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NICER Detection of Strong Photospheric Expansion during a Thermonuclear X-Ray Burst from 4U 1820–30The Neutron Star Interior Composition Explorer (NICER) on the International Space Station (ISS) observed strong photospheric expansion of the neutron star in 4U1820–30 during a Type I X-ray burst. A thermonuclear helium flash in the star’s envelope powered a burst that reached the Eddington limit. Radiation pressure pushed the photosphere out to ∼200 km, while the blackbody temperature dropped to 0.45 keV. Previous observations of similar bursts were performed with instruments that are sensitive only above 3 keV, and the burst signal was weak at low temperatures. NICERʼs 0.2–12 keV passband enables the first complete detailed observation of strong expansion bursts. The strong expansion lasted only 0.6 s, and was followed by moderate expansion with a 20 km apparent radius, before the photosphere finally settled back down at 3 s after the burst onset. In addition to thermal emission from the neutron star, the NICER spectra reveal a second component that is well fit by optically thick Comptonization. During the strong expansion, this component is six times brighter than prior to the burst, and it accounts for 71% of the flux. In the moderate expansion phase, the Comptonization flux drops, while the thermal component brightens, and the total flux remains constant at the Eddington limit. We speculate that the thermal emission is reprocessed in the accretion environment to form the Comptonization component, and that changes in the covering fraction of the star explain the evolution of the relative contributions to the total flux.
Document ID
20190002380
Acquisition Source
Goddard Space Flight Center
Document Type
Reprint (Version printed in journal)
Authors
Keek, L.
(Maryland Univ. College Park, MD, United States)
Arzoumanian, Z.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Chakrabarty, D.
(Massachusetts Institute of Technology (MIT) Cambridge, MA, United States)
Chenevez, J.
(Technical University of Denmark Kongens Lyngby, Denmark)
Gendreau, K. C.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Guillot, S.
(Centre National de la Recherche Scientifique Annecy-le-Vieux, France)
Güver, T.
(İstanbul Üniversitesi İstanbul, Turkey)
Homan, J.
(Eureka Scientific, Inc. Oakland, CA, United States)
Jaisawal, G. K.
(Technical University of Denmark Kongens Lyngby, Denmark)
LaMarr, B.
(Massachusetts Institute of Technology (MIT) Cambridge, MA, United States)
Lamb, F. K.
(Illinois Univ. at Urbana-Champaign Urbana, IL, United States)
Mahmoodifar, S.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Markwardt, C. B.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Okajima, T.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Strohmayer, T. E.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Zand, J. J. M. in 't
(SRON Netherlands Institute for Space Research Utrecht, Netherlands)
Date Acquired
April 11, 2019
Publication Date
April 4, 2018
Publication Information
Publication: The Astrophysical Journal Letters
Publisher: American Astronomical Society
Volume: 856
Issue: 2
ISSN: 2041-8205
e-ISSN: 2041-8213
Subject Category
Astrophysics
Report/Patent Number
GSFC-E-DAA-TN66164
Funding Number(s)
CONTRACT_GRANT: PHY-1430152
Distribution Limits
Public
Copyright
Use by or on behalf of the US Gov. Permitted.
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