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Keywords:

  • gamma-ray burst: general;
  • stars: neutron;
  • supernovae: general

ABSTRACT

Swift recently discovered an unusual gamma-ray and X-ray transient (Swift 1644+57) that was initially identified as a long-duration gamma-ray burst (GRB). However, the ∼10 keV X-ray emission has persisted for over approximately a month with a luminosity comparable to its peak value. The astrometric coincidence of the source with the centre of its host galaxy, together with other considerations, motivated the interpretation that Swift 1644+57 was produced by an outburst from a ∼106–107 M black hole at the centre of the galaxy. Here we consider the alternate possibility that Swift 1644+57 is indeed a long-duration GRB, albeit a particularly long one! We discuss the general properties of very long-duration, low-power GRB-like transients associated with the core-collapse of a massive star. Both neutron star (magnetar) spin-down and black hole accretion can power such events. The requirements for producing low-power, very long duration GRBs by magnetar spin-down are similar to those for powering extremely luminous supernovae by magnetar spin-down, suggesting a possible connection between these two unusual types of transients. Alternatively, Swift 1644+57 could be associated with the faintest core-collapse explosions: the collapse of a rotating red supergiant in a nominally failed supernova can power accretion on to a solar-mass black hole for up to ∼100 d; the jet produced by black hole accretion inevitably unbinds the outer envelope of the progenitor, leading to a weak ∼1049 erg explosion. In both neutron-star and black hole models, a jet can burrow through the host star in a few days, with a kinetic luminosity ∼1045–1046 erg s−1, sufficient to power the observed emission of Swift 1644+57.