X-ray identification of the soft γ-ray repeater 1806 – 20

Detection of a radio counterpart to the 27 December 2004 giant flare from SGR 1806–20

It was established over a decade ago that the remarkable high-energy transients known as soft gamma-ray repeaters (SGRs) are located in our Galaxy and originate from neutron stars with intense (< or = 10(15)G) magnetic fields-so-called 'magnetars'. On 27 December 2004, a giant flare with a fluence exceeding 0.3 erg cm(-2) was detected from SGR 1806-20. Here we report the detection of a fading radio counterpart to this event. We began a monitoring programme from 0.2 to 250 GHz and obtained a high-resolution 21-cm radio spectrum that traces the intervening interstellar neutral hydrogen clouds. Analysis of the spectrum yields the first direct distance measurement of SGR 1806-20: the source is located at a distance greater than 6.4 kpc and we argue that it is nearer than 9.8 kpc. If correct, our distance estimate lowers the total energy of the explosion and relaxes the demands on theoretical models. The energetics and the rapid decay of the radio source are not compatible with the afterglow model that is usually invoked for gamma-ray bursts. Instead, we suggest that the rapidly decaying radio emission arises from the debris ejected during the explosion.

The Distance to the Soft Gamma Repeater SGR 1627-41

We report millimeter observations of the line of sight to the recently discovered soft gamma repeater SGR 1627-41, which has been tentatively associated with the supernova remnant (SNR) G337.0-0.1. Among the eight molecular clouds along the line of sight to SGR 1627-41, we show that SNR G337.0-0.1 is probably interacting with one of the most massive giant molecular clouds (GMCs) in the Galaxy, at a distance of 11 kpc from the Sun. Based on the high extinction to the persistent X-ray counterpart of SGR 1627-41, we present evidence for an association of this new soft gamma repeater (SGR) with the SNR G337.0-0.1; they both appear to be located on the near side of the GMC. This is the second SGR located near an extraordinarily massive GMC. We suggest that SGR 1627-41 is a neutron star with a high transverse velocity ( approximately 1000 km s-1) escaping the young ( approximately 5000 yr) SNR G337.0-0.1.

Magnetar Spin-Down

We examine the effects of a relativistic wind on the spin-down of a neutron star and apply our results to the study of soft gamma repeaters (SGRs), which are thought to be neutron stars with magnetic fields greater than 1014 G. We derive a spin-down formula that includes torques from both dipole radiation and episodic or continuous particle winds. We find that if SGR 1806-20 puts out a continuous particle wind of 1037 ergs s-1, then the pulsar age is consistent with that of the supernova remnant, but the derived surface dipole magnetic field is only 3x1013 G, in the range of normal radio pulsars. If instead the particle wind flows are episodic with small duty cycle, then the observed period derivatives imply magnetar-strength fields, while still allowing characteristic ages within a factor of 2 of the estimated supernova remnant age. Close monitoring of the periods of SGRs will allow us to establish or place limits on the wind duty cycle and thus the magnetic field and age of the neutron star.