NICER Discovers Spectral Lines during Photospheric Radius Expansion Bursts from 4U 1820−30: Evidence for Burst-driven Winds
© 2019. The American Astronomical Society. All rights reserved. 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...
| Հիմնական հեղինակներ: | , , , , , , , , , , , , , , , , |
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| Այլ հեղինակներ: | |
| Ձևաչափ: | Հոդված |
| Լեզու: | English |
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American Astronomical Society
2021
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| Առցանց հասանելիություն: | https://hdl.handle.net/1721.1/134771 |
| _version_ | 1826201763903963136 |
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| author | Strohmayer, TE Altamirano, D Arzoumanian, Z Bult, PM Chakrabarty, D Chenevez, J Fabian, AC Gendreau, KC Guillot, S in ’t Zand, JJM Jaisawal, GK Keek, L Kosec, P Ludlam, RM Mahmoodifar, S Malacaria, Christian Miller, JM |
| author2 | MIT Kavli Institute for Astrophysics and Space Research |
| author_facet | MIT Kavli Institute for Astrophysics and Space Research Strohmayer, TE Altamirano, D Arzoumanian, Z Bult, PM Chakrabarty, D Chenevez, J Fabian, AC Gendreau, KC Guillot, S in ’t Zand, JJM Jaisawal, GK Keek, L Kosec, P Ludlam, RM Mahmoodifar, S Malacaria, Christian Miller, JM |
| author_sort | Strohmayer, TE |
| collection | MIT |
| description | © 2019. The American Astronomical Society. All rights reserved. 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. |
| first_indexed | 2024-09-23T11:56:23Z |
| format | Article |
| id | mit-1721.1/134771 |
| institution | Massachusetts Institute of Technology |
| language | English |
| last_indexed | 2024-09-23T11:56:23Z |
| publishDate | 2021 |
| publisher | American Astronomical Society |
| record_format | dspace |
| spelling | mit-1721.1/1347712023-11-08T21:50:29Z NICER Discovers Spectral Lines during Photospheric Radius Expansion Bursts from 4U 1820−30: Evidence for Burst-driven Winds Strohmayer, TE Altamirano, D Arzoumanian, Z Bult, PM Chakrabarty, D Chenevez, J Fabian, AC Gendreau, KC Guillot, S in ’t Zand, JJM Jaisawal, GK Keek, L Kosec, P Ludlam, RM Mahmoodifar, S Malacaria, Christian Miller, JM MIT Kavli Institute for Astrophysics and Space Research © 2019. The American Astronomical Society. All rights reserved. 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. 2021-10-27T20:09:04Z 2021-10-27T20:09:04Z 2019 2020-09-21T18:41:54Z Article http://purl.org/eprint/type/JournalArticle https://hdl.handle.net/1721.1/134771 en 10.3847/2041-8213/AB25EB Astrophysical Journal Letters Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. application/pdf American Astronomical Society The American Astronomical Society |
| spellingShingle | Strohmayer, TE Altamirano, D Arzoumanian, Z Bult, PM Chakrabarty, D Chenevez, J Fabian, AC Gendreau, KC Guillot, S in ’t Zand, JJM Jaisawal, GK Keek, L Kosec, P Ludlam, RM Mahmoodifar, S Malacaria, Christian Miller, JM NICER Discovers Spectral Lines during Photospheric Radius Expansion Bursts from 4U 1820−30: Evidence for Burst-driven Winds |
| title | NICER Discovers Spectral Lines during Photospheric Radius Expansion Bursts from 4U 1820−30: Evidence for Burst-driven Winds |
| title_full | NICER Discovers Spectral Lines during Photospheric Radius Expansion Bursts from 4U 1820−30: Evidence for Burst-driven Winds |
| title_fullStr | NICER Discovers Spectral Lines during Photospheric Radius Expansion Bursts from 4U 1820−30: Evidence for Burst-driven Winds |
| title_full_unstemmed | NICER Discovers Spectral Lines during Photospheric Radius Expansion Bursts from 4U 1820−30: Evidence for Burst-driven Winds |
| title_short | NICER Discovers Spectral Lines during Photospheric Radius Expansion Bursts from 4U 1820−30: Evidence for Burst-driven Winds |
| title_sort | nicer discovers spectral lines during photospheric radius expansion bursts from 4u 1820 30 evidence for burst driven winds |
| url | https://hdl.handle.net/1721.1/134771 |
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