Recent Submissions

  • Predicting the heaviest black holes below the pair instability gap

    Armagh Observatory and Planetarium (AOP), Armagh, College Hill, BT61 9DB, UK; School of Maths and Physics, Queen's University Belfast, Northern Ireland, University Road, BT7 1NN, UK; Armagh Observatory and Planetarium (AOP), Armagh, College Hill, BT61 9DB, UK; Winch, Ethan R. J.; Vink, Jorick S.; Higgins, Erin R.; Sabhahitf, Gautham N. (Monthly Notices of the Royal Astronomical Society, 2024-04-01)
    Traditionally, 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> &lt; Z/Z<SUB>⊙</SUB> &lt; 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 DR3

    Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany; Department of Physics and Astronomy, Texas Tech University, P.O. Box 41051, Lubbock, TX 79409, USA; Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85741 Garching, Germany; Institute for Gravitational Wave Astronomy, School of Physics and Astronomy, University of Birmingham, Birmingham, B15 2TT, UK; NASA Marshall Space Flight Center, Huntsville, AL 35811, USA; Max Planck Institute for Gravitational Physics (Albert Einstein Institute), Callinstrasse 38, 30167 Hannover, Germany; Leibniz Universität Hannover, Institut für Gravitationsphysik, Callinstrasse 38, 30167 Hannover, Germany; Université de Paris, CNRS, Astroparticule et Cosmologie, 75013 Paris, France; IRFU, CEA, Université Paris-Saclay, F-91191, Gif-sur-Yvette, France; Department of Astrophysics/IMAPP, Radboud University, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands; South African Astronomical Observatory, P.O. Box 9, Observatory, 7935, Cape Town, South Africa; Department of Astronomy &amp; Inter-University Institute for Data Intensive Astronomy, University of Cape Town, Private Bag X3, 7701 Rondebosch, South Africa; Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK; Université de Paris, CNRS, Astroparticule et Cosmologie, 75013 Paris, France; Department of Astrophysics/IMAPP, Radboud University, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands; SRON, Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands; Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium; European Space Agency, European Space Astronomy Centre, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain; et al. (The Astrophysical Journal, 2024-03-01)
    Galactic 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 Gap

    Armagh Observatory and Planetarium (AOP), Armagh, College Hill, BT61 9DB; School of Maths and Physics, Queen's University Belfast, Northern Ireland, University Road, BT7 1NN; Armagh Observatory and Planetarium (AOP), Armagh, College Hill, BT61 9DB; Winch, Ethan R. J.; Vink, Jorick S.; Higgins, Erin R.; Sabhahit, Gautham N. (Monthly Notices of the Royal Astronomical Society, 2024-02-01)
    Traditionally, 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> &lt; Z/Z<SUB>⊙</SUB> &lt; 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 extension

    Astronomical Institute, St.-Petersburg State University, 198504 St.-Petersburg, Russia; Pulkovo Observatory, 196140 St.-Petersburg, Russia; INAF, Osservatorio Astrofisico di Torino, via Osservatorio 20, I-10025 Pino Torinese, Italy; Astronomical Institute, St.-Petersburg State University, 198504 St.-Petersburg, Russia; Institute for Astrophysical Research, Boston University, Boston, MA, 22015 USA; Institute for Astrophysical Research, Boston University, Boston, MA, 22015 USA; Instituto de Astrofisíca de Andalucía, CSIC, E-18080 Granada, Spain; Steward Observatory, University of Arizona, Tucson, AZ 85721, USA; Instituto de Astrofisica de Canarias (IAC), La Laguna, E-38200 Tenerife, Spain; Departamento de Astrofisica, Universidad de La Laguna, La Laguna, Tenerife, Spain; Pulkovo Observatory, 196140 St.-Petersburg, Russia; Institute of Astronomy, Bulgarian Academy of Sciences, BG-1784 Sofia, Bulgaria; Astronomical Institute, St.-Petersburg State University, 198504 St.-Petersburg, Russia; Department of Physics and Institute for Plasma Physics, University of Crete, GR-71003 Heraklion, Greece; Foundation for Research and Technology - Hellas, IESL, Voutes, GR-7110 Heraklion, Greece; et al. (Monthly Notices of the Royal Astronomical Society, 2016-09-01)
    After 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 characterisation

