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ABSTRACT We present the deepest and highest-resolution radio continuum imaging of the Galactic globular cluster Terzan 5, one of the most crowded locations in the radio sky. In these new 2–4 GHz Karl G. Jansky Very Large Array images, we detect 38 of the 49 confirmed pulsars, including extensive multi-frequency eclipse mapping of the luminous redback Ter5A. Nonetheless, there is still a large amount of diffuse residual flux from pulsars that are fainter than our 2.5 GHz continuum detection limit of $$\sim 11\, \mu$$Jy. Using a range of approaches including image-based simulations, we model the fluxes of the detected pulsars together with the residual flux. We find a minimum total population of $$N\sim 250$$ detectable pulsars in Terzan 5 and perhaps substantially more, though the luminosity function remains very uncertain. Consideration of the $$\gamma$$-ray properties of the cluster, though also not unambiguous to interpret, leads to consistent conclusions. These pulsar population estimates are larger than inferred from previous work and highlight Terzan 5 as a keystone target for next-generation radio facilities. 

ABSTRACT The fate of massive stars above 20‌ $$\rm M$$_{\normalsize \odot }$$ remains uncertain. Debate persists about whether they die as supernovae (SNe), or if they collapse directly into black holes (BHs) with little or no optical outburst – so-called failed supernovae. The source M31-2014-DS1 experienced an optical outburst in 2014 and has remained faint at visual wavelengths since then. Due to its persistent faintness, it has been proposed as a failed SN candidate. We present new observations of this candidate obtained using the James Webb Space Telescope (JWST), the Submillimeter Array, and Chandra. The JWST observations demonstrate that a luminous mid-infrared source persists at the same location a decade after the star faded at visual wavelengths. We model its current spectral energy distribution as a dust-enshrouded star. No X-ray emission is detected, disfavouring the hypothesis that the late-time luminosity is powered by accretion on to a BH. We find that the remaining source is highly obscured by an asymmetric distribution of circumstellar dust, making it difficult to quantify its physical properties using spherically symmetric radiative transfer codes. The dust geometry requires that the inferred bolometric luminosity is only a lower limit, as a significant fraction of the central source’s radiation may escape without being reprocessed by dust. We discuss the implications of these findings in the context of failed SN models and consider the potential overlap with signatures expected from a stellar merger, which also seems to provide a plausible explanation of this source. 

ABSTRACT While nova eruptions produce some of the most common and dramatic dust formation episodes among astrophysical transients, the demographics of dust-forming novae remain poorly understood. Here, we present a statistical study of dust formation in 40 novae with high-quality optical/IR light curves, quantitatively distinguishing dust-forming from non-dust-forming novae while exploring the properties of the dust events. We find that 50–70 per cent of novae produce dust, significantly higher than previous estimates. Dust-forming novae can be separated from those that do not show dust formation by using the largest redward ($V-K$) colour change from peak visible brightness; ($V-J$) or ($V-H$) offer useful but less sensitive constraints. This makes optical+IR photometry a powerful tool to quantify dust formation in novae. We find that novae detected in GeV $$\gamma$$-rays by Fermi-LAT appear to form dust more often than novae not detected by Fermi, implying a possible connection between $$\gamma$$-ray-producing shocks and dust production. We also find that novae that evolve very quickly ($$t_2 < 10$$ d) are much less likely to form dust, in agreement with previous findings. We confirm a correlation between $$t_2$$ and the time of the onset of dust formation (which occurs $$\sim$$1 week–3 months after maximum light), but conclude that it is primarily an observational artefact driven by dust formation determining when a nova drops 2 mag below peak. The significant fraction of novae that form dust make them ideal laboratories in our Galactic backyard to tackle the puzzle of dust formation around explosive transients. 

We report the discovery of two new Galactic accreting compact objects consistent with the respective positions of the unassociated Fermi-LATgamma-ray sources 4FGL J0639.1-8009 and 4FGL J1824.2+1231. A combination of new and archival X-ray data from Chandra, XMM-Newton, Swift/XRT, and eROSITA reveals a variable X-ray source in each gamma-ray error ellipse. Both candidate counterparts show power-law spectra with photon indices Gamma ∼ 1.7 to 1.9. Optical follow-up photometry and spectroscopy show rapid high-amplitude variability unrelated to orbital motion and persistent accretion disk spectra for both objects. We demonstrate that the properties of these X-ray/optical sources are at odds with the known phenomenology of accreting white dwarfs, but are consistent with the observed properties of the subluminous disk state of transitional millisecond pulsars. This brings the census of confirmed or candidate transitional millisecond pulsars in the Galactic field to nine. We show this potentially represents <~ 10% of the total population of transitional millisecond pulsars within 8 kpc. 

