebula, but the powering engine remains uncertain. In this work, we present new integral-field observations of NGC 5408 X-1, supplemented by archival long-slit spectroscopy and Hubble Space Telescope (HST) imaging, and confirm the presence of a composite nebula, with a small He III region centered on the ULX and a large, shell-like H II region. We also confirm that the broad He II emission is point-like and most likely associated with the ULX binary system. Photoionization simulations with Cloudy show that the ULX spectral energy distribution (SED), with a total luminosity of $2.4 \times 10^{40}\ {\rm erg\ s^{-1}}$ obtained by fitting the optical/UV/X-ray data, overpredicts both the luminosity and size of the He III region. Instead, adopting the same SED shape with a reduced luminosity of $1.0 \times 10^{39}\ {\rm erg\ s^{-1}}$ together with a blackbody of temperature $30000\ {\rm K}$ and luminosity $1.3 \times 10^{39}\ {\rm erg\ s^{-1}}$ successfully reproduces both the He III and H II regions in terms of their luminosities and sizes. Such a dual-component ionizing spectrum is consistent with HST measurements of the ULX in the optical and UV, while being a factor of 24 lower than the inferred isotropic X-ray luminosity. This implies that the EUV and X-ray emission from the ULX may be mildly beamed toward our line of sight, consistent with the picture of supercritical accretion.

astro-ph.HE↗

arXiv2026

Multiwavelength Analysis of the Einstein Probe X-ray Transient EP240305a

We report multiwavelength observations of EP240305a, an uncatalogued X-ray transient detected by the Einstein Probe on March 5, 2024. The source exhibits distinct characteristics across the X-ray, optical, near-infrared, and radio bands. The soft X-ray observations show two significant flares lasting ~100-250 s, accompanied by rapid flux decay in a few days, and the optical and near-infrared data reveal a faint, candidate counterpart. In contrast, the radio observations expose a long-term spectral evolution from a self-absorbed to an optically thin state within two months, implying discrete jet ejection. We compare EP240305a with known classes of X-ray transients and find that it is unlikely to be associated with long-timescale transients such as jetted tidal disruption events or X-ray binaries. Its properties also disfavor a short-timescale stellar flare origin. Although the absence of optical spectroscopy prevents a redshift determination, the source exhibits properties similar to those of gamma-ray-dark gamma-ray burst-like transients, which may be associated with relativistic jets viewed off-axis or with choked jets. The discovery of EP240305a, along with other uncataloged transients detected by the Einstein Probe, underscores the scientific potential of highly sensitive X-ray survey telescopes and rapid-response multiwavelength follow-up observations in exploring the nature of atypical astronomical transients.

astro-ph.HE↗

arXiv2026

A Multiwavelength Assessment Disfavoring the X-ray Binary Origin of He III Regions in Metal-Poor Star-Forming Dwarf Galaxies

Recent observations of metal-poor, star-forming dwarf galaxies reveal He III regions, traced by nebular He II 4686 emission that require a strong source of extreme-ultraviolet (EUV) radiation. The origin of this hard ionizing radiation remains poorly understood, as standard stellar populations fail to account for it, posing key implications for the understanding of early galaxy formation. We present a systematic Chandra X-ray study of 21 nearby star-forming galaxies with He II emission but lacking Wolf-Rayet spectral signatures. Using 7 new and 36 archival Chandra X-ray observations combined with optical stellar population synthesis modelling, we constrain the ionizing continuum required to sustain the observed He II line, the ionizing continuum available from X-ray objects, and the properties of the host H II regions. We find that the inferred EUV output from accreting X-ray sources in our sample is systematically lower than what is required to produce the observed He II emission. Our sample is consistent with established empirical scaling relations for X-ray luminosity, indicating that this discrepancy cannot be attributed to an anomalously low number or luminosity of X-ray sources. These results indicate that accreting X-ray sources alone cannot account for the observed He II-ionizing photon budget, pointing to additional or alternative sources of hard EUV radiation in metal-poor star-forming environments. Potential alternative or additional contributors are discussed.

