RSS 1.0Refereed
Browse by
Recent Submissions
-
A Broadband X-Ray Investigation of Fast-spinning Intermediate Polar CTCV J2056–3014We report on XMM-Newton, NuSTAR, and NICER X-ray observations of CTCV J2056–3014, a cataclysmic variable (CV) with one of the fastest-spinning white dwarfs (WDs) at P = 29.6 s. While previously classified as an intermediate polar, CJ2056 also exhibits the properties of WZ Sge–type CVs, such as dwarf novae and superoutbursts. With XMM-Newton and NICER, we detected the spin period up to ∼2 keV with 7σ significance. We constrained its derivative to <inline-formula> <mml:math overflow=scroll><mml:mo stretchy=false>|</mml:mo><mml:mrow><mml:mover><mml:mi>P</mml:mi><mml:mo>̇</mml:mo></mml:mover></mml:mrow><mml:mo stretchy=false>|</mml:mo><mml:mo><</mml:mo><mml:mn>1.8</mml:mn><mml:mo>×</mml:mo><mml:msup><mml:mrow><mml:mn>10</mml:mn></mml:mrow><mml:mrow><mml:mo>‑</mml:mo><mml:mn>12</mml:mn></mml:mrow></mml:msup></mml:math> </inline-formula> s s<SUP>‑1</SUP> after correcting for binary orbital motion. The pulse profile is characterized by a single broad peak with ∼25% modulation. NuSTAR detected a fourfold increase in unabsorbed X-ray flux coincident with an optical flare, in 2022 November. The XMM-Newton and NICER X-ray spectra at 0.310 keV are best characterized by an absorbed, optically thin three-temperature thermal plasma model (kT = 0.3, 1.0, and 4.9 keV), while the NuSTAR spectra at 3–30 keV are best fit by a single-temperature thermal plasma model (kT = 8.4 keV), both with Fe abundance Z <SUB>Fe</SUB>/Z <SUB>⊙</SUB> = 0.3. CJ2056 exhibits similarities to other fast-spinning CVs, such as low plasma temperatures and no significant X-ray absorption at low energies. As the WD's magnetic field strength is unknown, we applied both nonmagnetic and magnetic CV spectral models (MKCFLOW and MCVSPEC) to determine the WD mass. The derived WD mass range (M = 0.7–1.0 M <SUB>⊙</SUB>) is above the centrifugal breakup mass limit of 0.56 M <SUB>⊙</SUB> and consistent with the mean WD mass of local CVs (M ≈ 0.8–0.9 M <SUB>⊙</SUB>).
-
X-Shooting ULLYSES: Massive stars at low metallicity: VIII. Stellar and wind parameters of newly revealed stripped stars in Be binariesOn the route toward merging neutron stars and stripped-envelope supernovae, binary population synthesis predicts a large number of post-interaction systems with massive stars that have been stripped of their outer layers. However, observations of such stars in the intermediate-mass regime below the Wolf-Rayet masses are rare. Using X-Shooting ULLYSES (XShootU) data, we have discovered three partially stripped star + Be/Oe binaries in the Magellanic Clouds. We analyzed the UV and optical spectra using the Potsdam Wolf-Rayet (PoWR) model atmosphere code by superimposing model spectra that correspond to each component. The estimated current masses of the partially stripped stars fall within the intermediate-mass range of ≈4 ‑ 8 M<SUB>⊙</SUB>. These objects are found to be over-luminous for their corresponding stellar masses, which aligns with the luminosities during core He-burning. Their accompanying Be/Oe secondaries are found to have much higher masses than their stripped primaries (mass ratio ≳2). The surfaces of all three partially stripped stars exhibit clear indications of significant nitrogen enrichment as well as a depletion of carbon and oxygen. Furthermore, one of our sample stars shows signs of substantial helium enrichment. Our study provides the first comprehensive determination of the wind parameters of partially stripped stars in the intermediate-mass range. The wind mass-loss rates of these stars are estimated to be on the order of 10<SUP>‑7</SUP> M<SUB>⊙</SUB> yr<SUP>‑1</SUP>, which is more than ten times higher than that of OB stars with the same luminosity. The current mass-loss recipes commonly employed in evolutionary models to characterize this phase are based on OB or WR mass-loss rates, and they significantly underestimate or overestimate the observed mass-loss rates of (partially) stripped stars by an order of magnitude. Binary evolution models suggest that the observed primaries had initial masses in the range of 12‑17 M<SUB>⊙</SUB>, and are potential candidates for stripped-envelope supernovae resulting in the formation of a neutron star. If these systems survive the explosion, they will likely evolve to become Be X-ray binaries and later double neutron stars.