The explosion of a white dwarf (WD) in a close binary can launch a surviving runaway star at velocities of . Such runaways provide a direct probe of thermonuclear supernovae (SNe) in double-degenerate binaries. Several candidate runaways are known, but their evolutionary states and the demographics of the broader population are uncertain. To enable robust population inference, we carry out a systematic survey for hypervelocity runaways with a simple selection function, selecting candidates based on large Gaia-inferred tangential velocities and blue colors. We classify 100% of the resulting 92 candidates using a combination of spectroscopic follow-up and archival data. The search yields ten suspected D stars and three LP 40-365 stars. Three D stars are new discoveries, including two hot ( K) objects and one cool ( K) object. We forward-model our survey under several proposed D star evolutionary models, coupling each to a Galactic model and the survey selection function. No single model reproduces the observed diversity of D stars, which likely reflects a range of remnant masses, ages, and heating mechanisms. Models in which runaway companions are heated by SN shocks alone are too faint and short-lived to explain most of the observed sample, while fully reheated models are too luminous and long-lived. Models with intermediate heating, as occurs in some simulations of violent mergers and partially disrupted remnants, best match the observed magnitude, distance, and kinematic-age distributions. The inferred D star birth rate is model dependent, but the models that best match the observed population require rates of only a few percent of the Galactic SN Ia rate, perhaps implying that most SNe Ia result from WD binaries in which both components explode. 

Abstract A subset of galaxies have dense nuclei, and when these galaxies are accreted and tidally stripped, the nuclei can masquerade as globular clusters in the halos of large galaxies. If these nuclei contain massive central black holes, some may accrete gas and become observable as active galactic nuclei. Previous studies have found that candidate stripped nuclei rarely host luminous X-ray sources, but these studies were typically restricted to both the most massive candidate nuclei and the most luminous X-ray sources. Here we use new and archival Chandra and XMM-Newton data to search for X-ray emission in a near-complete sample of massive globular clusters and candidate stripped nuclei in the nearest accessible elliptical galaxy, NGC 5128. This sample has the unique advantage that the candidate stripped nuclei are identified dynamically via elevated mass-to-light ratios (M/L_V). Our central result is that 5/22 ( $23_{-6}^{+11}$ %) of the candidate stripped nuclei have X-ray sources down to a typical limit ofL_X∼ 5 × 10³⁶erg s⁻¹, a fraction lower than or comparable to that among massive clusters with normalM/L_V(16/41; $39_{-7}^{+8}$ %). Hence we confirm and extend the result that nearly all X-ray sources in stripped nuclei are likely to be X-ray binaries rather than active galactic nuclei. If the candidate stripped nuclei have black holes of typical masses ∼2 × 10⁵M_⊙needed to explain their elevatedM/L_V, then they have typical Eddington ratios of ≲ 2 × 10⁻⁶. This suggests that it will be challenging to conduct an accretion census of wandering black holes around even nearby galaxies. 

Abstract We present a systematic study of the BVRI colours of novae over the course of their eruptions. Where possible, interstellar reddening was measured using the equivalent widths of Diffuse Interstellar Bands (DIBs). Some novae lack spectra with sufficient resolution and signal-to-noise ratios; therefore, we supplement as necessary with 3D and 2D dust maps. Utilising only novae with DIB- or 3D-map-based E(B − V), we find an average intrinsic (B − V)0 colour of novae at V-band light curve peak of 0.20 with a standard deviation of 0.31, based on 25 novae. When the light curve has declined by 2 magnitudes (t2), we find an average (B − V)0 = −0.03 with a standard deviation of 0.19. These average colours are consistent with previous findings, although the spreads are larger than previously found due to more accurate reddening estimates. We also examined the intrinsic (R − I)0 and (V − R)0 colours across our sample. These colours behave similarly to (B − V)0, except that the (V − R)0 colour gets redder after peak, likely due to the contributions of emission line flux. We searched for correlations between nova colours and t2, peak V-band absolute magnitude, and GeV γ-ray luminosity, but find no statistically significant correlations. Nova colours can therefore be used as standard “crayons” to estimate interstellar reddening from photometry alone, with 0.2–0.3 mag uncertainty. We present a novel Bayesian strategy for estimating distances to Galactic novae based on these E(B − V) measurements, independent of assumptions about luminosity, built using 3D dust maps and a stellar mass model of the Milky Way. 

