Hydrogen-triggered X-Ray Bursts from SAX J1808.4−3658? The Onset of Nuclear Burning
We present a study of weak, thermonuclear X-ray bursts from the accreting millisecond X-ray pulsar SAX J1808.4−3658. We focus on a burst observed with the Neutron Star Interior Composition Explorer on 2019 August 9, and describe a similar burst observed with the Rossi X-ray Timing Explorer in 2005 J...
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| Format: | Article |
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IOP Publishing
2023-01-01
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| Series: | The Astrophysical Journal |
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| Online Access: | https://doi.org/10.3847/1538-4357/acc24f |
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| author | Sierra Casten Tod E. Strohmayer Peter Bult |
| author_facet | Sierra Casten Tod E. Strohmayer Peter Bult |
| author_sort | Sierra Casten |
| collection | DOAJ |
| description | We present a study of weak, thermonuclear X-ray bursts from the accreting millisecond X-ray pulsar SAX J1808.4−3658. We focus on a burst observed with the Neutron Star Interior Composition Explorer on 2019 August 9, and describe a similar burst observed with the Rossi X-ray Timing Explorer in 2005 June. These bursts occurred soon after outburst onset, 2.9 and 1.1 days, after the first indications of fresh accretion. We measure peak burst bolometric fluxes of 6.98 ± 0.50 × 10 ^−9 and 1.54 ± 0.10 × 10 ^−8 erg cm ^−2 s ^−1 , respectively, which are factors of ≈30 and 15 less than the peak flux of the brightest, helium-powered bursts observed from this source. From spectral modeling we estimate the accretion rates and accreted columns at the time of each burst. For the 2019 burst we estimate an accretion rate of $\dot{M}\approx 1.4\mbox{--}1.6\times {10}^{-10}$ M _⊙ yr ^−1 , and a column in the range 3.9–5.1 × 10 ^7 g cm ^−2 . For the 2005 event the accretion rate was similar, but the accreted column was half of that estimated for the 2019 burst. The low accretion rates, modest columns, and evidence for a cool neutron star in quiescence, suggest these bursts are triggered by thermally unstable CNO cycle hydrogen burning. The post-burst flux level in the 2019 event appears offset from the pre-burst level by an amount consistent with quasi-stable hydrogen burning due to the temperature-insensitive, hot-CNO cycle, further suggesting hydrogen burning as the primary fuel source. This provides strong observational evidence for hydrogen-triggered bursts. We discuss our results in the context of previous theoretical modeling. |
| first_indexed | 2024-03-12T03:19:54Z |
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| id | doaj.art-29e011e79df042cb9300a5fe5a75a158 |
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| issn | 1538-4357 |
| language | English |
| last_indexed | 2024-03-12T03:19:54Z |
| publishDate | 2023-01-01 |
| publisher | IOP Publishing |
| record_format | Article |
| series | The Astrophysical Journal |
| spelling | doaj.art-29e011e79df042cb9300a5fe5a75a1582023-09-03T14:02:56ZengIOP PublishingThe Astrophysical Journal1538-43572023-01-01948211710.3847/1538-4357/acc24fHydrogen-triggered X-Ray Bursts from SAX J1808.4−3658? The Onset of Nuclear BurningSierra Casten0https://orcid.org/0000-0003-3872-1703Tod E. Strohmayer1https://orcid.org/0000-0001-7681-5845Peter Bult2https://orcid.org/0000-0002-7252-0991Department of Physics and Astronomy, Michigan State University , 567 Wilson Road, East Lansing, MI 48824, USAAstrophysics Science Division and Joint Space-Science Institute , NASA’s Goddard Space Flight Center, Greenbelt, MD 20771, USADepartment of Astronomy, University of Maryland , College Park, MD 20742, USA; Astrophysics Science Division, NASA Goddard Space Flight Center , Greenbelt, MD 20771, USAWe present a study of weak, thermonuclear X-ray bursts from the accreting millisecond X-ray pulsar SAX J1808.4−3658. We focus on a burst observed with the Neutron Star Interior Composition Explorer on 2019 August 9, and describe a similar burst observed with the Rossi X-ray Timing Explorer in 2005 June. These bursts occurred soon after outburst onset, 2.9 and 1.1 days, after the first indications of fresh accretion. We measure peak burst bolometric fluxes of 6.98 ± 0.50 × 10 ^−9 and 1.54 ± 0.10 × 10 ^−8 erg cm ^−2 s ^−1 , respectively, which are factors of ≈30 and 15 less than the peak flux of the brightest, helium-powered bursts observed from this source. From spectral modeling we estimate the accretion rates and accreted columns at the time of each burst. For the 2019 burst we estimate an accretion rate of $\dot{M}\approx 1.4\mbox{--}1.6\times {10}^{-10}$ M _⊙ yr ^−1 , and a column in the range 3.9–5.1 × 10 ^7 g cm ^−2 . For the 2005 event the accretion rate was similar, but the accreted column was half of that estimated for the 2019 burst. The low accretion rates, modest columns, and evidence for a cool neutron star in quiescence, suggest these bursts are triggered by thermally unstable CNO cycle hydrogen burning. The post-burst flux level in the 2019 event appears offset from the pre-burst level by an amount consistent with quasi-stable hydrogen burning due to the temperature-insensitive, hot-CNO cycle, further suggesting hydrogen burning as the primary fuel source. This provides strong observational evidence for hydrogen-triggered bursts. We discuss our results in the context of previous theoretical modeling.https://doi.org/10.3847/1538-4357/acc24fNeutron starsX-ray binary starsX-ray burstsX-ray sources |
| spellingShingle | Sierra Casten Tod E. Strohmayer Peter Bult Hydrogen-triggered X-Ray Bursts from SAX J1808.4−3658? The Onset of Nuclear Burning The Astrophysical Journal Neutron stars X-ray binary stars X-ray bursts X-ray sources |
| title | Hydrogen-triggered X-Ray Bursts from SAX J1808.4−3658? The Onset of Nuclear Burning |
| title_full | Hydrogen-triggered X-Ray Bursts from SAX J1808.4−3658? The Onset of Nuclear Burning |
| title_fullStr | Hydrogen-triggered X-Ray Bursts from SAX J1808.4−3658? The Onset of Nuclear Burning |
| title_full_unstemmed | Hydrogen-triggered X-Ray Bursts from SAX J1808.4−3658? The Onset of Nuclear Burning |
| title_short | Hydrogen-triggered X-Ray Bursts from SAX J1808.4−3658? The Onset of Nuclear Burning |
| title_sort | hydrogen triggered x ray bursts from sax j1808 4 3658 the onset of nuclear burning |
| topic | Neutron stars X-ray binary stars X-ray bursts X-ray sources |
| url | https://doi.org/10.3847/1538-4357/acc24f |
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