AXP 1E 1547.0-5408 - a magnetar observed through the shields
1E1547.0-5408 is one of only 9 confirmed Anomalous X-ray Pulsars (AXP) -
isolated, young neutron stars with unusually strong magnetic fields
(10^14G - 10^15G). Together with Soft Gamma Repeaters they make up a class
of celestial object known as magnetars. It has exhibited a small number
of outbursts over the past few years but at relatively weak levels.
On January 22, 2009, extremely strong flaring activity of this source
was detected by different satellites. The Anti-Coincidence Shield (ACS)
of the Integral spectrometer SPI alone detected more than 200 bursts
within a few hours. The upper panel of the image shows SPI ACS lightcurve
(50 ms time bins during the most active period (Fig. 1 of Mereghetti et al.,
2009, ApJL accepted,
arXiv:0903.1974 ).
The flux of the soft gamma ray photons produced by the source was
so strong that it penetrated through the walls of the IBIS telescope
and produced a strong signal in the ISGRI detector, sufficient to even
saturate the telemetry flow for short intervals. The lower panel shows
a comparison of the lightcurves of the strongest bursts from the source
as seen by the SPI/ACS and ISGRI detectors (Savchenko et al., 2009, A&A
submitted). Red solid and black dotted lines show the ISGRI lightcurves in,
respectively, 20-60 keV and 60-200 keV energy bands. The blue dotted line
shows the lightcurves of the same bursts detected by the SPI/ACS detector.
The bright short hard spikes of the gamma-ray emission, which are,
most probably, produced by the fractures of the crust of the neutron
star with extremely strong magnetic field (2x10^14 G) or by the
reconnection of magnetic field lines, are followed by the softer
"afterglows", which occasionally exhibit pulsations with the neutron
star's spin period.
This is a rare example of the situation when having the source outside
the field of view of IBIS telescope is an advantage, from the viewpoint
of the data analysis. If the source would be inside the FoV, the absence
of suppression of the signal by the walls of the telescope would lead to
immediate telemetry saturation, so that even the "piece-wise" information
about the evolution of the source spectrum would not be available.
Besides these serendipitous bursts, Integral also obtained a lot of
information about the source and bursts through
a series of dedicated pointed observations
from 25 January onwards. The analysis of these
observations is ongoing.