Abstract Metals in the diffuse, ionized gas at the boundary between the Milky Way’s interstellar medium (ISM) and circumgalactic medium, known as the disk–halo interface (DHI), are valuable tracers of the feedback processes that drive the Galactic fountain. However, metallicity measurements in this region are challenging due to obscuration by the Milky Way ISM and uncertain ionization corrections that affect the total hydrogen column density. In this work, we constrain ionization corrections to neutral hydrogen column densities using precisely measured electron column densities from the dispersion measures of pulsars that lie in the same globular clusters as UV-bright targets with high-resolution absorption spectroscopy. We address the blending of absorption lines with the ISM by jointly fitting Voigt profiles to all absorption components. We present our metallicity estimates for the DHI of the Milky Way based on detailed photoionization modeling of the absorption from ionized metal lines and ionization-corrected total hydrogen columns. Generally, the gas clouds show a large scatter in metallicity, ranging between 0.04 and 3.2Z_⊙, implying that the DHI consists of a mixture of gaseous structures having multiple origins. We estimate the inflow and outflow timescales of the DHI ionized clouds to be 6–35 Myr. We report the detection of an infalling cloud with supersolar metallicity that suggests a Galactic fountain mechanism, whereas at least one low-metallicity outflowing cloud (Z< 0.1Z_⊙) poses a challenge for Galactic fountain and feedback models. 

Abstract We present new Hubble Space Telescope (HST) imaging of three recently discovered star-forming dwarf galaxies beyond the Local Group: Pavo, Corvus A, and Kamino. The discovery of Kamino is reported here for the first time. They rank among the most isolated faint dwarf galaxies known; hence they provide unique opportunities to study galaxy evolution at the smallest scales, free from the environmental effects of more massive galaxies. Our HST data reach ∼2–4 magnitudes below the tip of the red giant branch (TRGB) for each dwarf, allowing us to measure their distances, structural properties, and recent star formation histories (SFHs). All three galaxies contain a complex stellar population of young and old stars, and are typical of field galaxies in this mass regime (M_V = −10.62 ± 0.08 and $D=2.16_{-0.07}^{+0.08}$ Mpc for Pavo,M_V = −10.91 ± 0.10 andD = 3.34 ± 0.11 Mpc for Corvus A, andM_V = −12.02 ± 0.12 and $D=6.50_{-0.11}^{+0.15}$ Mpc for Kamino). Our HST-derived SFHs reveal differences among the three dwarfs: Pavo and Kamino show relatively steady, continuous star formation, while Corvus A formed ∼60% of its stellar mass by 10 Gyr ago. These results align with theoretical predictions of diverse evolutionary pathways for isolated low-mass galaxies. 

ABSTRACT Classical novae in the Milky Way have now been well-established as high-energy GeV $$\gamma$$-ray sources. In novae with main-sequence companions, this emission is believed to result from shocks internal to the nova ejecta, as a later fast wind collides with an earlier slow outflow. To test this model and constrain the $$\gamma$$-ray production mechanism, we present a systematic study of a sample of recent Galactic novae, comparing their $$\gamma$$-ray properties ($$\gamma$$-ray luminosity and duration) with their outflow velocities, peak V-band magnitudes, and the decline times of their optical light curves ($$t_2$$). We uniformly estimate distances in a luminosity-independent manner, using spectroscopic reddening estimates combined with three-dimensional Galactic dust maps. Across our sample, $$\gamma$$-ray luminosities ($>$100 MeV) vary by three orders of magnitude, spanning $$10^{34}{{-}}10^{37}$$ erg s$$^{-1}$$. Novae with larger velocity of the fast outflow (or larger differential between the fast and slow outflow) have larger $$\gamma$$-ray luminosities, but are detectable for a shorter duration. The optical and $$\gamma$$-ray fluxes are correlated, consistent with substantial thermal emission in the optical from shock-heated gas. Across six novae with $$\gamma$$-ray and infrared light curves, evidence for dust formation appears soon after the end of the detected $$\gamma$$-ray emission. Dusty and non-dusty novae appear to have similar $$\gamma$$-ray luminosities, though novae that have more material processed by the shocks may be more likely to form dust. We find that the properties of the $$\gamma$$-ray emission in novae depend heavily on the ejecta properties, and are consistent with expectations for internal shocks.