-
Radio signatures of star-planet interactions, exoplanets and space weatherRadio detections of stellar systems provide a window onto stellar magnetic activity and the space weather conditions of extrasolar planets — information that is difficult to obtain at other wavelengths. The maturation of low-frequency radio instruments and the plethora of wide-field radio surveys have driven recent advances in observing auroral emissions from radio-bright low-mass stars and exoplanets. To guide us in putting these recent results in context, we introduce the foremost local analogues for the field: solar bursts and the aurorae found on Jupiter. We detail how radio bursts associated with stellar flares are foundational to the study of stellar coronae, and time-resolved radio dynamic spectra offer one of the best prospects for detecting and characterizing coronal mass ejections from other stars. We highlight the possibility of directly detecting coherent radio emission from exoplanetary magnetospheres, as well as early tentative results. We bridge this discussion with the field of brown dwarf radio emission — the larger and stronger magnetospheres of these stars are amenable to detailed study with current instruments. Bright, coherent radio emission is also predicted from magnetic interactions between stars and close-in planets. We discuss the underlying physics of these interactions and the implications of recent provisional detections for exoplanet characterization. We conclude with an overview of outstanding questions in the theory of stellar, star-planet interaction and exoplanet radio emission and the potential of future facilities to answer them.
-
Prospects for a survey of the galactic plane with the Cherenkov Telescope ArrayApproximately one hundred sources of very-high-energy (VHE) gamma rays are known in the Milky Way, detected with a combination of targeted observations and surveys. A survey of the entire Galactic Plane in the energy range from a few tens of GeV to a few hundred TeV has been proposed as a Key Science Project for the upcoming Cherenkov Telescope Array Observatory (CTAO). This article presents the status of the studies towards the Galactic Plane Survey (GPS). We build and make publicly available a sky model that combines data from recent observations of known gamma-ray emitters with state-of-the-art physically-driven models of synthetic populations of the three main classes of established Galactic VHE sources (pulsar wind nebulae, young and interacting supernova remnants, and compact binary systems), as well as of interstellar emission from cosmic-ray interactions in the Milky Way. We also perform an optimisation of the observation strategy (pointing pattern and scheduling) based on recent estimations of the instrument performance. We use the improved sky model and observation strategy to simulate GPS data corresponding to a total observation time of 1620 hours spread over ten years. Data are then analysed using the methods and software tools under development for real data. Under our model assumptions and for the realisation considered, we show that the GPS has the potential to increase the number of known Galactic VHE emitters by almost a factor of five. This corresponds to the detection of more than two hundred pulsar wind nebulae and a few tens of supernova remnants at average integral fluxes one order of magnitude lower than in the existing sample above 1 TeV, therefore opening the possibility to perform unprecedented population studies. The GPS also has the potential to provide new VHE detections of binary systems and pulsars, to confirm the existence of a hypothetical population of gamma-ray pulsars with an additional TeV emission component, and to detect bright sources capable of accelerating particles to PeV energies (PeVatrons). Furthermore, the GPS will constitute a pathfinder for deeper follow-up observations of these source classes. Finally, we show that we can extract from GPS data an estimate of the contribution to diffuse emission from unresolved sources, and that there are good prospects of detecting interstellar emission and statistically distinguishing different scenarios. Thus, a survey of the entire Galactic plane carried out from both hemispheres with CTAO will ensure a transformational advance in our knowledge of Galactic VHE source populations and interstellar emission.
