We report the first detection of hard (>10 keV) X-ray emission simultaneous with gamma-rays in a nova eruption. Observations of the nova V5855 Sgr carried out with the NuSTAR satellite on Day 12 of ...the eruption revealed faint, highly absorbed thermal X-rays. The extreme equivalent hydrogen column density toward the X-ray emitting region (∼3 × 1024 cm−2) indicates that the shock producing the X-rays was deeply embedded within the nova ejecta. The slope of the X-ray spectrum favors a thermal origin for the bulk of the emission, and the constraints of the temperature in the shocked region suggest a shock velocity compatible with the ejecta velocities inferred from optical spectroscopy. While we do not claim the detection of nonthermal X-rays, the data do not allow us to rule out an additional, fainter component dominating at energies above 20 keV, for which we obtained upper limits. The inferred luminosity of the thermal X-rays is too low to be consistent with the gamma-ray luminosities if both are powered by the same shock under standard assumptions regarding the efficiency of nonthermal particle acceleration and the temperature distribution of the shocked gas.
ABSTRACT
We present time-series linear-polarization observations of the bright O4 supergiant ζ Puppis. The star is found to show polarization variation on time-scales of around an hour and longer. ...Many of the observations were obtained contemporaneously with Transiting Exoplanet Survey Satellite (TESS) photometry. We find that the polarization varies on similar time-scales to those seen in the TESS light curve. The previously reported 1.78-d photometric periodicity is seen in both the TESS and polarization data. The amplitude ratio of photometry to polarization is ∼9 for the periodic component and the polarization variation is oriented along position angle ∼70°–160°. Higher frequency stochastic variability is also seen in both data sets with an amplitude ratio of ∼19 and no preferred direction. We model the polarization expected for a rotating star with bright photospheric spots and find that models that fit the photometric variation produce too little polarization variation to explain the observations. We suggest that the variable polarization is more likely the result of scattering from the wind, with corotating interaction regions producing the periodic variation and a clumpy outflow producing the stochastic component. The Hα emission line strength was seen to increase by 10 per cent in 2021 with subsequent observations showing a return to the pre-2018 level.
Abstract
Nova V5856 Sagittarii is unique for having remained more than nine magnitudes above its pre-outburst brightness for more than 6 yr. Extensive visible and infrared (IR) spectra from the time ...of outburst to the present epoch reveal separate emitting regions with distinct spectral characteristics. Permitted emission lines have both broad and narrow components, whereas the forbidden line profiles are almost entirely broad. The permitted line components frequently display P Cygni profiles indicating high optical depth, whereas the broad components do not show detectable absorption. The densities and velocities deduced from the spectra, including differences in the O
i
λ
7773 and
λ
8446 lines, are not consistent with an ongoing wind. Instead, the prolonged high luminosity and spectral characteristics are indicative of a post-outburst common envelope that enshrouds the binary, and is likely the primary source of the visible and IR emission.
Now in its sixth year of operation, the SPIRIT initiative remains unique in Australia, as a robust web-enabled robotic telescope initiative funded for education and outreach. With multiple modes of ...operation catering for a variety of usage scenarios and a fully supported education program, SPIRIT provides free access to contemporary astronomical tools for students and educators in Western Australia and beyond. The technical solution itself provides an excellent model for low cost robotic telescope installations, and the education program has evolved over time to include a broad range of student experiences - from engagement activities to authentic science. This paper details the robotic telescope solution, student interface and educational philosophy, summarises achievements and lessons learned and examines the possibilities for future enhancement including spectroscopy.
We present time-series linear-polarization observations of the bright O4
supergiant $\zeta$ Puppis. The star is found to show polarization variation on
timescales of around an hour and longer. Many ...of the observations were obtained
contemporaneously with Transiting Exoplanet Survey Satellite (TESS) photometry.
We find that the polarization varies on similar timescales to those seen in the
TESS light-curve. The previously reported 1.78-day photometric periodicity is
seen in both the TESS and polarization data. The amplitude ratio of photometry
to polarization is ~9 for the periodic component and the polarization variation
is oriented along position angle ~70 deg-160 deg. Higher-frequency stochastic
variability is also seen in both datasets with an amplitude ratio of ~19 and no
preferred direction. We model the polarization expected for a rotating star
with bright photospheric spots and find that models that fit the photometric
variation produce too little polarization variation to explain the
observations. We suggest that the variable polarization is more likely the
result of scattering from the wind, with corotating interaction regions
producing the periodic variation and a clumpy outflow producing the stochastic
component. The H$\alpha$ emission line strength was seen to increase by 10% in
2021 with subsequent observations showing a return to the pre-2018 level.
Nova V5856 Sagittarii is unique for having remained more than nine magnitudes above its pre-outburst brightness for more than six years. Extensive visible and IR spectra from the time of outburst to ...the present epoch reveal separate emitting regions with distinct spectral characteristics. Permitted emission lines have both broad and narrow components, whereas the forbidden line profiles are almost entirely broad. The permitted line components frequently display P Cygni profiles indicating high optical depth, whereas the broad components do not show detectable absorption. The densities and velocities deduced from the spectra, including differences in the O I 7773 and 8446 lines, are not consistent with an on-going wind. Instead, the prolonged high luminosity and spectral characteristics are indicative of a post-outburst common envelope that enshrouds the binary, and is likely the primary source of the visible and IR emission.
PDF