astro-ph.HE↗

arXiv2026

Deep VLBI constraints on compact radio cores in four ultraluminous X-ray sources

We present high-sensitivity Very Long Baseline Interferometry (VLBI) observations of four ultraluminous X-ray sources (ULXs): Holmberg II X-1, IC 342 X-1, NGC 6946 X-1, and NGC 925 X-1. No compact emission was detected on milliarcsecond scales, with rms noise levels reaching approximately 5--20 $μ$Jy. The corresponding $5σ$ flux density upper limits reach $\sim 26\,μ\mathrm{Jy}$, implying radio luminosity limits $L_{\rm R} \lesssim 2 \times 10^{33}\,\mathrm{erg\,s^{-1}}$. This disfavors any persistently bright hard-state-like compact core at our sensitivity level. The previously reported VLBI core in Holmberg II X-1 exhibits significant long-term variability, broadly consistent with an overall decline over the past decades. This behavior is consistent with emission from optically-thin ejecta undergoing adiabatic expansion. The VLBI non-detections may reflect intrinsically weak/intermittent compact emission, and/or low--surface--brightness structure that is resolved out by VLBI, and/or absorption/propagation effects such as free--free absorption in dense, ionized winds.

astro-ph.HE↗

arXiv2026

ASKAP J005512.2-255834: A Luminous, Long-Lived Radio Transient at z = 0.1 -- an Orphan Afterglow or an off-nuclear TDE from an IMBH?

We report the discovery of a slowly evolving, extragalactic radio transient, ASKAP J005512.2--255834 (hereafter ASKAP J0055-2558), identified using the Australian SKA Pathfinder in a search for orphan afterglows associated with archival gravitational wave events. Although discovered in this context, there is no evidence that the transient is associated with any known gravitational wave event. Nonetheless, this source exhibits a 20-fold increase in flux density over $<250$ days, and it remains in a declining, detectable state more than 1000 days after the initial detection. Follow-up observations from 0.3 to 9 GHz reveal an evolving spectrum consistent with synchrotron emission. ASKAP J0055-2558 is spatially coincident with a low-mass, star-forming galaxy at redshift $z = 0.116$ ($d_{\rm L}$= 543 Mpc), placing its peak radio luminosity at $νL_ν\sim 10^{39}\,\rm erg\,s^{-1}$. Analysis of its radio light curve, inferred blastwave velocity, energetics, host galaxy properties and the absence of counterparts at other wavelengths suggest that ASKAP J0055-2558 is most consistent with either the late-time phase of an orphan long gamma-ray burst afterglow or a tidal disruption event involving an intermediate-mass black hole spatially offset from the galaxy nucleus. The radio discovery of either of these phenomena is extremely rare, with only a few or no confirmed examples to date.

astro-ph.HE↗

arXiv2026

Radio counterparts of ultraluminous X-ray sources in nearby galaxies

Radio surveys of ultraluminous X-ray sources (ULXs) allow us to find supercritically accreting compact objects (SS 433/W50 like systems) or stripped nuclear black holes in nearby galaxies. We identified 21 such objects by crossmatching a ULX catalog with the Rapid ASKAP Continuum Survey (RACS) and Very Large Array Sky Survey (VLASS). They may have a diverse population. (i) Three have a double lobed radio structure with a compact core found in two of them, and could be quasars. (ii) Five are associated with extended radio structure and star forming regions in optical, where the radio emission is likely due to star forming activities, although the steep radio spectrum up to several GHz casts doubt on that. Two of them show X-ray variability suggesting that they are ULXs embedded in star forming regions. (iii) Thirteen are associated with an unresolved radio source, with a steep spectrum seen in eight, a flat or inverted spectrum seen in two. Those with a steep spectrum are arguably candidates for SS 433/W50 like objects, with radio emission due to optically thin synchrotron radiation in a surrounding jet/wind powered nebula. Remarkable cases include NGC 925 ULX1 and NGC 6946 ULX1, which are associated with an optical nebula. Those with a flat or inverted spectrum could be accreting black holes with a compact jet, while the black hole mass is estimated to be several $10^6 - 10^8$ $M_\odot$ based on the fundamental plane. Redshift measurements are needed to firmly determine the association with their apparent host galaxy.

