Flares from a candidate Galactic magnetar suggest a missing link to dim isolated neutron stars

Abstract

Magnetars are young neutron stars with very strong magnetic fields of the order of 10(14)-10(15) G. They are detected in our Galaxy either as soft gamma-ray repeaters or anomalous X-ray pulsars. Soft gamma-ray repeaters are a rare type of gamma-ray transient sources that are occasionally detected as bursters in the high-energy sky. No optical counterpart to the gamma-ray flares or the quiescent source has yet been identified. Here we report multi-wavelength observations of a puzzling source, SWIFT J195509+261406. We detected more than 40 flaring episodes in the optical band over a time span of three days, and a faint infrared flare 11 days later, after which the source returned to quiescence. Our radio observations confirm a Galactic nature and establish a lower distance limit of approximately 3.7 kpc. We suggest that SWIFT J195509+261406 could be an isolated magnetar whose bursting activity has been detected at optical wavelengths, and for which the long-term X-ray emission is short-lived. In this case, a new manifestation of magnetar activity has been recorded and we can consider SWIFT J195509+261406 to be a link between the 'persistent' soft gamma-ray repeaters/anomalous X-ray pulsars and dim isolated neutron stars.

A Radio-Selected Sample of Gamma-Ray Burst Afterglows

“…Among the 33 SHBs, 28 were detected by Swift alone. We also have one candidate galactic transient in our sample (GRB 070610; Kasliwal et al 2008;Castro-Tirado et al 2008).…”

TWO RAPIDLY VARIABLE GALACTIC X-RAY TRANSIENTS OBSERVED WITHCHANDRA,XMM-NEWTON, ANDSUZAKU

“…However, some rapidly variable X-ray transient behavior may be produced by other systems, such as symbiotic stars (with white dwarf accretors, e.g., Smith et al 2008), symbiotic X-ray binaries (with NS accretors, e.g., Masetti et al 2007), some unusual black hole X-ray binaries (in 't Zand et al 2000), or magnetars (Castro-Tirado et al 2008;Stefanescu et al 2008). Identifying the optical/infrared (IR) counterparts of rapidly variable X-ray transients is important for understanding these systems, and requires accurate positions from focusing X-ray instruments.…”

The unusual gamma-ray burst GRB 101225A explained as a minor body falling onto a neutron star

“…Tight upper limits were also derived in the optical and IR with H > 23, R > 26.0 and i > 24.5 (ref. 89,87). Given this observational framework Rea et al 80 questioned the magnetar interpretation, based on the too low quiescent X-ray flux.…”