    CONICET-Universidad de Buenos Aires, Instituto de Astronomía y Física del Espacio (IAFE), Buenos Aires, Argentina ; Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires, Argentina; CONICET-Universidad de Buenos Aires, Instituto de Astronomía y Física del Espacio (IAFE), Buenos Aires, Argentina; Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires, Argentina; School of Physics, University of New South Wales, Sydney, NSW 2052, Australia; Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, UK; School of Physics, University of New South Wales, Sydney, NSW 2052, Australia; School of Computing Enginnering and Mathematics, Western Sydney University, Locked Bay 1797, Penrith, NSW 2751, Australia; School of Physics, University of New South Wales, Sydney, NSW 2052, Australia; Supan, L.; Castelletti, G.; Supanitsky, A. D.; Burton, M. G.; Wong, G. F.; et al. (Astronomy and Astrophysics, 2018-11-01)
    Using 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 revisited

    CONICET - Universidad de Buenos Aires, Instituto de Astronomía y Física del Espacio (IAFE), Buenos Aires, Argentina ; Universidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales, Buenos Aires, Argentina; CONICET - Universidad de Buenos Aires, Instituto de Astronomía y Física del Espacio (IAFE), Buenos Aires, Argentina; Universidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales, Buenos Aires, Argentina; School of Physics, University of New South Wales, Sydney, NSW, 2052, Australia; Armagh Observatory and Planetarium, College Hill, Armagh, BT61 9DG, UK; Supan, L.; Castelletti, G.; Supanitsky, A. D.; Burton, M. G. (Astronomy and Astrophysics, 2018-11-01)
    Kes 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 J1616-508

    School of Physical Sciences, University of Adelaide, Adelaide, SA 5005, Australia; School of Physics, University of New South Wales, Sydney, NSW 2052, Australia; School of Physics, University of New South Wales, Sydney, NSW 2052, Australia; Armagh Observatory and Planetarium, College Hill, Armagh, BT61 9DG, Northern Ireland, UK; Department of Physics, University of Nagoya, Furo-cho, Chikusa-ku, Nagoya, 464-8601, Japan; Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia; International Centre for Radio Astronomy Research, Curtin University, Bentley, WA 6845, Australia; Lau, J. C.; Rowell, G.; Voisin, F.; Braiding, C.; et al. (Publications of the Astronomical Society of Australia, 2017-12-01)
    HESS 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 J1801-233

    School of Physics, University of New South Wales, Sydney 2052, Australia; School of Physical Sciences, Adelaide University, Adelaide 5005, Australia; School of Physics, University of New South Wales, Sydney 2052, Australia; Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, Northern Ireland, UK; International Centre for Radio Astronomy Research, Curtin University, GPO Box U1987, Perth, Australia; Department of Astrophysics, Nagoya University, Furocho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan; National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan; Maxted, Nigel I.; de Wilt, Phoebe; Rowell, Gavin P.; Nicholas, Brent P.; et al. (Monthly Notices of the Royal Astronomical Society, 2016-10-01)
    We present 12 mm Mopra observations of the dense (&gt;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 3XMMJ051034.6-670353

    Armagh Observatory and Planetarium, College Hill, Armagh, BT61 9DG, UK; Department of Physics, University of Warwick, Coventry, CV4 7AL, UK; Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA, 91125, USA; Department of Physics &amp; Astronomy, University of Sheffield, Sheffield, S3 7RH, UK; Instituto de Astrofísica de Canarias, 38205, La Laguna, Tenerife, Spain; Inter-University Institute for Data Intensive Astronomy, Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch, 7701, South Africa; Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL, Nijmegen, Netherlands; Ramsay, G.; Marsh, T. R.; Kupfer, T.; Dhillon, V. S.; et al. (Astronomy and Astrophysics, 2018-09-01)
    We 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 outburst

    Armagh Observatory, College Hill, Armagh BT61 9DG, UK; Columbia Astrophysics Lab, 550 W120th St., 1027 Pupin Hall, MC 5247 Columbia University, NY 10027, USA; Instituto de Astronomía y Física del Espacio (IAFE, CONICET-UBA), Av. Inte. Güiraldes 2620, C1428ZAA, Buenos Aires, Argentina; Instituto de Ciencias Astronómicas de la Tierra y el Espacio (ICATE-UNSJ), Av. Espanã (sur) 1512, 5400 San Juan, Argentina; Ramsay, Gavin; Sokoloski, J. L.; Luna, G. J. M.; Nuñez, N. E. (Monthly Notices of the Royal Astronomical Society, 2016-10-01)
    Symbiotic 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 quiescence

    Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, UK; Department of Physics and Astronomy, Texas A&amp;M University-Commerce, Commerce, TX 75429, USA; CRESST and Astroparticle Physics Laboratory NASA/GSFC, Greenbelt, MD 20771, USA; Department of Physics, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA; NASA Ames Research Center, Moffett Field, CA 94095, USA; NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA; Ramsay, Gavin; Wood, Matt A.; Cannizzo, John K.; Howell, Steve B.; Smale, Alan (Monthly Notices of the Royal Astronomical Society, 2017-07-01)
    We 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.
  • Interstellar gas towards the TeV γ-ray sources HESS J1640-465 and HESS J1641-463

    School of Physical Sciences, University of Adelaide, Adelaide, SA 5005, Australia; School of Physics, University of New South Wales, Sydney, NSW 2052, Australia; Armagh Observatory and Planetarium, College Hill, Armagh, BT61 9DG, Northern Ireland, UK; Department of Physics, University of Nagoya, Furo-cho, Chikusa-ku, Nagoya, 464-8601, Japan; Max-Planck-Institut für Kernphysik, PO Box 103980, D-69029 Heidelberg, Germany; Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland; National Research Nuclear University (MEPHI), 115409, Moscow, Russia; DESY, D-15738 Zeuthen, Germany; GRAPPA, Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, NL-1098 XH Amsterdam, the Netherlands; Max-Planck-Institut für Kernphysik, PO Box 103980, D-69029 Heidelberg, Germany; Instytut Fizyki Ja¸drowej PAN, ul. Radzikowskiego 152, PL-31-342 Krakow, Poland; Lau, J. C.; Rowell, G.; Burton, M. G.; et al. (Monthly Notices of the Royal Astronomical Society, 2017-01-01)
    We present a detailed analysis of the interstellar medium towards the tera electron volt (TeV) γ-ray sources HESS J1640-465 and HESS J1641-463 using results from the Mopra Southern Galactic Plane CO Survey and from a Mopra 7 mm-wavelength study. The γ-ray sources are positionally coincident with two supernova remnants (SNRs) G338.3-0.0 and G338.5+0.1, respectively. A bright complex of H II regions connect the two SNRs and TeV objects. Observations in the CO(1-0) transition lines reveal substantial amounts of diffuse gas positionally coincident with the γ-ray sources at multiple velocities along the line of sight, while 7 mm observations in CS, SiO, HC<SUB>3</SUB>N and CH<SUB>3</SUB>OH transition lines reveal regions of dense, shocked gas. Archival H I data from the Southern Galactic Plane Survey was used to account for the diffuse atomic gas. Physical parameters of the gas towards the TeV sources were calculated from the data. We find that for a hadronic origin for the γ-ray emission, the cosmic ray enhancement rates are ∼10<SUP>3</SUP> and 10<SUP>2</SUP> times the local solar value for HESS J1640-465 and HESS J1641-463, respectively.
  • The Superoutburst Duration versus Orbital Period Relation for AM CVn Stars

    CRESST/Joint Center for Astrophysics, University of Maryland, Baltimore County, Baltimore, MD 21250, USA ; Astroparticle Physics Laboratory, NASA-Goddard Space Flight Center, Greenbelt, MD 20771, USA ;;; Armagh Observatory, College Hill, Armagh BT61 9DG, UK; Cannizzo, J. K.; Ramsay, G. (The Astronomical Journal, 2019-03-01)
    We examine the relationship between superoutburst duration t <SUB>dur</SUB> and orbital period P <SUB>orb</SUB> in AM CVn ultra-compact binary systems. We show that the previously determined steep relation derived by Levitan et al. was strongly influenced by the inclusion of upper limits for systems with a relatively long orbital period in their fit. Excluding the upper limit values and including t <SUB>dur</SUB> values for three systems at long P <SUB>orb</SUB> that were not considered previously, then d{log}({t}<SUB>dur</SUB>})/d{log}({P}<SUB>orb</SUB>}) is flat as predicted by Cannizzo &amp; Nelemans.
  • Mass loss and stellar superwinds

