Dips and eclipses in the X-ray binary Swift J1858.6–0814 observed with NICER

Abstract

We present the discovery of eclipses in the X-ray light curves of the X-ray binary Swift J1858.6–0814. From these, we find an orbital period of $P=76841.3_{-1.4}^{+1.3}$ s (≈21.3 hours) and an eclipse duration of $t_{\rm ec}=4098_{-18}^{+17}$ s (≈1.14 hours). We also find several absorption dips during the pre-eclipse phase. From the eclipse duration to orbital period ratio, the inclination of the binary orbit is constrained to i > 70○. The most likely range for the companion mass suggests that the inclination is likely to be closer to this value than 90○. The eclipses are also consistent with earlier data, in which strong variability (‘flares’) and the long orbital period prevent clear detection of the period or eclipses. We also find that the bright flares occurred preferentially in the post-eclipse phase of the orbit, likely due to increased thickness at the disc-accretion stream interface preventing flares being visible during the pre-eclipse phase. This supports the notion that variable obscuration is responsible for the unusually strong variability in Swift J1858.6–0814.

A persistent ultraviolet outflow from an accreting neutron star binary transient

“…The X-ray, far-UV and optical variability are clearly correlated, with any lags between these time series being 3 s (Vincentelli et al in prep). In agreement with previously reported X-ray light curves 13,14 , there is no evidence of thermonuclear runaway events during the campaign. This suggests that the multi-wavelength flaring is driven by a variable central X-ray source.…”

“…J1858 displayed extreme variability during its outburst in all energy bands, with the X-ray luminosity changing by 1-2 orders of magnitude on time-scales of seconds (see Figure 1-a) 11,14 . The X-ray spectrum consisted of a heavily absorbed thermal accretion disc component plus a very shallow non-thermal power law tail (photon flux N ph (E) ∝ E −Γ , with Γ < 1) 15 .…”

A persistent ultraviolet outflow from the accretion disc in a transient neutron star binary

“…In line with data obtained at other wavelengths 10,13,23 , the far-UV light curve exhibits dramatic flaring activity (Figure 1-b).…”

“…The disc in Swift J1858.6−0814 is certainly large enough to drive such an outflow. The orbital period of the system is P orb ≃ 21.3 h 13 . From Kepler's third law, and assuming that q = M 2 /M 1 1, the binary separation is a bin ≃ 3 × 10 11 cm.…”

“…The system was also found to undergo eclipses, implying a nearly edge-on viewing angle with respect to the disc (i 70 • ) and revealing the orbital period to be P orb ≃ 21.3 h 13 . J1858 displayed extreme variability during its outburst in all energy bands, with the X-ray luminosity changing by 1-2 orders of magnitude on time-scales of seconds (see Figure 1-a) [13][14][15][16] . The X-ray spectrum consisted of a heavily absorbed thermal accretion disc component plus a very steep non-thermal power law tail (photon flux N ph (E) ∝ E −Γ , with Γ < 1) 17,18 .…”