Magnetar outbursts: an observational review

TL;DR

This paper reviews observational data on magnetar outbursts, highlighting their multi-band emission, spectral evolution, and decay behaviors, and summarizes all known transient events to date.

Contribution

It provides a comprehensive observational review of magnetar outbursts, including detailed summaries of all known transient events and their multi-band emission characteristics.

Findings

Magnetar outbursts involve emission from radio to hard X-ray bands.

Outbursts cause a 10-1000 fold increase in soft X-ray flux.

Decay times vary from weeks to years with different decay laws.

Abstract

Transient outbursts from magnetars have shown to be a key property of their emission, and one of the main way to discover new sources of this class. From the discovery of the first transient event around 2003, we now count about a dozen of outbursts, which increased the number of these strongly magnetic neutron stars by a third in six years. Magnetar outbursts might involve their multi-band emission resulting in an increased activity from radio to hard X-ray, usually with a soft X-ray flux increasing by a factor of 10-1000 with respect to the quiescent level. A connected X-ray spectral evolution is also often observed, with a spectral softening during the outburst decay. The flux decay times vary a lot from source to source, ranging from a few weeks to several years, as also the decay law which can be exponential-like, a power-law or even multiple power-laws can be required to model the flux decrease. We review here on the latest observational results on the multi-band emission of magnetars, and summarize one by one all the transient events which could be studied to date from these sources.