    Armagh Observatory, College Hill, Armagh BT61 9DG, UK,; Vink, Jorick S. (Philosophical Transactions of the Royal Society of London Series A, 2017-09-01)
    Mass loss bridges the gap between massive stars and supernovae (SNe) in two major ways: (i) theoretically, it is the amount of mass lost that determines the mass of the star prior to explosion and (ii) observations of the circumstellar material around SNe may teach us the type of progenitor that made the SN. Here, I present the latest models and observations of mass loss from massive stars, both for canonical massive O stars, as well as very massive stars that show Wolf-Rayet type features. <P />This article is part of the themed issue 'Bridging the gap: from massive stars to supernovae'.
  • Searching for an interstellar medium association for HESS J1534 - 571

    School of Physics, The University of New South Wales, Sydney, 2052, Australia; Western Sydney University, Locked Bag 1797, Penrith, NSW 2751, Australia; School of Physics, The University of New South Wales, Sydney, 2052, Australia; School of Physical Sciences, The University of Adelaide, Adelaide, 5005, Australia; School of Physics, The University of New South Wales, Sydney, 2052, Australia; Armagh Observatory and Planetarium, College Hill, Armagh, BT61 9DG, Northern Ireland, UK; Western Sydney University, Locked Bag 1797, Penrith, NSW 2751, Australia; Department of Astronomy Faculty of Mathematics, University of Belgrade Studentski trg 16, 11000 Belgrade, Serbia; Isaac Newton Institute of Chile Yugoslavia Branch, Serbia; Astronomical Observatory, Volgina 7, POBox 74 11060 Belgrade, Serbia; Department of Astronomy Faculty of Mathematics, University of Belgrade Studentski trg 16, 11000 Belgrade, Serbia; Department of Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan; Maxted, Nigel I.; et al. (Monthly Notices of the Royal Astronomical Society, 2018-10-01)
    The Galactic supernova remnant HESS J1534-571 (also known as G323.7 - 1.0) has a shell-like morphology in TeV gamma-ray emission and is a key object in the study of cosmic ray origin. Little is known about its distance and local environment. We examine Mopra <SUP>12</SUP>CO/<SUP>13</SUP>CO(1-0) data, Australian Telescope Compact Array H I, and Parkes H I data towards HESS J1534-571. We trace molecular clouds in at least five velocity ranges, including clumpy interstellar medium structures near a dip in H I emission at a kinematic velocity consistent with the Scutum-Crux arm at ∼3.5 kpc. This feature may be a cavity blown-out by the progenitor star, a scenario that suggests HESS J1534-571 resulted from a core-collapse event. By employing parametrizations fitted to a sample of supernova remnants of known distance, we find that the radio continuum brightness of HESS J1534-571 is consistent with the ∼3.5 kpc kinematic distance of the Scutum-Crux arm H I dip. Modelling of the supernova evolution suggests a ∼8-24 kyr age for HESS J1534-571 at this distance.
  • A Supernova Remnant Counterpart for HESS J1832-085

    School of Science, The University of New South Wales, Australian Defence Force Academy, Canberra 2610, Australia; Western Sydney University, Locked Bag 1797, Penrith, NSW 2751, Australia; International Centre for Radio Astronomy Research, Curtin University, Bentley, WA 6102, Australia; School of Physical Sciences, The University of Adelaide, Adelaide 5005, Australia; Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, D-85748 Garching, Germany; Western Sydney University, Locked Bag 1797, Penrith, NSW 2751, Australia; School of Physics, The University of New South Wales, Sydney 2052, Australia; School of Physics, The University of New South Wales, Sydney 2052, Australia; Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, UK; Institut für Astronomie und Astrophysik, Universität Tübingen, D-72076 Tübingen, Germany; Department of Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan; et al. (The Astrophysical Journal, 2019-11-01)
    We examine the new Galactic supernova remnant (SNR) candidate, G23.11+0.18, as seen by the Murchison Widefield Array radio telescope. We describe the morphology of the candidate and find a spectral index of -0.63 ± 0.05 in the 70-170 MHz domain. Coincident TeV gamma-ray detection in High Energy Stereoscopic System (HESS) data supports the SNR nature of G23.11+0.18 and suggests that G23.11+0.18 is accelerating particles beyond TeV energies, thus making this object a promising new cosmic-ray hadron source candidate. The remnant cannot be seen in current optical, infrared and X-ray data sets. We do find, however, a dip in CO-traced molecular gas at a line-of-sight velocity of ∼85 km s<SUP>-1</SUP>, suggesting the existence of a G23.11+0.18 progenitor wind-blown bubble. Furthermore, the discovery of molecular gas clumps at a neighboring velocity toward HESS J1832-085 adheres to the notion that a hadronic gamma-ray production mechanism is plausible toward the north of the remnant. Based on these morphological arguments, we propose an interstellar medium association for G23.11+0.18 at a kinematic distance of 4.6 ± 0.8 kpc.
  • Wolf-Rayet spin at low metallicity and its implication for black hole formation channels

    Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, Northern Ireland, UK; Department of Physics and Astronomy, University of Exeter, Exeter, EX4 4QL, UK; Vink, Jorick S.; Harries, Tim J. (Astronomy and Astrophysics, 2017-07-01)
    Context. The spin of Wolf-Rayet (WR) stars at low metallicity (Z) is most relevant for our understanding of gravitational wave sources, such as GW 150914, and of the incidence of long-duration gamma-ray bursts (GRBs). Two scenarios have been suggested for both phenomena: one of them involves rapid rotation and quasi-chemical homogeneous evolution (CHE) and the other invokes classical evolution through mass loss in single and binary systems. <BR /> Aims: The stellar spin of WR stars might enable us to test these two scenarios. In order to obtain empirical constraints on black hole progenitor spin we infer wind asymmetries in all 12 known WR stars in the Small Magellanic Cloud (SMC) at Z = 1 / 5 Z<SUB>⊙</SUB> and within a significantly enlarged sample of single and binary WR stars in the Large Magellanic Cloud (LMC at Z = 1 / 2 Z<SUB>⊙</SUB>), thereby tripling the sample of Vink from 2007. This brings the total LMC sample to 39, making it appropriate for comparison to the Galactic sample. <BR /> Methods: We measured WR wind asymmetries with VLT-FORS linear spectropolarimetry, a tool that is uniquely poised to perform such tasks in extragalactic environments. <BR /> Results: We report the detection of new line effects in the LMC WN star BAT99-43 and the WC star BAT99-70, along with the well-known WR LBV HD 5980 in the SMC, which might be undergoing a chemically homogeneous evolution. With the previous reported line effects in the late-type WNL (Ofpe/WN9) objects BAT99-22 and BAT99-33, this brings the total LMC WR sample to four, I.e. a frequency of 10%. Perhaps surprisingly, the incidence of line effects amongst low Z WR stars is not found to be any higher than amongst the Galactic WR sample, challenging the rotationally induced CHE model. <BR /> Conclusions: As WR mass loss is likely Z-dependent, our Magellanic Cloud line-effect WR stars may maintain their surface rotation and fulfill the basic conditions for producing long GRBs, both via the classical post-red supergiant or luminous blue variable channel, or resulting from CHE due to physics specific to very massive stars.
  • Arcminute-scale studies of the interstellar gas towards HESS J1804-216: Still an unidentified TeV γ-ray source

    School of Physical Sciences, University of Adelaide, Adelaide, SA, 5005, Australia; School of Physics, University of New South Wales, Sydney, NSW, 2052, Australia; School of Physics, University of New South Wales, Sydney, NSW, 2052, Australia; Armagh Observatory and Planetarium, College Hill, Armagh, BT61 9DG, UK; School of Science, The University of New South Wales, Australian Defence Force Academy, Canberra, 2600, Australia; Western Sydney University, Locked Bag 1797, Penrith South DC, NSW, 2751, Australia; Western Sydney University, Locked Bag 1797, Penrith South DC, NSW, 2751, Australia; Pawsey Supercomputing Centre, 26 Dick Perry Ave, Kensington 6151, WA, Australia; School of Physics, The University of New South Wales, Sydney, 2052, Australia; Feijen, Kirsty; Rowell, Gavin; Einecke, Sabrina; Braiding, Catherine; Burton, Michael G.; et al. (Publications of the Astronomical Society of Australia, 2020-12-01)
    The Galactic TeV ${γ}$ -ray source ${HESS J1804{-}216}$ is currently an unidentified source. In an attempt to unveil its origin, we present here the most detailed study of interstellar gas using data from the Mopra Southern Galactic Plane CO Survey, 7- and 12-mm wavelength Mopra surveys and Southern Galactic Plane Survey of HI. Several components of atomic and molecular gas are found to overlap ${HESS J1804{-}216}$ at various velocities along the line of sight. The CS(1-0) emission clumps confirm the presence of dense gas. Both correlation and anti-correlation between the gas and TeV ${γ}$ -ray emission have been identified in various gas tracers, enabling several origin scenarios for the TeV ${γ}$ -ray emission from ${HESS J1804{-}216}$ . For a hadronic scenario, ${SNR G8.7{-}0.1}$ and the progenitor supernova remnant (SNR) of ${PSR J1803{-}2137}$ require cosmic ray (CR) enhancement factors of ${\mathord{∼} 50}$ times the solar neighbour CR flux value to produce the TeV ${γ}$ -ray emission. Assuming an isotropic diffusion model, CRs from both these SNRs require a slow diffusion coefficient, as found for other TeV SNRs associated with adjacent ISM gas. The morphology of gas located at 3.8 kpc (the dispersion measure distance to ${PSR J1803{-}2137}$ ) tends to anti-correlate with features of the TeV emission from ${HESS J1804{-}216}$ , making the leptonic scenario possible. Both pure hadronic and pure leptonic scenarios thus remain plausible.
  • TESS observations of the asynchronous polar CD Ind: mapping the changing accretion geometry

