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

Abstract

We report on a near infrared, optical and X-ray simultaneous campaign performed in 2017 with the XMM-Newton and Swift satellites and the HAWK-I instrument mounted on the VLT on the transitional millisecond pulsar PSR J1023+0038. Near infrared observations were performed in fast-photometric mode (0.5s exposure time) in order to detect any fast variation of the flux and correlate them with the optical and X-ray light curves. The optical light curve shows the typical sinusoidal modulation at the system orbital period (4.75hr). No significant flaring or flickering is found in the optical, neither signs of transitions between active and passive states. On the contrary, the near infrared light curve displays a bimodal behaviour, showing strong flares in the first part of the curve, and an almost flat trend in the rest. The X-ray light curves instead show a few low/high mode transitions, but no flaring activity is detected. Interestingly, one of the low/high mode transition is found to happen at the same time as the emission of an infrared flare. This can be interpreted in terms of the emission of an outflow or a jet: the infrared flare could be due to the evolving spectrum of the jet, which possesses a break frequency that moves from higher (near infrared) to lower (radio) frequencies after the launching, that has to happen at the low/high mode transition. We also present the cross correlation function between the optical and near infrared curves. Due to the bimodality of the near infrared curve, we divided it in two parts (flaring and quiet). While the cross correlation function of the quiet part is found to be flat, the one referring to the flaring part shows a narrow peak at ∼10s, which indicates a delay of the near infrared emission with respect to the optical. This lag can be interpreted as reprocessing of the optical emission at the light cylinder radius with a stream of matter spiraling around the system due to a phase of radio-ejection. This strongly supports a different origin of the infrared flares observed for PSR J1023+0038 with respect to the optical and X-ray flaring activity reported in other works on the same source.

Transitional millisecond pulsars

“…Outflows -The bright continuous radio emission [54,135,55] and the obscuration of the disc emission lines at certain orbital phases [46] suggested that tMSPs launch outflows of plasma. The radio emission observed in the high mode is compatible with self-absorbed synchrotron emission from a compact jet, whose spectral break would be beyond the near-infrared band given the low accretion luminosity [128]. Less collimated outflows could also be launched by the propelling magnetosphere [181,182] or by the pulsar wind [56].…”

“…The emergence of an additional polarised component during the flares, possibly due to Thomson scattering from ejected matter, also supported this hypothesis [138]. Flaring variability observed in the near-infrared lagged the optical variability by ∼ 10 s, and was tentatively attributed to the reprocessing of the optical emission produced close to the light cylinder by a stream of matter ejected by the system further out [128].…”

“…A bi-modal distribution of the optical flux was also found from a lower cadence Kepler K-2 monitoring [85]. However, simultaneous optical/X-ray XMM-Newton observations have not revealed such dips in B-band data [83,128], possibly because the optical low modes are energy dependent. The lack of simultaneous detection of optical dips and X-ray low modes has prevented to establish the relationship between these phenomena, so far.…”

Optical photometry of two transitional millisecond pulsars in the radio pulsar state

“…Ambrosino et al (2017) report observations of optical pulsations of J1023 in its AP state, originating from inside the magnetosphere, and the flickering behaviour seen in the AP state is observed to occur simultaneously in optical and X-ray wavelengths (Papitto et al 2019). Near-infrared flaring is also seen (Baglio et al 2019). Lastly, pulsed X-ray and UV emission has been detected (Jaodand et al 2016(Jaodand et al , 2021.…”

UV and X-ray pulse amplitude variability in the transitional millisecond pulsar PSR J1023+0038

“…visible in the X-ray and UV bands and occurs simultaneously (Coti Zelati et al 2018;Papitto et al 2019;Jaodand et al 2021). In the optical and near-infrared bands, the low-high mode transition has never been detected, but the flaring activity is clearly visible and could be related to the emission of collimated jets or outflows (Shahbaz et al 2018;Papitto et al 2019;Baglio et al 2019). The low-high X-ray and UV mode transition is anticorrelated with the radio emission (Bogdanov et al 2018).…”