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First search for an x-ray–optical reverberation signal in an ultraluminous x-ray source

First search for an x-ray–optical reverberation signal in an ultraluminous x-ray source
First search for an x-ray–optical reverberation signal in an ultraluminous x-ray source
Using simultaneous optical (VLT/FORS2) and X-ray (XMM-Newton) data of NGC 5408, we present the first ever attempt to search for a reverberation signal in an ultraluminous X-ray source (NGC 5408 X-1). The idea is similar to active galactic nucleus broad line reverberation mapping where a lag measurement between the X-ray and the optical flux combined with a Keplerian velocity estimate should enable us to weigh the central compact object. We find that although NGC 5408 X-1's X-rays are variable on a timescale of a few hundred seconds (rms of 9.0 ± 0.5%), the optical emission does not show any statistically significant variations. We set a 3? upper limit on the rms optical variability of 3.3%. The ratio of the X-ray to the optical variability is an indicator of X-ray reprocessing efficiency. In X-ray binaries, this ratio is roughly 5. Assuming a similar ratio for NGC 5408 X-1, the expected rms optical variability is ?2%, which is still a factor of roughly two lower than what was possible with the VLT observations in this study. We find marginal evidence (3?) for optical variability on a ~24 hr timescale. Our results demonstrate that such measurements can be made, but photometric conditions, low sky background levels, and longer simultaneous observations will be required to reach optical variability levels similar to those of X-ray binaries.
1538-4357
1-10
Pasham, Dheeraj R.
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Strohmayer, Tod E.
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Cenko, S. Bradley
e7a32a77-161f-4df3-8563-1b3fd474410b
Trippe, Margaret L.
1edd166b-5e06-4701-aa52-5db0a3d0e50b
Mushotzky, Richard F.
0b8f11ee-414b-40ed-b677-6c4398c26a92
Gandhi, Poshak
5bc3b5af-42b0-4dd8-8f1f-f74048d4d4a9
Pasham, Dheeraj R.
0205c46d-c904-447e-9846-1b198d64ff7d
Strohmayer, Tod E.
e1a6de84-d23d-43de-b33e-2d49c4a5daa7
Cenko, S. Bradley
e7a32a77-161f-4df3-8563-1b3fd474410b
Trippe, Margaret L.
1edd166b-5e06-4701-aa52-5db0a3d0e50b
Mushotzky, Richard F.
0b8f11ee-414b-40ed-b677-6c4398c26a92
Gandhi, Poshak
5bc3b5af-42b0-4dd8-8f1f-f74048d4d4a9

Pasham, Dheeraj R., Strohmayer, Tod E., Cenko, S. Bradley, Trippe, Margaret L., Mushotzky, Richard F. and Gandhi, Poshak (2016) First search for an x-ray–optical reverberation signal in an ultraluminous x-ray source. The Astrophysical Journal, 818 (1), 1-10. (doi:10.3847/0004-637X/818/1/85).

Record type: Article

Abstract

Using simultaneous optical (VLT/FORS2) and X-ray (XMM-Newton) data of NGC 5408, we present the first ever attempt to search for a reverberation signal in an ultraluminous X-ray source (NGC 5408 X-1). The idea is similar to active galactic nucleus broad line reverberation mapping where a lag measurement between the X-ray and the optical flux combined with a Keplerian velocity estimate should enable us to weigh the central compact object. We find that although NGC 5408 X-1's X-rays are variable on a timescale of a few hundred seconds (rms of 9.0 ± 0.5%), the optical emission does not show any statistically significant variations. We set a 3? upper limit on the rms optical variability of 3.3%. The ratio of the X-ray to the optical variability is an indicator of X-ray reprocessing efficiency. In X-ray binaries, this ratio is roughly 5. Assuming a similar ratio for NGC 5408 X-1, the expected rms optical variability is ?2%, which is still a factor of roughly two lower than what was possible with the VLT observations in this study. We find marginal evidence (3?) for optical variability on a ~24 hr timescale. Our results demonstrate that such measurements can be made, but photometric conditions, low sky background levels, and longer simultaneous observations will be required to reach optical variability levels similar to those of X-ray binaries.

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1601.02638v1.pdf - Accepted Manuscript
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Accepted/In Press date: 4 January 2016
Published date: 9 February 2016
Organisations: Astronomy Group

Identifiers

Local EPrints ID: 399908
URI: http://eprints.soton.ac.uk/id/eprint/399908
ISSN: 1538-4357
PURE UUID: 9a51146a-d1c4-47c3-b665-5ce1096a7faf
ORCID for Poshak Gandhi: ORCID iD orcid.org/0000-0003-3105-2615

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Date deposited: 02 Sep 2016 11:17
Last modified: 20 Jul 2019 00:35

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