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X-ray transients, such as accreting neutron stars, periodically undergo outbursts, thought to be caused by a thermal-viscous instability in the accretion disc. Usually outbursts of accreting neutron stars are identified when the accretion disc has undergone an instability, and the persistent X-ray flux has risen to a threshold detectable by all sky monitors on X-ray space observatories. Here, we present the earliest known combined optical, UV, and X-ray monitoring observations of the outburst onset of an accreting neutron star low-mass X-ray binary (LMXB) system. We observed a significant, continuing increase in the optical i ′-band magnitude starting on July 25, 12 d before the first X-ray detection with Swift /XRT and NICER (August 6), during the onset of the 2019 outburst of SAX J1808.4−3658. We also observed a 4 d optical to X-ray rise delay, and a 2 d UV to X-ray delay, at the onset of the outburst. We present the multiwavelength observations that were obtained, discussing the theory of outbursts in X-ray transients, including the disc instability model, and the implications of the delay. This work is an important confirmation of the delay in optical to X-ray emission during the onset of outbursts in LMXBs, which has only previously been measured with less sensitive all sky monitors. We find observational evidence that the outburst is triggered by ionization of hydrogen in the disc. [ABSTRACT FROM AUTHOR]

We present phase-resolved spectroscopy, photometry, and circular spectropolarimetry of the eclipsing polar UZ Fornacis. Doppler tomography of the strongest emission lines using the inside-out projection revealed the presence of three emission regions: from the irradiated face of the secondary star, the ballistic stream and the threading region, and the magnetically confined accretion stream. The total intensity spectrum shows broad emission features and a continuum that rises in the blue. The circularly polarized spectrum shows the presence of three cyclotron emission harmonics at ∼4500, 6000, and 7700 Å, corresponding to harmonic numbers 4, 3, and 2, respectively. These features are dominant before the eclipse and disappear after the eclipse. The harmonics are consistent with a magnetic field strength of ∼57 MG. We also present phase-resolved circular and linear photopolarimetry to complement the spectropolarimetry around the times of eclipse. MeerKAT radio observations show a faint source that has a peak flux density of 30.7 ± 5.4 |$\mu$| Jy beam−1 at 1.28 GHz at the position of UZ For. [ABSTRACT FROM AUTHOR]

We report the discovery of the first transient with MeerKAT, MKT J170456.2–482100, discovered in ThunderKAT images of the low-mass X-ray binary GX339–4. MKT J170456.2–482100 is variable in the radio, reaching a maximum flux density of |$0.71\pm 0.11\, \mathrm{mJy}$| on 2019 October 12, and is undetected in 15 out of 48 ThunderKAT epochs. MKT J170456.2–482100 is coincident with the chromospherically active K-type sub-giant TYC 8332-2529-1, and |$\sim 18\, \mathrm{yr}$| of archival optical photometry of the star shows that it varies with a period of |$21.25\pm 0.04\, \mathrm{d}$|⁠. The shape and phase of the optical light curve changes over time, and we detect both X-ray and UV emission at the position of MKT J170456.2–482100, which may indicate that TYC 8332-2529-1 has large star spots. Spectroscopic analysis shows that TYC 8332-2529-1 is in a binary, and has a line-of-sight radial velocity amplitude of |$43\, \mathrm{km\, s^{-1}}$|⁠. We also observe a spectral feature in antiphase with the K-type sub-giant, with a line-of-sight radial velocity amplitude of |$\sim 12\pm 10\, \mathrm{km\, s^{-1}}$|⁠ , whose origins cannot currently be explained. Further observations and investigation are required to determine the nature of the MKT J170456.2–482100 system. [ABSTRACT FROM AUTHOR]

We present the first ever X-ray data taken of an intermediate polar, FO Aquarii (FO Aqr), when in a low accretion state and during the subsequent recovery. Both the Swift and Chandra X-ray data taken during the low accretion state in 2016 July show a softer spectrum when compared to archival data taken when FO Aqr was in a high state. The X-ray spectrum in the low state showed a significant increase in the ratio of the soft X-ray flux to the hard X-ray flux due to a change in the partial covering fraction of the white dwarf from >85 per cent to 70+5 -8 per cent and a change in the hydrogen column density within the disc from 19+1.2 -0.9 × 1022 to 1.3+0.6 -0.3 × 1022 cm-2. XMM–Newton observations of FO Aqr during the subsequent recovery suggest that the system had not yet returned to its typical high state by 2016 November, with the hydrogen column density within the disc found to be 15+3.0 -2.0 cm-2. The partial covering fraction varied in the recovery state between 85 per cent and 95 per cent. The spin period of the white dwarf in 2014 and 2015 has also been refined to 1254.3342(8) s. Finally, we find an apparent phase difference between the high state X-ray pulse and recovery X-ray pulse of 0.17, which may be related to a restructuring of the X-ray-emitting regions within the system. [ABSTRACT FROM AUTHOR]