A ‘superoutburst’ in XTEJ1118+480

I propose that the properties of the two outbursts observed in the X‐ray transient XTEJ1118+480 in 2000 are akin to superoutbursts of SU UMa stars. In these systems a ‘normal’ outburst immediately precedes a 5–10 times longer (‘super’) outburst. The optical light curve of the outbursts of XTEJ1118+480 is remarkably similar to that seen in some SU UMa stars, such as UVPer and TLeo, where the precursor outburst is distinct from the superoutburst, but the time scales are a factor of ∼15 different. The first outburst of XTEJ1118+480 was relatively short (∼1 month) while the second outburst was ∼5 times longer. During the second outburst superhumps were seen, a feature characteristic for superoutbursts. The gap of about a month between the two outbursts is longer in X‐rays with respect to the optical, a feature not previously recognized for X‐ray transients. Also in SU UMa stars the precursor outburst becomes more distinct at shorter wavelengths. Finally, I show that the time of appearance of the superhumps in XTEJ1118+480 is consistent with the expected superhump growth time, if the superhump mechanism was triggered during the first outburst. I conclude that the similarity in outburst behaviour in the two types of systems provides further support that a common mechanism is at work to start the long (‘super’) outbursts.