Multicolour optical light curves of the companion star to the millisecond pulsar PSR J2051−0827

ABSTRACT We present simultaneous, multicolour optical light curves of the companion star to black-widow pulsar PSR J2051−0827, obtained approximately 10 yr apart using ULTRACAM and HiPERCAM, respectively. The confirm previously reported asymmetry in which leading hemisphere appears be brighter than trailing hemisphere. HiPERCAM curves, however, do not show this asymmetry, demonstrating that whatever mechanism is responsible for it varies on time-scales a decade or less. fit symmetrical with direct-heating model derive system parameters, finding an orbital inclination $55.9^{+4.8}_{-4.1}$ degrees, good agreement radio-eclipse constraints. find half pulsar’s spin-down energy converted luminosity, resulting temperatures ranging from $5150^{+190}_{-190}$ K day side $2750^{+130}_{-150}$ night star. close filling its Roche lobe ($f_{\rm RL} =0.88^{+0.02}_{-0.02}$) has mass $0.039^{+0.010}_{-0.011}$ M⊙, giving mean density $20.24^{+0.59}_{-0.44}$ g cm−3 apsidal motion constant range 0.0036 < k2 0.0047. values are consistent those brown dwarfs, but implies significantly more centrally condensed internal structure typical such objects.