-
Prospects for γ-ray observations of the Perseus galaxy cluster with the Cherenkov Telescope ArrayGalaxy clusters are expected to be both dark matter (DM) reservoirs and storage rooms for the cosmic-ray protons (CRp) that accumulate along the cluster's formation history. Accordingly, they are excellent targets to search for signals of DM annihilation and decay at γ-ray energies and are predicted to be sources of large-scale γ-ray emission due to hadronic interactions in the intracluster medium (ICM). In this paper, we estimate the sensitivity of the Cherenkov Telescope Array (CTA) to detect diffuse γ-ray emission from the Perseus galaxy cluster. We first perform a detailed spatial and spectral modelling of the expected signal for both the DM and the CRp components. For each case, we compute the expected CTA sensitivity accounting for the CTA instrument response functions. The CTA observing strategy of the Perseus cluster is also discussed. In the absence of a diffuse signal (non-detection), CTA should constrain the CRp to thermal energy ratio X <SUB>500</SUB> within the characteristic radius R <SUB>500</SUB> down to about X <SUB>500</SUB> < 3 × 10<SUP>-3</SUP>, for a spatial CRp distribution that follows the thermal gas and a CRp spectral index α<SUB>CRp</SUB> = 2.3. Under the optimistic assumption of a pure hadronic origin of the Perseus radio mini-halo and depending on the assumed magnetic field profile, CTA should measure α<SUB>CRp</SUB> down to about Δα<SUB>CRp</SUB> ≃ 0.1 and the CRp spatial distribution with 10% precision, respectively. Regarding DM, CTA should improve the current ground-based γ-ray DM limits from clusters observations on the velocity-averaged annihilation cross-section by a factor of up to ∼ 5, depending on the modelling of DM halo substructure. In the case of decay of DM particles, CTA will explore a new region of the parameter space, reaching models with τ <SUB>χ</SUB> > 10<SUP>27</SUP> s for DM masses above 1 TeV. These constraints will provide unprecedented sensitivity to the physics of both CRp acceleration and transport at cluster scale and to TeV DM particle models, especially in the decay scenario.
-
New Wolf-Rayet wind yields and nucleosynthesis of Helium starsStrong metallicity-dependent winds dominate the evolution of core He-burning, classical Wolf-Rayet (cWR) stars, which eject both H and He-fusion products such as $^{14}$N, $^{12}$C, $^{16}$O, $^{19}$F, $^{22}$Ne, and $^{23}$Na during their evolution. The chemical enrichment from cWRs can be significant. cWR stars are also key sources for neutron production relevant for the weak s-process. We calculate stellar models of cWRs at solar metallicity for a range of initial Helium star masses (12-50 $\rm M_{\odot }$), adopting recent hydrodynamical wind rates. Stellar wind yields are provided for the entire post-main sequence evolution until core O-exhaustion. While literature has previously considered cWRs as a viable source of the radioisotope $^{26}$Al, we confirm that negligible $^{26}$Al is ejected by cWRs since it has decayed to $^{26}$Mg or proton-captured to $^{27}$Al. However, in Paper I, we showed that very massive stars eject substantial quantities of $^{26}$Al, among other elements including N, Ne, and Na, already from the zero-age-main-sequence. Here, we examine the production of $^{19}$F and find that even with lower mass-loss rates than previous studies, our cWR models still eject substantial amounts of $^{19}$F. We provide central neutron densities (N$_{n}$) of a 30 $\rm M_{\odot }$ cWR compared with a 32 $\rm M_{\odot }$ post-VMS WR and confirm that during core He-burning, cWRs produce a significant number of neutrons for the weak s-process via the $^{22}$Ne($\alpha$,n)$^{25}$Mg reaction. Finally, we compare our cWR models with observed [Ne/He], [C/He], and [O/He] ratios of Galactic WC and WO stars.
-
Kilonova Seekers: the GOTO project for real-time citizen science in time-domain astrophysicsTime-domain astrophysics continues to grow rapidly, with the inception of new surveys drastically increasing data volumes. Democratized, distributed approaches to training sets for machine learning classifiers are crucial to make the most of this torrent of discovery - with citizen science approaches proving effective at meeting these requirements. In this paper, we describe the creation of and the initial results from the Kilonova Seekers citizen science project, built to find transient phenomena from the GOTO telescopes in near real-time. Kilonova Seekers launched in 2023 July and received over 600 000 classifications from approximately 2000 volunteers over the course of the LIGO-Virgo-KAGRA O4a observing run. During this time, the project has yielded 20 discoveries, generated a 'gold-standard' training set of 17 682 detections for augmenting deep-learned classifiers, and measured the performance and biases of Zooniverse volunteers on real-bogus classification. This project will continue throughout the lifetime of GOTO, pushing candidates at ever-greater cadence, and directly facilitate the next-generation classification algorithms currently in development.