astro-ph.HE↗

arXiv2025

A fast powerful X-ray transient from possible tidal disruption of a white dwarf

Stars captured by black holes (BHs) can be torn apart by strong tidal forces, producing electromagnetic flares. To date, more than 100 tidal disruption events (TDEs) have been observed, each involving invariably normal gaseous stars whose debris falls onto the BH, sustaining the flares over years. White dwarfs (WDs), which are the most prevalent compact stars and a million times denser--and therefore tougher--than gaseous stars, can only be disrupted by intermediate-mass black holes (IMBHs) of 10^2--10^5 solar masses. WD-TDEs are considered to generate more powerful and short-lived flares, but their evidence has been lacking. Here we report observations of a fast and luminous X-ray transient EP250702a detected by Einstein Probe. Its one-day-long X-ray peak as luminous as 10^(47-49) erg/s showed strong recurrent flares with hard spectra extending to several tens of MeV gamma-rays, as detected by Fermi/GBM and Konus-Wind, indicating relativistic jet emission. The jet's X-ray dropped sharply from 3 x 10^49 erg/s to around 10^44 erg/s within 20 days (10 days in the source rest frame). These characteristics are inconsistent with any known transient phenomena other than a jetted-TDE evolving over an unprecedentedly short timescale, indicating the disruption of a WD by an IMBH. At late times, a new soft component progressively dominates the X-ray spectrum, exhibiting an extreme super-Eddington luminosity, which possibly originates from an accretion disc. WD-TDEs open a new window for investigating the elusive IMBHs and their surrounding stellar environments, and they are prime sources of gravitational waves in the band of space-based interferometers.

astro-ph.HE↗

arXiv2025

A spectral library and census of near-infrared stellar large-amplitude variables from Palomar Gattini-IR

We present a near-infrared census of stellar large-amplitude variables (LAVs) observed by the Palomar Gattini-IR (PGIR) surveyor from 2019-2021. Over the three-year time period, PGIR performed a brightness-limited survey of the Northern sky (18,000 sq. deg) to J-band AB magnitudes of $\sim 13$ within and $\sim 15$ outside the Galactic plane. From 70 million stars detected in PGIR reference images, we provide a spectral and photometric library of the 128 largest amplitude stellar variables detected to median SNR > 10 for more than 50 epochs with more than 5 high-amplitude detections, peak-to-peak magnitudes $\geq$ 2, and von Neumann ratios $\leq$ 0.2. We obtained medium-resolution near-infrared spectra with TripleSpec on the 200-inch Hale Telescope at Palomar Observatory and SpeX at NASA's Infrared Telescope Facility. The spectral census consists of 82 evolved and dust-obscured Asymptotic Giant Branch stars, 16 R Coronae Borealis stars, 13 young-stellar or pre-main-sequence objects, 8 symbiotic binaries, 7 erratic carbon- and oxygen-rich giants, and 2 RV Tauri supergiants. The spectral and photometric dataset serves as an atlas of near-infrared LAVs and a repository of evolved stars, eruptive variables, and binary systems for future deeper infrared surveys.

astro-ph.SR↗

arXiv2025

The Complex Multi-Wavelength Morphology of the Peculiar Compact Galaxy Group IC 2431

