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The radiative efficiency of a radiatively inefficient accretion flow

D'Angelo, C. R. and Fridriksson, J. K. and Messenger, C. and Patruno, A. (2015) The radiative efficiency of a radiatively inefficient accretion flow. Monthly Notices of the Royal Astronomical Society, 449 (3). pp. 2803-2817. ISSN 0035-8711. doi:10.1093/mnras/stv465.

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A recent joint XMM–Newton/Nuclear Spectroscopic Telescope Array (NuSTAR) observation of the accreting neutron star Cen X-4 (LX ∼ 10^(33) erg s^(−1)) revealed a hard power-law component (Γ ∼ 1–1.5) with a relatively low cut-off energy (∼10 keV), suggesting bremsstrahlung emission. The physical requirements for bremsstrahlung combined with other observed properties of Cen X-4 suggest the emission comes from a boundary layer rather than the accretion flow. The accretion flow itself is thus undetected (with an upper limit of L_(flow) ≲ 0.3LX). A deep search for coherent pulsations (which would indicate a strong magnetic field) places a 6 per cent upper limit on the fractional amplitude of pulsations, suggesting the flow is not magnetically regulated. Considering the expected energy balance between the accretion flow and the boundary layer for different values of the neutron star parameters (size, magnetic field, and spin) we use the upper limit on L_(flow) to set an upper limit of ε ≲ 0.3 for the intrinsic radiative efficiency of the accretion flow for the most likely model of a fast-spinning, non-magnetic neutron star. The non-detection of the accretion flow provides the first direct evidence that this flow is indeed ‘radiatively inefficient’, i.e. most of the gravitational potential energy lost by the flow before it hits the star is not emitted as radiation.

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Additional Information:© 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2015 February 27. Received 2015 February 26. In original form 2014 October 14. First published online April 4, 2015. CRD'A and AP are financially supported by an NWO Vidi grant (PI: Patruno). We are grateful to Daniel Wik for sharing his nuskybgd background analysis tools for NuSTAR. AP acknowledges very useful conversations with Anne Archibald. CRD'A acknowledges very useful conversations with Deepto Chakrabarty, Marat Gilfanov, Craig Heinke, Andrew King, John Raymond, and Rudy Wijnands.
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Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)UNSPECIFIED
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ID Code:59864
Deposited By: Joy Painter
Deposited On:24 Aug 2015 22:39
Last Modified:10 Nov 2021 22:26

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