The University of Southampton
University of Southampton Institutional Repository

Simultaneous optical/X-ray study of GS 1354-64 (=BW Cir) during hard outburst: evidence for optical cyclo-synchrotron emission from the hot accretion flow

Simultaneous optical/X-ray study of GS 1354-64 (=BW Cir) during hard outburst: evidence for optical cyclo-synchrotron emission from the hot accretion flow
Simultaneous optical/X-ray study of GS 1354-64 (=BW Cir) during hard outburst: evidence for optical cyclo-synchrotron emission from the hot accretion flow
We present results from simultaneous optical (SALT) and X-ray (Swift and INTEGRAL) observations of GS 1354-64/BW Cir during the 2015 hard state outburst. During the rising phase, optical/X-ray time series show a strong anti-correlation with X-ray photons lagging optical. Optical and X-ray power spectra show quasi-periodic oscillations at a frequency of ~18 mHz with a confidence level of at least 99%. Simultaneous fitting of Swift/XRT and INTEGRAL spectra in the range 0.5-1000 keV shows non-thermal, power-law dominated (> 90%) spectra with a hard power-law index of 1.48 +/- 0.03, inner disc temperature of 0.12 +/- 0.01 keV and inner disc radius of ~3000 km. All evidence is consistent with cyclo-synchrotron radiation in a non-thermal, hot electron cloud extending to ~100 Schwarzschild radii being a major physical process for the origin of optical photons. At outburst peak about one month later, when the X-ray flux rises and the optical drops, the apparent features in the optical/X-ray correlation vanish and the optical auto correlation widens. Although ~0.19 Hz QPO is observed from the X-ray power spectra, the optical variability is dominated by the broadband noise, and the inner disc temperature increases. These results support a change in the dominant optical emission source between outburst rise and peak, consistent with a weakening of hot flow as the disc moves in.
0035-8711
193-205
Pahari, Mayukh
37a1bc53-be32-4688-91a3-2469a4bf0e78
Gandhi, Poshak
5bc3b5af-42b0-4dd8-8f1f-f74048d4d4a9
Charles, Philip A.
0429b380-0754-4dc1-8def-885c7fa6a086
Kotze, Marissa M.
8af66bd3-8352-4867-9ade-a0c890283952
Altamirano, Diego
d5ccdb09-0b71-4303-9538-05b467be075b
Misra, Ranjeev
2285adb3-8a4c-47f7-b073-e5f36c7dd6ad
Pahari, Mayukh
37a1bc53-be32-4688-91a3-2469a4bf0e78
Gandhi, Poshak
5bc3b5af-42b0-4dd8-8f1f-f74048d4d4a9
Charles, Philip A.
0429b380-0754-4dc1-8def-885c7fa6a086
Kotze, Marissa M.
8af66bd3-8352-4867-9ade-a0c890283952
Altamirano, Diego
d5ccdb09-0b71-4303-9538-05b467be075b
Misra, Ranjeev
2285adb3-8a4c-47f7-b073-e5f36c7dd6ad

Pahari, Mayukh, Gandhi, Poshak, Charles, Philip A., Kotze, Marissa M., Altamirano, Diego and Misra, Ranjeev (2017) Simultaneous optical/X-ray study of GS 1354-64 (=BW Cir) during hard outburst: evidence for optical cyclo-synchrotron emission from the hot accretion flow. Monthly Notices of the Royal Astronomical Society, 469 (1), 193-205. (doi:10.1093/mnras/stx840).

Record type: Article

Abstract

We present results from simultaneous optical (SALT) and X-ray (Swift and INTEGRAL) observations of GS 1354-64/BW Cir during the 2015 hard state outburst. During the rising phase, optical/X-ray time series show a strong anti-correlation with X-ray photons lagging optical. Optical and X-ray power spectra show quasi-periodic oscillations at a frequency of ~18 mHz with a confidence level of at least 99%. Simultaneous fitting of Swift/XRT and INTEGRAL spectra in the range 0.5-1000 keV shows non-thermal, power-law dominated (> 90%) spectra with a hard power-law index of 1.48 +/- 0.03, inner disc temperature of 0.12 +/- 0.01 keV and inner disc radius of ~3000 km. All evidence is consistent with cyclo-synchrotron radiation in a non-thermal, hot electron cloud extending to ~100 Schwarzschild radii being a major physical process for the origin of optical photons. At outburst peak about one month later, when the X-ray flux rises and the optical drops, the apparent features in the optical/X-ray correlation vanish and the optical auto correlation widens. Although ~0.19 Hz QPO is observed from the X-ray power spectra, the optical variability is dominated by the broadband noise, and the inner disc temperature increases. These results support a change in the dominant optical emission source between outburst rise and peak, consistent with a weakening of hot flow as the disc moves in.

Other
pdf - Accepted Manuscript
Download (828kB)

More information

Accepted/In Press date: 3 April 2017
e-pub ahead of print date: 5 April 2017
Published date: 1 July 2017
Organisations: Astronomy Group, Physics & Astronomy

Identifiers

Local EPrints ID: 411345
URI: http://eprints.soton.ac.uk/id/eprint/411345
ISSN: 0035-8711
PURE UUID: bb1b983d-55ac-44d8-a2e8-b477c99f45dc
ORCID for Poshak Gandhi: ORCID iD orcid.org/0000-0003-3105-2615
ORCID for Diego Altamirano: ORCID iD orcid.org/0000-0002-3422-0074

Catalogue record

Date deposited: 19 Jun 2017 16:31
Last modified: 07 Oct 2020 02:06

Export record

Altmetrics

Contributors

Author: Mayukh Pahari
Author: Poshak Gandhi ORCID iD
Author: Marissa M. Kotze
Author: Ranjeev Misra

University divisions

Download statistics

Downloads from ePrints over the past year. Other digital versions may also be available to download e.g. from the publisher's website.

View more statistics

Atom RSS 1.0 RSS 2.0

Contact ePrints Soton: eprints@soton.ac.uk

ePrints Soton supports OAI 2.0 with a base URL of http://eprints.soton.ac.uk/cgi/oai2

This repository has been built using EPrints software, developed at the University of Southampton, but available to everyone to use.

We use cookies to ensure that we give you the best experience on our website. If you continue without changing your settings, we will assume that you are happy to receive cookies on the University of Southampton website.

×