    Finnish Centre for Astronomy with ESO (FINCA), Quantum, University of Turku, FI-20014, Turku, Finland; Armagh Observatory and Planetarium, College Hill, Armagh, Northern Ireland, BT61 9DG, UK; South African Astronomical Observatory, PO Box 9, Observatory, 7935 Cape Town, South Africa; Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK; Hakala, Pasi; Ramsay, Gavin; Potter, Stephen B.; Beardmore, Andrew; Buckley, David A. H.; Wynn, Graham (Monthly Notices of the Royal Astronomical Society, 2019-06-01)
    We present the results of near continuous TESS optical observations of the asynchronous polar CD Ind (RX J2115-5840). The 27.9 d long light curve, with 2 min resolution, reveals remarkable changes in the magnetic accretion geometry of the system over the 7.3 d beat period. We have modelled the changes in the optical spin period pulse shape using a cyclotron emission mapping technique. The resulting cyclotron emission maps of the magnetic white dwarf reveal how the accretion geometry changes from single- to two-pole accretion and back over the beat cycle. Finally, we present the results from particle-based numerical magnetic accretion simulations, which agree with our interpretation of the changing accretion scenario.
  • Probing the local environment of the supernova remnant HESS J1731-347 with CO and CS observations

    School of Physics, The University of New South Wales, Sydney 2052, Australia; School of Physics, The University of New South Wales, Sydney 2052, Australia; Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, UK; School of Physical Sciences, The University of Adelaide, Adelaide 5005, Australia; Department of Astrophysics, Nagoya University, Furocho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan; Institut für Astronomie und Astrophysik, Universität Tübingen, Tübingen D-72076, Germany; Maxted, N.; Burton, M.; Braiding, C.; Rowell, G.; Sano, H.; et al. (Monthly Notices of the Royal Astronomical Society, 2018-02-01)
    The shell-type supernova remnant HESS J1731 - 347 emits TeV gamma-rays, and is a key object for the study of the cosmic ray acceleration potential of supernova remnants. We use 0.5-1 arcmin Mopra CO/CS(1-0) data in conjunction with H I data to calculate column densities towards the HESS J1731 - 347 region. We trace gas within at least four Galactic arms, typically tracing total (atomic+molecular) line-of-sight H column densities of 2-3× 10<SUP>22</SUP> cm<SUP>-2</SUP>. Assuming standard X-factor values and that most of the H I/CO emission seen towards HESS J1731 - 347 is on the near-side of the Galaxy, X-ray absorption column densities are consistent with H I+CO-derived column densities foreground to, but not beyond, the Scutum-Crux Galactic arm, suggesting a kinematic distance of ∼3.2 kpc for HESS J1731 - 347. At this kinematic distance, we also find dense, infrared-dark gas traced by CS(1-0) emission coincident with the north of HESS J1731 - 347, the nearby H II region G353.43-0.37 and the nearby unidentified gamma-ray source HESS J1729 - 345. This dense gas lends weight to the idea that HESS J1729 - 345 and HESS J1731 - 347 are connected, perhaps via escaping cosmic-rays.

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