Sometimes they come back: pulsations in A 0538-66 at 69 ms rediscovered with NICER
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arXiv
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| Format: | Preprint |
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2025
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| _version_ | 1866909698981101568 |
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| author | Ducci, Lorenzo Mereghetti, Sandro |
| author_facet | Ducci, Lorenzo Mereghetti, Sandro |
| contents | We report on NICER X-ray observations of the Be X-ray binary A 0538-66 located in the Large Magellanic Cloud. Fast pulsations (69 ms) in this source were discovered in 1980 during a bright outburst in which it reached a luminosity of ~8E38 erg/s, but were never reobserved since then. We clearly detected the pulsations at P=69.3055 +/- 0.0005 ms with a pulsed fraction of ~20% during a short time interval (~11 minutes) on 2023 January 9, when A 0538-66 had a luminosity of ~8E36 erg/s (0.3-10 keV). The pulsations were not detected in other NICER observations (total exposure ~162.7 ks), during which A 0538-66 had a similar or lower luminosity. On 2023 February 8-9 the source exhibited a strong variability, with short flares reaching ~1E38 erg/s, but no periodic pulsations were detected. Assuming the magnetospheric radius lies within the corotation radius during pulsations, we estimate the neutron star magnetic field is below ~2.7E10 G. This would make A 0538-66 the high-mass X-ray binary with the weakest known magnetic field. We discuss implications for magnetic field evolution in accreting pulsars and propose that, alternatively, A 0538-66 has a stronger magnetic field and during the NICER detection, a centrifugal barrier may have been active while part of the plasma accumulated at the magnetosphere sporadically leaked through it via an instability mechanism, allowing accretion onto the polar caps. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2507_16420 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Sometimes they come back: pulsations in A 0538-66 at 69 ms rediscovered with NICER Ducci, Lorenzo Mereghetti, Sandro High Energy Astrophysical Phenomena We report on NICER X-ray observations of the Be X-ray binary A 0538-66 located in the Large Magellanic Cloud. Fast pulsations (69 ms) in this source were discovered in 1980 during a bright outburst in which it reached a luminosity of ~8E38 erg/s, but were never reobserved since then. We clearly detected the pulsations at P=69.3055 +/- 0.0005 ms with a pulsed fraction of ~20% during a short time interval (~11 minutes) on 2023 January 9, when A 0538-66 had a luminosity of ~8E36 erg/s (0.3-10 keV). The pulsations were not detected in other NICER observations (total exposure ~162.7 ks), during which A 0538-66 had a similar or lower luminosity. On 2023 February 8-9 the source exhibited a strong variability, with short flares reaching ~1E38 erg/s, but no periodic pulsations were detected. Assuming the magnetospheric radius lies within the corotation radius during pulsations, we estimate the neutron star magnetic field is below ~2.7E10 G. This would make A 0538-66 the high-mass X-ray binary with the weakest known magnetic field. We discuss implications for magnetic field evolution in accreting pulsars and propose that, alternatively, A 0538-66 has a stronger magnetic field and during the NICER detection, a centrifugal barrier may have been active while part of the plasma accumulated at the magnetosphere sporadically leaked through it via an instability mechanism, allowing accretion onto the polar caps. |
| title | Sometimes they come back: pulsations in A 0538-66 at 69 ms rediscovered with NICER |
| topic | High Energy Astrophysical Phenomena |
| url | https://arxiv.org/abs/2507.16420 |