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Evidence for pulsar-like emission components in the broadband ULX sample

Evidence for pulsar-like emission components in the broadband ULX sample
Evidence for pulsar-like emission components in the broadband ULX sample

We present broadband X-ray analyses of a sample of bright ultraluminous X-ray sources (ULX) with the goal of investigating the spectral similarity of this population to the known ULX pulsars, M82 X-2, NGC 7793 P13, and NGC 5907 ULX. We perform a phase-resolved analysis of the broadband XMM-Newton+NuSTAR data set of NGC 5907 ULX, finding that the pulsed emission from the accretion column in this source exhibits a similar spectral shape to that seen in both M82 X-2 and NGC 7793 P13, and that this is responsible for the excess emission observed at the highest energies when the spectra are fit with accretion disk models. We then demonstrate that similar "hard" excesses are seen in all ULXs in the broadband sample. Finally, for ULXs where the nature of the accretor is currently unknown, we test whether the hard excesses are all consistent with being produced by an accretion column similar to those present in M82 X-2, NGC 7793 P13, and NGC 5907 ULX. Based on the average shape of the pulsed emission, we find that in all cases a similar accretion column can successfully reproduce the observed data, consistent with the hypothesis that this ULX sample may be dominated by neutron star accretors. Compared to the known pulsar ULXs, our spectral fits for the remaining ULXs suggest that the non-pulsed emission from the accretion flow beyond the magnetosphere makes a stronger relative contribution than the component associated with the accretion column. If these sources do also contain neutron star accretors, this may help to explain the lack of detected pulsations.

stars: neutron, X-rays: binaries, X-rays: individual (NGC 5907 ULX)
0004-637X
Walton, D.J.
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Fürst, F.
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Heida, M.
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Harrison, F.A.
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Barret, D.
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Stern, D.
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Bachetti, M.
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Brightman, M.
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Fabian, A.C.
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Middleton, M.J.
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Walton, D.J.
84f34f90-2aef-454b-b20e-98dc8a8b4b65
Fürst, F.
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Heida, M.
fc66bb37-4c8d-4fc5-9af4-2e7438e45fcd
Harrison, F.A.
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Barret, D.
343cdacd-9683-4f70-910b-6539201daf06
Stern, D.
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Bachetti, M.
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Brightman, M.
eb110ae1-9a2c-4968-b9ef-e7955feba535
Fabian, A.C.
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Middleton, M.J.
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Walton, D.J., Fürst, F., Heida, M., Harrison, F.A., Barret, D., Stern, D., Bachetti, M., Brightman, M., Fabian, A.C. and Middleton, M.J. (2018) Evidence for pulsar-like emission components in the broadband ULX sample. The Astrophysical Journal, 856 (2), [128]. (doi:10.3847/1538-4357/aab610).

Record type: Article

Abstract

We present broadband X-ray analyses of a sample of bright ultraluminous X-ray sources (ULX) with the goal of investigating the spectral similarity of this population to the known ULX pulsars, M82 X-2, NGC 7793 P13, and NGC 5907 ULX. We perform a phase-resolved analysis of the broadband XMM-Newton+NuSTAR data set of NGC 5907 ULX, finding that the pulsed emission from the accretion column in this source exhibits a similar spectral shape to that seen in both M82 X-2 and NGC 7793 P13, and that this is responsible for the excess emission observed at the highest energies when the spectra are fit with accretion disk models. We then demonstrate that similar "hard" excesses are seen in all ULXs in the broadband sample. Finally, for ULXs where the nature of the accretor is currently unknown, we test whether the hard excesses are all consistent with being produced by an accretion column similar to those present in M82 X-2, NGC 7793 P13, and NGC 5907 ULX. Based on the average shape of the pulsed emission, we find that in all cases a similar accretion column can successfully reproduce the observed data, consistent with the hypothesis that this ULX sample may be dominated by neutron star accretors. Compared to the known pulsar ULXs, our spectral fits for the remaining ULXs suggest that the non-pulsed emission from the accretion flow beyond the magnetosphere makes a stronger relative contribution than the component associated with the accretion column. If these sources do also contain neutron star accretors, this may help to explain the lack of detected pulsations.

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Evidence for Pulsar-like Emission Components in the Broadband ULX Sample - Accepted Manuscript
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More information

Accepted/In Press date: 8 March 2018
e-pub ahead of print date: 2 April 2018
Published date: April 2018
Keywords: stars: neutron, X-rays: binaries, X-rays: individual (NGC 5907 ULX)

Identifiers

Local EPrints ID: 420322
URI: http://eprints.soton.ac.uk/id/eprint/420322
ISSN: 0004-637X
PURE UUID: a7cc7c77-30fb-4e59-9caf-f1ffe6b66284

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Date deposited: 04 May 2018 16:30
Last modified: 17 Dec 2019 05:27

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