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NICER discovers spectral lines during photospheric radius expansion bursts from 4U 1820-30: Evidence for burst-driven winds

NICER discovers spectral lines during photospheric radius expansion bursts from 4U 1820-30: Evidence for burst-driven winds
NICER discovers spectral lines during photospheric radius expansion bursts from 4U 1820-30: Evidence for burst-driven winds
We report the discovery with the Neutron Star Interior Composition Explorer (NICER) of narrow emission and absorption lines during photospheric radius expansion (PRE) X-ray bursts from the ultracompact binary 4U 1820−30. NICER observed 4U 1820−30 in 2017 August during a low-flux, hard spectral state, accumulating about 60 ks of exposure. Five thermonuclear X-ray bursts were detected, of which four showed clear signs of PRE. We extracted spectra during the PRE phases and fit each to a model that includes a Comptonized component to describe the accretion-driven emission, and a blackbody for the burst thermal radiation. The temperature and spherical emitting radius of the fitted blackbody are used to assess the strength of PRE in each burst. The two strongest PRE bursts (burst pair 1) had blackbody temperatures of ≈0.6 keV and emitting radii of ≈100 km (at a distance of 8.4 kpc). The other two bursts (burst pair 2) had higher temperatures (≈0.67 keV) and smaller radii (≈75 km). All of the PRE bursts show evidence of narrow line emission near 1 keV. By coadding the PRE phase spectra of burst pairs 1 and, separately, 2, we find, in both coadded spectra, significant, narrow, spectral features near 1.0 (emission), 1.7, and 3.0 keV (both in absorption). Remarkably, all the fitted line centroids in the coadded spectrum of burst pair 1 appear systematically blueshifted by a factor of 1.046 ± 0.006 compared to the centroids of pair 2, strongly indicative of a gravitational shift, a wind-induced blueshift, or more likely some combination of both effects. The observed shifts are consistent with this scenario in that the stronger PRE bursts in pair 1 reach larger photospheric radii, and thus have weaker gravitational redshifts, and they generate faster outflows, yielding higher blueshifts. We discuss possible elemental identifications for the observed features in the context of recent burst-driven wind models.
2041-8205
Strohmayer, T. E.
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Altamirano, D.
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Arzoumanian, Z.
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Bult, Peter M.
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Chakrabarty, D.
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Chenevez, J.
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Fabian, A.C.
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Gendreau, K.C.
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Guillot, S.
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in 't Zand, J.J.M.
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Jaisawal, G.K.
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Keek, L.
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Kosec, P.
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Ludlam, R.M.
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Mahmoodifar, S.
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Malacaria, C.
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Miller, J.M.
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Strohmayer, T. E.
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Altamirano, D.
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Arzoumanian, Z.
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Bult, Peter M.
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Chakrabarty, D.
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Chenevez, J.
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Fabian, A.C.
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Gendreau, K.C.
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Guillot, S.
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in 't Zand, J.J.M.
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Jaisawal, G.K.
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Keek, L.
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Kosec, P.
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Ludlam, R.M.
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Mahmoodifar, S.
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Malacaria, C.
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Miller, J.M.
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Strohmayer, T. E., Altamirano, D., Arzoumanian, Z., Bult, Peter M., Chakrabarty, D., Chenevez, J., Fabian, A.C., Gendreau, K.C., Guillot, S., in 't Zand, J.J.M., Jaisawal, G.K., Keek, L., Kosec, P., Ludlam, R.M., Mahmoodifar, S., Malacaria, C. and Miller, J.M. (2019) NICER discovers spectral lines during photospheric radius expansion bursts from 4U 1820-30: Evidence for burst-driven winds. The Astrophysical Journal Letters. (doi:10.3847/2041-8213/ab25eb).

Record type: Article

Abstract

We report the discovery with the Neutron Star Interior Composition Explorer (NICER) of narrow emission and absorption lines during photospheric radius expansion (PRE) X-ray bursts from the ultracompact binary 4U 1820−30. NICER observed 4U 1820−30 in 2017 August during a low-flux, hard spectral state, accumulating about 60 ks of exposure. Five thermonuclear X-ray bursts were detected, of which four showed clear signs of PRE. We extracted spectra during the PRE phases and fit each to a model that includes a Comptonized component to describe the accretion-driven emission, and a blackbody for the burst thermal radiation. The temperature and spherical emitting radius of the fitted blackbody are used to assess the strength of PRE in each burst. The two strongest PRE bursts (burst pair 1) had blackbody temperatures of ≈0.6 keV and emitting radii of ≈100 km (at a distance of 8.4 kpc). The other two bursts (burst pair 2) had higher temperatures (≈0.67 keV) and smaller radii (≈75 km). All of the PRE bursts show evidence of narrow line emission near 1 keV. By coadding the PRE phase spectra of burst pairs 1 and, separately, 2, we find, in both coadded spectra, significant, narrow, spectral features near 1.0 (emission), 1.7, and 3.0 keV (both in absorption). Remarkably, all the fitted line centroids in the coadded spectrum of burst pair 1 appear systematically blueshifted by a factor of 1.046 ± 0.006 compared to the centroids of pair 2, strongly indicative of a gravitational shift, a wind-induced blueshift, or more likely some combination of both effects. The observed shifts are consistent with this scenario in that the stronger PRE bursts in pair 1 reach larger photospheric radii, and thus have weaker gravitational redshifts, and they generate faster outflows, yielding higher blueshifts. We discuss possible elemental identifications for the observed features in the context of recent burst-driven wind models.

Text
NICER Discovers Spectral Lines during Photospheric Radius Expansion Bursts from 4U 1820-30: Evidence for Burst-driven Winds - Accepted Manuscript
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Accepted/In Press date: 30 May 2019
e-pub ahead of print date: 14 June 2019
Additional Information: Arxiv copy 1906.00974 author confirmed is AM.

Identifiers

Local EPrints ID: 432391
URI: http://eprints.soton.ac.uk/id/eprint/432391
ISSN: 2041-8205
PURE UUID: 1577a64c-ba75-41cf-9d1f-fe1359bc4d59
ORCID for D. Altamirano: ORCID iD orcid.org/0000-0002-3422-0074

Catalogue record

Date deposited: 12 Jul 2019 16:30
Last modified: 22 Nov 2021 07:30

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Contributors

Author: T. E. Strohmayer
Author: D. Altamirano ORCID iD
Author: Z. Arzoumanian
Author: Peter M. Bult
Author: D. Chakrabarty
Author: J. Chenevez
Author: A.C. Fabian
Author: K.C. Gendreau
Author: S. Guillot
Author: J.J.M. in 't Zand
Author: G.K. Jaisawal
Author: L. Keek
Author: P. Kosec
Author: R.M. Ludlam
Author: S. Mahmoodifar
Author: C. Malacaria
Author: J.M. Miller

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