AR-2019-2020

&KDQJH LQ 0DJQLWXGH U¶ í J¶ X¶ í í í í í í í 7LPH 2IIVHW )URP $SH[ V &RXQWV 5HODWLYH WR 0HGLDQ í í í Figure 17: The top panel shows the superposition of twelve dips in the optical, in three bands - r (red), g (green) and u (blue). Note that how the blue light dips much less than the red. Also on average, the dips have a V-shaped profile, reminiscent of partially-eclipsing systems. The bottom panel shows the average X-ray response - the dashed horizontal lines show the 5-95% significance lines. Further, how the lightcurve rarely strays out of this region across the optical dip, confirming a lack of X-ray response. A black hole X-ray binary at ∼ 100 Hz: Multiwavelength timing of MAXI J1820+070 with HiPERCAM and NICER In 2018, a new black hole was discovered in our galaxy, briefly flaring to become the brightest X-ray source in the sky after the Sun. Capitalising on the opportunity, John A. Paice , Poshak Gandhi, Ranjeev Misra and collaborators studied the source (known as MAXI J1820+070, or J1820) in both optical (HiPERCAM/GTC, La Palma) and X-ray (NICER, ISS) light at over 300 frames per second. What they saw was a complex source that emitted red-coloured flares over a broad range of timescales, down to 10ms long. By looking at how the two wavelengths related to each other, it was found that while the two bands were mostly anti-correlated (a rise in X-rays would lead to a decline in optical), there was also a correlated optical lag at roughly 170ms (so an X-ray flare would lead to an optical flare after that delay). Such behaviour, it is being found, is fairly normal for black holes - but close analysis of the optical lag found it differ, increasing with wavelength; so, following an X-ray flare, blue light would flare before green, and both would flare before red, at a time difference just visible to the telescope (see Fig. 18). Such a result has never been seen before; the authors have postulated that it could be the result of waves of plasma colliding within a jet, with higher-energy blue and green light being emitted closer to the black hole itself, before the lower-energy red light is emitted further away. If so, this could be the next step in understanding jet physics, and the phenomena around these enigmatic and violent sources. Determination of log-normal flux distributions for astrophysical systems Determining whether the flux distribution of an astrophysical source is a Gaussian or a log-normal, provides key insight into the nature of its variability. For example, the light curve produced by an additive sequence of many independent emission components results in a normal distribution of flux. However, if the underlying physical process produces random emission components in a multiplica- tive sequence, then resulting time series distribution would be log-normal. Zahir Shah , Ranjeev Misra and Atreyee Sinha carried extensive simulations of lightcurves with different lengths, vari-