An X-ray pulsar with a superstrong magnetic field in the soft γ-ray repeater SGR1806 − 20

Abstract

Soft ␥-ray repeaters (SGRs) emit multiple, brief (ϳ0.1-s), intense outbursts of low-energy ␥-rays. They are extremely rare 1 -three 2-4 are known in our Galaxy and one 5 in the Large Magellanic Cloud. Two SGRs are associated 5-7 with young supernova remnants (SNRs), and therefore most probably with neutron stars, but it remains a puzzle why SGRs are so different from 'normal' radio pulsars. Here we report the discovery of pulsations in the persistent X-ray flux of SGR1806 ؊ 20, with a period of 7.47 s and a spindown rate of 2:6 ؋ 10 ؊ 3 s yr ؊ 1 . We argue that the spindown is due to magnetic dipole emission and find that the pulsar age and (dipolar) magnetic field strength are ϳ1,500 years and 8 ؋ 10 14 gauss, respectively. Our observations demonstrate the existence of 'magnetars' , neutron stars with magnetic fields about 100 times stronger than those of radio pulsars, and support earlier suggestions 8,9 that SGR bursts are caused by neutron-star 'crustquakes' produced by magnetic stresses. The 'magnetar' birth rate is about one per millennium-a substantial fraction of that of radio pulsars. Thus our results may explain why some SNRs have no radio pulsars.SGR1806 Ϫ 20 became extremely active between October 1996 and November 1997, when over 40 intense bursts and numerous weaker ones were detected 10 with the Burst And Transient Source Experiment (BATSE) on board the Compton Gamma-Ray Observatory (CGRO). We observed SGR1806 Ϫ 20 with the Rossi X-Ray Timing Explorer (RXTE) five times between 5 and 18 November 1996, starting five days after the first triggered burst detection with BATSE. (Information on the archival data from RXTE/PCA and ASCA is available at http://heasarc.gsfc.nasa.gov.) During these observations 11 , the source emitted series of outbursts in a 'bunching' mode, never seen before. The intensity of the outbursts, as well as the 'bunching' mode, varied significantly: mini-outbursts were interlaced with very intense ones and the rate of bursts varied from bunch to bunch (S. Dieters et al., manuscript in preparation).We made a period search of the data after excluding all bursts from the time series. The data were then energy-selected for 2-24 keV X-rays, background subtracted and binned at 0.5-s resolution. The resulting light curve was searched for periodicities between 0.03 and 1 Hz, by calculating a fast-Fourier-transform power spectrum (Fig. 1). The peaks in the spectrum are centred on the fundamental frequency of 0.13375 Hz (period of 7.47655 s) and its first harmonic at 0.26750 Hz. We find no significant power in any other frequency in the searched range. The probability that we detect a signal at the fundamental frequency this strong by chance coincidence is 1 ϫ 10 Ϫ 13 (taking into account the number of trials, 1:9 ϫ 10 6 , and the probability per trial, 5 ϫ 10 Ϫ 20 ).To determine the fundamental period, all data sets were then corrected to the Solar System barycentre and separately folded at the longest detected period of 7.47655 s, and sub-harmonics thereof. These sub-harmonic folds showed...

XMM–Newton observations of soft gamma-ray repeaters

“…Unfortunately the source faintness did not allow a sensitive search for periodic pulsations. Both a steep power law (photon index Γ = 3.7 ± 0.5) and a blackbody with temperature 1983-1985, 1996, 1979Mar, 1992, 1998, 1979Mar-Apr, 2003, 20061981Dec-1983 Apr R:…”

“…Subsequent observations with RossiXTE led to the discovery of pulsations with P=7.5 s andṖ=8×10 −11 s s −1 (Kouveliotou et al 1998). Possible associations of SGR 1806−20 with the variable non-thermal core of a putative radio supernova remnant (Frail et al 1997) and with a luminous blue variable star (van Kerkwijk et al 1995) were disproved when a more precise localization of the SGR could be obtained with the Interplanetary Network (Hurley et al 1999b) and later improved with Chandra (Kaplan et al 2002).…”

Galactic evolution of rapid neutron capture process abundances: the inhomogeneous approach

“…The binary merger rates are estimated by van den Heuvel & Lorimer (1996) as well as Kalogera et al (2004). The rate of Jet-SNe is related to the fact that about 1% of neutron stars are found with magnetic fields of the order 10 15 Gauss (magnetars, see, e.g., Kouveliotou et al 1998, Kramer et al 2009). …”

Discovery and monitoring of the likely IR counterpart of SGR 1806–20 during the 2004 γ-ray burst-active state

“…The detection of a ∼8 s periodicity in the decaying tail of a very intense (∼10 44 erg) and long (several minutes) event, known as giant flare, from SGR 0526-66 on 1979 March 5th (Mazets et al 1979) suggested the association of SGRs with neutron stars. A small sample of peculiar X-ray pulsars, namely the Anomalous X-ray Pulsars (AXPs) has been proposed to be closely related to SGRs based on similar properties, namely their period P (in the 5-12 s range), their period derivativeṖ (10 −10 -10 −13 s s −1 range), and X-ray bursts (Kouveliotou et al 1998;Kaspi et al 2003;Gavriil et al 2002).…”