Last summer, NASA's Swift satellite was doing its thing when it picked up γ-ray burst GRB 070610. The first thought that would go through an astronomer's mind is that a star died somewhere in the far reaches of the universe. However, in this case, some strangeness followed. A pair of papers appearing in this week's edition of Nature report on what came next and what astronomers believe was behind those observations.
The initial burst of GRB 070610 came with a "bizarre X-ray counterpart," and within a minute, researchers began collecting data on this in various other wavelengths. Over the next three days, they observed a series of around 40 flareups in the optical region of the electromagnetic spectrum, which is not something typically associated with high-energy star deaths. Renamed SWIFT J195509+1261406, the event went quiet over the next few days until a single near-IR flare was observed by the ESO's Very Large Telescope. Almost six months later, no further bursts were measured by the XMM-Newton spacecraft.
Top row: optical view of one of the 40 flare ups over the course of a few seconds.
Bottom row: Similar observation, but in the near-IR region of the spectrum
Image Credit: A. de Ugarte Postigo, A. J. Castro-Tirado et al.
Analysis of the energy spectrum from the events showed that this was not the result of a far off star dying, but, in fact, happened in our own backyard. SWIFT J1955 is approximately 3.7 kpc (15,000 lightyears) away from Earth, within the Milky Way. One of the two papers suggests four possibilities for the behavior of SWIFT J1955. First, they suggest that it could be a 'bursting pulsar,' although the lack of subsequent γ-ray bursts from the source make this highly unlikely. A second possibility is that this is a black hole, but again, the flux measurements and lack of γ- and radio-waves make that an unlikely scenario. The third possibility is that SWIFT J1955 is an "an ultra-compact low-mass X-ray binary" surrounded by a few kilometers of plasma—this could explain the flux measurements during quiet periods and the X-ray and optical outbursts that were seen.
Artist's rendering of a magnetar
Image Credit: ESO/L.Calcada
Finally, the authors postulate that the SWIFT source could be an isolated compact object known as a magnetar. Magnetars are young neutron stars with staggeringly strong magnetic fields, typically on the order of 1014 to 1015 Gauss—Earth's magnetic field, for reference, varies from about 0.3 Gauss to 0.6 Gauss. As co-author Antonio de Ugarte Postigo said in an interview, "A magnetar would wipe the information from all credit cards on Earth from a distance halfway to the Moon." This type of object would explain all the data seen by this group of researchers. A second group, whose paper is also in this week's Nature, measured the optical light curve of the event and also concluded that it was derived from an optical magnetar.
If observations of SWIFT J195509+1261406 can be made in both the optical and X-ray spectrums simultaneously, they could shed further light onto the nature of the object and may be capable of distinguishing between the hypotheses that it's either a low mass binary system or an optical magnetar. However, since the initial observations its final near-IR burp 11 days later, SWIFT J1955 has gone quiet and there is no telling when it will flare up again.