-
The maximum black hole mass at solar metallicityWe analyse the current knowledge and uncertainties in detailed stellar evolution and wind modelling to evaluate the mass of the most massive stellar black hole (BH) at solar metallicity. Contrary to common expectations that it is the most massive stars that produce the most massive BHs, we find that the maximum M<SUB>BH</SUB><SUP>Max</SUP> ≃ 30 ± 10 M<SUB>⊙</SUB> is found in the canonical intermediate range between M<SUB>ZAMS</SUB> ≃ 30 and 50 M<SUB>⊙</SUB> instead. The prime reason for this seemingly counter-intuitive finding is that very massive stars (VMS) have increasingly high mass-loss rates that lead to substantial mass evaporation before they expire as stars and end as lighter BHs than their canonical O-star counterparts.
-
Dark matter line searches with the Cherenkov Telescope ArrayMonochromatic gamma-ray signals constitute a potential smoking gun signature for annihilating or decaying dark matter particles that could relatively easily be distinguished from astrophysical or instrumental backgrounds. We provide an updated assessment of the sensitivity of the Cherenkov Telescope Array (CTA) to such signals, based on observations of the Galactic centre region as well as of selected dwarf spheroidal galaxies. We find that current limits and detection prospects for dark matter masses above 300 GeV will be significantly improved, by up to an order of magnitude in the multi-TeV range. This demonstrates that CTA will set a new standard for gamma-ray astronomy also in this respect, as the world's largest and most sensitive high-energy gamma-ray observatory, in particular due to its exquisite energy resolution at TeV energies and the adopted observational strategy focussing on regions with large dark matter densities. Throughout our analysis, we use up-to-date instrument response functions, and we thoroughly model the effect of instrumental systematic uncertainties in our statistical treatment. We further present results for other potential signatures with sharp spectral features, e.g. box-shaped spectra, that would likewise very clearly point to a particle dark matter origin.
-
Predicting the heaviest black holes below the pair instability gapTraditionally, the pair instability (PI) mass gap is located between 50 and 130 M<SUB>⊙</SUB>, with stellar mass black holes (BHs) expected to 'pile up' towards the lower PI edge. However, this lower PI boundary is based on the assumption that the star has already lost its hydrogen (H) envelope. With the announcement of an 'impossibly' heavy BH of 85 M<SUB>⊙</SUB> as part of GW 190521 located inside the traditional PI gap, we realized that blue supergiant (BSG) progenitors with small cores but large hydrogen envelopes at low metallicity (Z) could directly collapse to heavier BHs than had hitherto been assumed. The question of whether a single star can produce such a heavy BH is important, independent of gravitational wave events. Here, we systematically investigate the masses of stars inside the traditional PI gap by way of a grid of 336 detailed MESA stellar evolution models calculated across a wide parameter space, varying stellar mass, overshooting, rotation, semiconvection, and Z. We evolve low Z stars in the range 10<SUP>-3</SUP> < Z/Z<SUB>⊙</SUB> < Z<SUB>SMC</SUB>, making no prior assumption regarding the mass of an envelope, but instead employing a wind mass-loss recipe to calculate it. We compute critical carbon-oxygen and helium core masses to determine our lower limit to PI physics, and we provide two equations for M<SUB>core</SUB> and M<SUB>final</SUB> that can also be of use for binary population synthesis. Assuming the H envelope falls into the BH, we confirm the maximum BH mass below PI is M<SUB>BH</SUB> ≃ 93.3 M<SUB>⊙</SUB>. Our grid allows us to populate the traditional PI gap, and we conclude that the distribution of BHs above the traditional boundary is not solely due to the shape of the initial mass function, but also to the same stellar interior physics (i.e. mixing) that which sets the BH maximum.
