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[Abridged] Classical novae (CNe) represent the main class of supersoft X-raysources (SSSs) in the central region of our neighbouring galaxy M 31. Onlythree confirmed novae and three SSSs have been discovered in globular clusters(GCs) of any galaxy so far, of which one nova and two SSSs (including the nova)were found in M 31 GCs. To study the SSS state of CNe we carried out ahigh-cadence X-ray monitoring of the M 31 central area with XMM-Newton andChandra. We analysed X-ray and optical data of a new transient X-ray source inthe M 31 GC Bol 126, discovered serendipitously in Swift observations. Ouroptical data set was based on regular M 31 monitoring programmes from fivedifferent small telescopes. Additionally, we made use of Pan-STARRS 1 dataobtained during the PAndromeda survey. Our observations reveal that the X-raysource in Bol 126 is the third SSS in an M 31 GC and can be confirmed as thesecond CN in the M 31 GC system. This nova is named M31N 2010-10f. Itsproperties in the X-ray and optical regimes agree with a massive white dwarf(M_WD >~ 1.3 M_sun) in the binary system. Incorporating the data on previouslyfound (suspected) novae in M 31 GCs we used our high-cadence X-ray monitoringobservations to estimate a tentative nova rate in the M 31 GC system of 0.05/yr/GC. An optical estimate, based on the recent 10.5-year WeCAPP survey, givesa lower nova rate, which is compatible with the X-ray rate on the 95%confidence level. There is growing evidence that the nova rate in GCs is higherthan expected from primordial binary formation and under conditions as in thefield. Dynamical binary formation and/or additional accretion from theintracluster medium are possible scenarios for an increased nova rate, butobservational confirmation for this enhancement has been absent, so far.Regular X-ray monitoring observations of M 31 provide a promising strategy tofind these novae.

Supersoft X-rays reveal a classical nova in the M 31 globular cluster Bol 126