We present new Chandra X-ray imaging spectroscopy of the compact galaxy group IC 2431, and compare with archival ultraviolet, optical, infrared, and radio images. IC 2431 is a starburst system containing three tidally-distorted disk galaxies. All three galaxies may have active nuclei. One galaxy is classified as an AGN based on its optical spectrum, a second is identified as a possible X-ray AGN based on the Chandra data, and the third galaxy may host a radio AGN. In optical images, a prominent dust lane crosses the southern galaxy, while Spitzer infrared images show a dusty bridge connecting the two brightest galaxies. Chandra maps reveal a massive (2 x 10^7 M(sun)) concentration of hot gas between these two galaxies, as well as several other knots of hot gas and non-thermal emission. The unabsorbed X-ray luminosity of the hot gas in IC 2431 is ~ 1 x 10^42 erg/s, which is enhanced by about a factor of four relative to the star formation rate, compared to other star-forming galaxies. In radio maps, a bright jet/ridge of radio continuum emission extends 4 kpc from one nucleus. We compare the properties of IC 2431 with those of other interacting galaxy systems, and discuss two different scenarios that may account for the peculiarities of IC 2431: ram pressure stripping of the interstellar medium during a head-on collision between two galaxies, or an AGN-powered radio jet that has been distorted by an interaction with interstellar gas during a tidal encounter between galaxies.

astro-ph.GA↗

arXiv2025

Four ages of rotating stars in the rotation--activity relationship and gyrochronology

Gyrochronology and the rotation--activity relationship are standard techniques used to determine the evolution phase.The mismatch in the definition of the evolutionary phases has so far raised many issues in physics and mathematics and hampered the understanding of how the internal dynamo processes affect the observable properties.To address this problem, we seek a unified scheme that shows a one-to-one mapping from gyrochronology to the rotation--activity relationship.We obtained the chromospheric activity $R'_{\rm HK}$ of 6846 stars and their rotation periods to investigate the rotation--activity relationship.We apply a three-interval model to fit the relationship and find that it is best fitted by three intervals in the range of Ro$<0.7$.We associate those intervals to the convective, gap and interface phases of gyrochronology.Furthermore,we suggest an additional epoch at late times of the I phase.We further use the three-interval models to fit the period--activity relationship in temperature bins and determine the duration of the transition phase as a function of effective temperature.By comparing the critical temperature and period of the g-to-I transition with the slowly rotating sequence of 10 young open clusters whose ages range from 1 Myr to 2.5 Gyr, we conclude that our new model finds the pure I sequence without fast rotating outliers, which defines the zero-age I sequence (ZAIS).We propose that there is an ambiguous consensus on when the I sequence starts to work.This ambiguity is from the visually convergent sequence of the color--period diagrams in open clusters.This visually convergent sequence is younger than the ZAIS and is actually the pre-I sequence that can be associated with the stall of the spin-down. Our results unify the rotation--activity relationship and gyrochonology for the stellar evolution of low-mass stars, for which we coined the ``CgIW" scenario.

astro-ph.SR↗

arXiv2025

Comprehensive Radio Monitoring of the Black Hole X-ray Binary Swift J1727.8$-$1613 during its 2023$-$2024 Outburst

This work presents comprehensive multi-frequency radio monitoring of the black hole low-mass X-ray binary Swift J1727.8$-$1613, which underwent its first recorded outburst after its discovery in August 2023. Through a considerable community effort, we have coalesced the data from multiple, distinct observing programs; the light curves include ${\sim} 10$ months and 197 epochs of monitoring from 7 radio facilities with observing frequencies ranging from (approximately) 0.3$-$230GHz. The primary purpose of this work is to provide the broader astronomical community with these light curves to assist with the interpretation of other observing campaigns, particularly non-radio observing frequencies. We discuss the phenomenological evolution of the source, which included: (i) multiple radio flares consistent with the launching of discrete jet ejections, the brightest of which reached $\sim$ 1 Jy; (ii) temporally evolving radio spectral indices ($α$), reaching values steeper than expected for optically-thin synchrotron emission ($α{<} -1$) and emission with significant radiative cooling ($α< -1.5$). We have published a digital copy of the data and intend for this work to set a precedent for the community to continue releasing comprehensive radio light curves of future low-mass X-ray binary outbursts.