-
LISA Galactic Binaries with Astrometry from Gaia DR3Galactic compact binaries with orbital periods shorter than a few hours emit detectable gravitational waves (GWs) at low frequencies. Their GW signals can be detected with the future Laser Interferometer Space Antenna (LISA). Crucially, they may be useful in the early months of the mission operation in helping to validate LISA's performance in comparison to prelaunch expectations. We present an updated list of 55 candidate LISA-detectable binaries with measured properties, for which we derive distances based on Gaia Data Release 3 astrometry. Based on the known properties from electromagnetic observations, we predict the LISA detectability after 1, 3, 6, and 48 months using Bayesian analysis methods. We distinguish between verification and detectable binaries as being detectable after 3 and 48 months, respectively. We find 18 verification binaries and 22 detectable sources, which triples the number of known LISA binaries over the last few years. These include detached double white dwarfs, AM CVn binaries, one ultracompact X-ray binary, and two hot subdwarf binaries. We find that across this sample the GW amplitude is expected to be measured to ≈10% on average, while the inclination is expected to be determined with ≈15° precision. For detectable binaries, these average errors increase to ≈50% and ≈40°, respectively.
-
Predicting the Heaviest Black Holes below the Pair Instability GapTraditionally, the pair instability (PI) mass gap is located between 50 and 130 M<SUB>⊙</SUB>, with stellar mass black holes (BHs) expected to pile up towards the lower PI edge. However, this lower PI boundary is based on the assumption that the star has already lost its hydrogen (H) envelope. With the announcement of an impossibly heavy BH of 85 M<SUB>⊙</SUB> as part of GW 190521 located inside the traditional PI gap, we realised that blue supergiant (BSG) progenitors with small cores but large Hydrogen envelopes at low metallicity (Z) could directly collapse to heavier BHs than had hitherto been assumed. The question of whether a single star can produce such a heavy BH is important, independent of gravitational wave events. Here, we systematically investigate the masses of stars inside the traditional PI gap by way of a grid of 336 detailed MESA stellar evolution models calculated across a wide parameter space, varying stellar mass, overshooting, rotation, semi-convection, and Z. We evolve low Z stars in the range 10<SUP>-3</SUP> < Z/Z<SUB>⊙</SUB> < Z<SUB>SMC</SUB>, making no prior assumption regarding the mass of an envelope, but instead employing a wind mass loss recipe to calculate it. We compute critical Carbon-Oxygen and Helium core masses to determine our lower limit to PI physics, and we provide two equations for M<SUB>core</SUB> and M<SUB>final</SUB> that can also be of use for binary population synthesis. Assuming the H envelope falls into the BH, we confirm the maximum BH mass below PI is M<SUB>BH</SUB> ≃ 93.3 M<SUB>⊙</SUB>. Our grid allows us to populate the traditional PI gap, and we conclude that the distribution of BHs above the gap is not solely due to the shape of the initial mass function (IMF), but also to the same stellar interior physics (i.e. mixing) that which sets the BH maximum.
-
Exceptional outburst of the blazar CTA 102 in 2012: the GASP-WEBT campaign and its extensionAfter several years of quiescence, the blazar CTA 102 underwent an exceptional outburst in 2012 September-October. The flare was tracked from γ-ray to near-infrared (NIR) frequencies, including Fermi and Swift data as well as photometric and polarimetric data from several observatories. An intensive Glast-Agile support programme of the Whole Earth Blazar Telescope (GASP-WEBT) collaboration campaign in optical and NIR bands, with an addition of previously unpublished archival data and extension through fall 2015, allows comparison of this outburst with the previous activity period of this blazar in 2004-2005. We find remarkable similarity between the optical and γ-ray behaviour of CTA 102 during the outburst, with a time lag between the two light curves of ≈1 h, indicative of cospatiality of the optical and γ-ray emission regions. The relation between the γ-ray and optical fluxes is consistent with the synchrotron self-Compton (SSC) mechanism, with a quadratic dependence of the SSC γ-ray flux on the synchrotron optical flux evident in the post-outburst stage. However, the γ-ray/optical relationship is linear during the outburst; we attribute this to changes in the Doppler factor. A strong harder-when-brighter spectral dependence is seen both the in γ-ray and optical non-thermal emission. This hardening can be explained by convexity of the UV-NIR spectrum that moves to higher frequencies owing to an increased Doppler shift as the viewing angle decreases during the outburst stage. The overall pattern of Stokes parameter variations agrees with a model of a radiating blob or shock wave that moves along a helical path down the jet.
