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The flaring X-ray corona in the quasar PDS 456

Monthly Notices of the Royal Astronomical Society, ISSN: 1365-2966, Vol: 500, Issue: 2, Page: 1974-1991
2020
  • 16
    Citations
  • 0
    Usage
  • 10
    Captures
  • 0
    Mentions
  • 0
    Social Media
Metric Options:   Counts1 Year3 Year

Metrics Details

  • Citations
    16
    • Citation Indexes
      16
  • Captures
    10

Article Description

New Swift monitoring observations of the variable, radio-quiet quasar, PDS 456, are presented. A bright X-ray flare was captured in 2018 September, the flux increasing by a factor of 4 and with a doubling time-scale of 2 d. From the light crossing argument, the coronal size is inferred to be ≤30 gravitational radii for a black hole mass of 109 M⊙ and the total flare energy exceeds 1051 erg. A hardening of the X-ray emission accompanied the flare, with the photon index decreasing from Γ = 2.2 to Γ = 1.7 and back again. The flare is produced in the X-ray corona, the lack of any optical or UV variability being consistent with a constant accretion rate. Simultaneous XMM-Newton and NuSTAR observations were performed, 1-3 d after the flare peak and during the decline phase. These caught PDS 456 in a bright, bare state, where no disc wind absorption features are apparent. The hard X-ray spectrum shows a high energy roll-over, with an e-folding energy of Efold=51+11-8 keV. The deduced coronal temperature, of kT = 13 keV, is one of the coolest measured in any AGN and PDS 456 lies well below the predicted pair annihilation line in X-ray corona. The spectral variability, becoming softer when fainter following the flare, is consistent with models of cooling X-ray coronae. Alternatively, an increase in a non-thermal component could contribute towards the hard X-ray flare spectrum.

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