Evidence for hot clumpy accretion flow in the transitional millisecond pulsar PSR J1023+0038

Abstract

We present simultaneous optical and near-infrared (IR) photometry of the millisecond pulsar PSR J1023+0038 during its low-mass X-ray binary phase. The r -and K s -band light curves show rectangular, flat-bottomed dips, similar to the X-ray mode-switching (active-passive state transitions) behaviour observed previously. The cross-correlation function (CCF) of the optical and near-IR data reveals a strong, broad negative anti-correlation at negative lags, a broad positive correlation at positive lags, with a strong, positive narrow correlation superimposed. The shape of the CCF resembles the CCF of black hole X-ray binaries but the time-scales are different. The features can be explained by reprocessing and a hot accretion flow close to the neutron star's magnetospheric radius. The optical emission is dominated by the reprocessed component, whereas the near-IR emission contains the emission from plasmoids in the hot accretion flow and a reprocessed component. The rapid active-passive state transition occurs when the hot accretion flow material is channelled onto the neutron star and is expelled from its magnetosphere. During the transition the optical reprocessing component decreases resulting in the removal of a blue spectral component. The accretion of clumpy material through the magnetic barrier of the neutron star produces the observed near-IR/optical CCF and variability. The dip at negative lags corresponds to the suppression of the near-IR synchrotron component in the hot flow, whereas the broad positive correlation at positive lags is driven by the increased synchrotron emission of the outflowing plasmoids. The narrow peak in the CCF is due to the delayed reprocessed component, enhanced by the increased X-ray emission.

Peering at the outflow mechanisms in the transitional pulsar PSR J1023+0038: simultaneous VLT, XMM-Newton, and Swift high-time resolution observations

“…The results reported in Fig. 9 of Shahbaz et al (2018) might explain the absence of any active/passive mode transition in our light curve. In fact, according to that analysis, the expected flux difference between the active and passive mode is supposed to vary depending on the wavelength of the observation, being more pronounced in the r ′ -band (i.e.…”

“…In addition, the cross-correlation function that we built between our NIR and optical light curves (Fig. 6) does not show many similarities with the one reported in Shahbaz et al (2018). In particular, we observe, for the portion of the light curve where NIR flares are detected (red curve in Fig.…”

“…In particular, we observe, for the portion of the light curve where NIR flares are detected (red curve in Fig. 6), a narrow, positive lag of the NIR with respect to the optical at 7.0 ± 1.3s, which is way higher (almost double) with respect to the one observed in Shahbaz et al (2018). Moreover, we do not observe in this CCF any broad correlation or anti-correlation.…”

“…In fact, according to that analysis, the expected flux difference between the active and passive mode is supposed to vary depending on the wavelength of the observation, being more pronounced in the r ′ -band (i.e. the band in which Shahbaz et al 2018 observed) than in our B-band. In particular, we should expect a variation of ∼ 0.1mJy in the B-band, that is comparable with the dimension of the error bars in our light curve (see Fig.…”

“…Optical, phase-resolved linear polarization measurements were also performed (Baglio et al 2016a;Hakala & Kajava 2018), revealing a degree of polarization of a few per cent which is better interpreted with being due to Thomson scattering from the accretion disc than to synchrotron emission from a jet. Strong flickering and flaring activity was also observed in the near-infrared (Hakala & Kajava 2018;Shahbaz et al 2018), and interpreted as due to reprocessing of the optical emission and to a combination of reprocessing and emission from plasmoids in the accretion flow in the near infrared. In addition, Kepler monitoring found a much stronger flaring activity in the optical than in the X-ray band (see Papitto et al 2018).…”