-
Natal molecular cloud of SNR Kes 41. Complete characterisationUsing high-resolution data of the <SUP>12</SUP>CO and <SUP>13</SUP>CO (J = 1-0) line emission from the Mopra Southern Galactic Plane CO Survey in conjunction with neutral hydrogen observations from the Southern Galactic Plane Survey (SGPS) and mid-infrared Spitzer data, we have explored the large-scale environment of the supernova remnant Kes 41. On the basis of these data, we identified for the first time the parent cloud of Kes 41 in its whole extension and surveyed the HII regions, masers, and the population of massive young stellar objects in the cloud. The whole unveiled giant cloud, located at the kinematic distance of 12.0 ± 3.6 kpc, whose average total mass and size are 10-30 × 10<SUP>5</SUP> M<SUB>⊙</SUB> and 26', also shines in γ-rays, as revealed by the Large Area Telescope on board the Fermi satellite. We determined a high average proton density 500-1000 cm<SUP>-3</SUP> in the large molecular complex, of which protons from the neutral atomic and ionised gases comprise only 15%.
-
Unidentified γ-ray emission towards the SNR Kes 41 revisitedKes 41 is one of the Galactic supernova remnants (SNRs) that are proposed to be physically linked to γ-ray emission at GeV energies. The nature of the γ-ray photons has been explained, but inconclusively, as hadronic collisions of particles accelerated at the SNR blast wave with target protons in an adjacent molecular clump. We performed an analysis of Fermi-Large Area Telescope (LAT) data of about nine years to assess the origin of the γ-ray emission. To investigate this matter, we also used spectral modelling constraints from the physical properties of the interstellar medium towards the γ-ray emitting region along with a revised radio continuum spectrum of Kes 41 (α = -0.54 ± 0.10, S ∝ ν<SUP>α</SUP>). We demonstrate that the γ-ray fluxes in the GeV range can be explained through bremsstrahlung emission from electrons interacting with the surrounding medium. We also considered a model in which the emission is produced by pion decay after hadronic collisions, and confirm that this mechanism cannot be excluded.
-
A Study of the Interstellar Medium Towards the Unidentified Dark TeV γ-Ray Sources HESS J1614-518 and HESS J1616-508HESS J1614-518 and HESS J1616-508 are two tera-electron volt γ-ray sources that are not firmly associated with any known counterparts at other wavelengths. We investigate the distribution of interstellar medium towards the tera-electron volt γ-ray sources using results from a 7-mm-wavelength Mopra study, the Mopra Southern Galactic Plane CO Survey, the Millimetre Astronomer's Legacy Team-45 GHz survey and [C i] data from the HEAT telescope. Data in the CO(1-0) transition lines reveal diffuse gas overlapping the two tera-electron volt sources at several velocities along the line of sight, while observations in the CS(1-0) transition line reveal several interesting dense gas features. To account for the diffuse atomic gas, archival H i data was taken from the Southern Galactic Plane Survey. The observations reveal gas components with masses 10<SUP>3</SUP> to 10<SUP>5</SUP> M<SUB>⊙</SUB> and with densities 10<SUP>2</SUP> to 10<SUP>3</SUP> cm<SUP>-3</SUP> overlapping the two tera-electron volt sources. Several origin scenarios potentially associated with the tera-electron volt γ-ray sources are discussed in light of the distribution of the local interstellar medium. We find no strong convincing evidence linking any counterpart with HESS J1614-518 or HESS J1616-508.
