DARTS of the Month

► DARTS of the Month

Fig. 1: Light curves (count rates of each instrument) around the short burst from SGR 0501+4516. (a) X-ray count rate of the X-ray Imaging Spectrometer (XIS) in the 0.6-10 keV with 2 sec binning. (b) and (c) Count rates of silicon PIN diodes and GSO scintillators of the Hard X-ray Detector (HXD) with 0.5 sec binning, in the 10-50 keV and 50-250 keV energy band, respectively. (d) One-sec count rate of the Wide Band All-sky Monitor (WAM) surrounding the HXD, in the 28-111 keV energy band. (e) and (f) Expanded views of panel (b) and (c), respectively, with 8 msec bin.

Fig. 2: X-ray spectra of a short (~200 m sec) burst from SGR 0501+4516, recorded at 03:16:16.9 UT on 26 August 2008, by XIS-FI, HXD-PIN, and HXD-GSO. The HXD data cover only a precursor phase, because of an instrumental saturation during the main burst.

The Strongest Magnet in the Universe Revealed by Suzaku

Last month we introduced TOO observations by astronomical satellites. This month we show a successful and latest example of TOO discovery.

While the magnetism of our Earth has an intensity of only 0.5 Gauss, neutron stars, which are fast-rotating small (10 km !) stars left after supernovae, have strong magnetic fields up to 10¹² Gauss! More interestingly, a particular kind of neutron stars, called "Magnetars", seems to have much stronger magnetic fields (three orders of magnitude stronger than those of normal neutron stars)! In such a strong magnetic field, photons may spontaneously split or merge. Therefore, these "magnetars" can offer an ideal and cosmic laboratory for the study of extreme physics.

There are two subgroup of magnetars, "Soft Gamma-ray Repeaters (SGRs)" and "Anomalous X-ray Pulsars (AXPs)". Only about 4 SGRs and 9 AXPs are known to date. However, on 21 August 2008 (UT), the Swift satellite discovered a new soft gamma-ray repeater "SGR 0501+4516" for the first time in this century. Our X-ray satellite Suzaku also performed a ToO (Target of Opportunity) observation of this newly discovered star for 40 ks on 26 August. It took only 10 days since this observation until the data became publicly available through DARTS, and the analysis team successfully detected a persistent emission and a pulsation in the 0.5-10 keV energy band. In addition, about 32 short bursts were recorded in this observation.

Some of these bursts were also detected in the high energy band by silicon PIN diodes of the Hard X-ray Detector (HXD). In particular, the biggest burst at 03:16:16.9 (UT) on 26 August was successfully detected by all the instruments onboard Suzaku including HXD-GSO (Figs. 1-2). Although the HXD was saturated during the main burst, it was so sensitive that a precursor of this burst was detected with the HXD up to 200 keV only in ~200 msec.

These "persistent emission", "pulsation" and "burst activity" will become strong clues to clarifying the extreme physics at an ultra-strong magnetized "Magnetars".

Y. Enoto (Univ. of Tokyo) et al.

November 2008