astro-ph.HE↗

arXiv2025

Supermassive black hole mass measurement in the spiral galaxy NGC 4736 Using JWST/NIRSpec stellar kinematics

We present accurate mass measurements of the central supermassive black hole (SMBH) in NGC 4736 (M 94).\ We used the ``gold-standard" stellar absorption features (CO band heads) at $\sim$2.3 ${\rm μm}$, as opposed to gas emission lines, to trace the dynamics in the nuclear region, easily resolving the SMBH's sphere of influence. The analysis uses observations made with the integral field unit of the Near-Infrared Spectrograph (NIRSpec) on the {\it James Webb} Space Telescope and a surface brightness profile derived from {\it Hubble} Space Telescope archival images. We used Jeans anisotropic models within a Bayesian framework, and comprehensive Markov chain Monte Carlo optimization, to determine the best-fit black hole mass, orbital anisotropy, mass-to-light ratio, and nucleus kinematical inclination. We obtained a SMBH mass $M_{\rm BH}=(1.60\pm0.16)\times10^7$ M$_\odot$ (1$σ$ random error), which is consistent with the $M_{\rm BH}$-$σ$ and $M_{\rm BH}$-$M_\star$ relations. This is the first dynamical measurement of a $M_{\rm BH}$ in NGC 4736 based on the stellar kinematics observed with NIRSpec. We thus settle a longstanding inconsistency between estimates based on nuclear emission-line tracers and the $M_{\rm BH}$-$σ$ relation. Our analysis shows that NIRSpec can detect SMBHs with $M_{\rm BH,min}\approx 5\times10^6$ M$_\odot$ in galaxies within 5 Mpc and $σ\approx100$ km s$^{-1}$

astro-ph.GA↗

arXiv2025

Teleios (G305.4-2.2) -- the mystery of a perfectly shaped new Galactic supernova remnant

We present the serendipitous radio-continuum discovery of a likely Galactic supernova remnant (SNR) G305.4-2.2. This object displays a remarkable circular symmetry in shape, making it one of the most circular Galactic SNRs known. Nicknamed Teleios due to its symmetry, it was detected in the new Australian Square Kilometre Array Pathfinder (ASKAP) Evolutionary Map of the Universe (EMU) radio-continuum images with an angular size of 1320"x1260" and PA = 0 deg. While there is a hint of possible H$α$ and gamma-ray emission, Teleios is exclusively seen at radio-continuum frequencies. Interestingly, Teleios is not only almost perfectly symmetric, but it also has one of the lowest surface brightnesses discovered among Galactic SNRs and a steep spectral index of $α=-0.6\pm 0.3$. Our estimates from HI studies and the Sigma-D relation place Teleios as a type Ia SNR at a distance of either ~2.2 kpc of ~7.7 kpc. This indicates two possible scenarios, either a young (under 1000 yr) or an older SNR (over 10000 yr). With a corresponding diameter of 14/48 pc, our evolutionary studies place Teleios at the either early or late Sedov phase, depending on the distance estimate. However, our modelling also predicts X-ray emission, which we do not see in the present generation of eROSITA images. We also explored a type Iax explosion scenario that points to a much closer distance of <1 kpc and Teleios size of only ~3.3 pc, which would be similar to the only known type Iax remnant SN1181. Unfortunately, all examined scenarios have their challenges, and no definitive supernova (SN) origin type can be established at this stage. Teleios's symmetrical shape suggests expansion into a rarefied and isotropic ambient medium. The low radio surface brightness and the lack of pronounced polarisation can be explained by a high level of ambient rotation measure (RM), with the largest RM being observed at centre.

astro-ph.HE↗