-
Ammonia excitation imaging of shocked gas towards the W28 gamma-ray source HESS J1801-233We present 12 mm Mopra observations of the dense (>10<SUP>3</SUP> cm<SUP>-3</SUP>) molecular gas towards the north-east of the W28 supernova remnant (SNR). This cloud is spatially well matched to the TeV gamma-ray source HESS J1801-233 and is known to be an SNR-molecular cloud interaction region. Shock-disruption is evident from broad NH<SUB>3</SUB> (1,1) spectral linewidths in regions towards the W28 SNR, while strong detections of spatially extended NH<SUB>3</SUB> (3,3), NH<SUB>3</SUB>(4,4) and NH<SUB>3</SUB>(6,6) inversion emission towards the cloud strengthen the case for the existence of high temperatures within the cloud. Velocity dispersion measurements and NH<SUB>3</SUB>(n,n)/(1,1) ratio maps, where n = 2, 3, 4 and 6, indicate that the source of disruption is from the side of the cloud nearest to the W28 SNR, suggesting that it is the source of cloud-disruption. Towards part of the cloud, the ratio of ortho to para-NH<SUB>3</SUB> is observed to exceed 2, suggesting gas-phase NH<SUB>3</SUB> enrichment due to NH<SUB>3</SUB> liberation from dust-grain mantles. The measured NH<SUB>3</SUB> abundance with respect to H<SUB>2</SUB> is ∼(1.2 ± 0.5) × 10<SUP>-9</SUP>, which is not high, as might be expected for a hot, dense molecular cloud enriched by sublimated grain-surface molecules. The results are suggestive of NH<SUB>3</SUB> sublimation and destruction in this molecular cloud, which is likely to be interacting with the W28 SNR shock.
-
Detection of a 23.6 min periodic modulation in the optical counterpart of 3XMMJ051034.6-670353We present high speed optical photometric observations made using the NTT and ULTRACAM of the optical counterpart of 3XMMJ051034.6-670353, which was recently identified as an X-ray source showing a modulation on a period of 23.6 min. Although the optical counterpart is faint (g = 21.4), we find that the u'g'r' light curves show a periodic modulation on a period which is consistent with the X-ray period. We also obtained three low resolution spectra of 3XMMJ051034.6-670353 using the Gemini South Telescope and GMOS. There is no evidence for strong emission lines in the optical spectrum of 3XMMJ051034.6-670353. We compare and contrast the optical and X-ray observations of 3XMMJ051034.6-670353 with the ultra compact binaries HM Cnc and V407 Vul. We find we can identify a distribution of binary masses in which stable direct impact accretion can occur.
-
Swift observations of the 2015 outburst of AG Peg - from slow nova to classical symbiotic outburstSymbiotic stars often contain white dwarfs with quasi-steady shell burning on their surfaces. However, in most symbiotics, the origin of this burning is unclear. In symbiotic slow novae, however, it is linked to a past thermonuclear runaway. In 2015 June, the symbiotic slow nova AG Peg was seen in only its second optical outburst since 1850. This recent outburst was of much shorter duration and lower amplitude than the earlier eruption, and it contained multiple peaks - like outbursts in classical symbiotic stars such as Z And. We report Swift X-ray and UV observations of AG Peg made between 2015 June and 2016 January. The X-ray flux was markedly variable on a time-scale of days, particularly during four days near optical maximum, when the X-rays became bright and soft. This strong X-ray variability continued for another month, after which the X-rays hardened as the optical flux declined. The UV flux was high throughout the outburst, consistent with quasi-steady shell burning on the white dwarf. Given that accretion discs around white dwarfs with shell burning do not generally produce detectable X-rays (due to Compton-cooling of the boundary layer), the X-rays probably originated via shocks in the ejecta. As the X-ray photoelectric absorption did not vary significantly, the X-ray variability may directly link to the properties of the shocked material. AG Peg's transition from a slow symbiotic nova (which drove the 1850 outburst) to a classical symbiotic star suggests that shell burning in at least some symbiotic stars is residual burning from prior novae.
-
V729 Sgr: a long period dwarf nova showing negative superhumps during quiescenceWe report K2 observations of the eclipsing cataclysmic variable V729 Sgr which covered nearly 80 d in duration. We find five short outbursts and two long outbursts, one of which shows a clear plateau phase in the rise to maximum brightness. The mean time between successive short outbursts is ∼10 d while the time between the two long outbursts is ∼38 d. The frequency of these outbursts is unprecedented for a cataclysmic variable (CV) above the orbital period gap. We find evidence that the mid-point of the eclipse occurs systematically earlier in outburst than in quiescence. During five of the six quiescent epochs we find evidence for a second photometric period which is roughly 5 per cent shorter than the 4.16 h orbital period which we attribute to negative superhumps. V729 Sgr is therefore one of the longest period CVs to show negative superhumps during quiescence.
