INTEGRAL observes Soft Gamma-ray Repeaters as persistent hard X-ray sources
Soft Gamma-ray Repeaters are famous for their spectacular bursting and
flaring activity in the hard X-ray/soft gamma-ray range, but persistent
emission above 20 keV was never detected from SGRs before INTEGRAL.
Deep observations with IBIS showed that the two SGRs that have been
active in the last couple of years (SGR 1806-20 and SGR 1900+14) have
also a persistent hard emission extending up to 100 keV, with a
luminosity of about 10^36 erg/s in the 20-100 keV range.
Soft Gamma-ray Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs) are
believed to be magnetars, i.e. slowly rotating (P~5-10 s) neutron
stars powered by extremely high magnetic fields (B~10^15 G).
An important discovery was the fact that the AXP 1E 1841-045 showed
pulsed emission up to ~150 keV (Kuiper, Hermsen, Mendez, 2004, ApJ,
613, 1173). More recently it was found from INTEGRAL data that 4 AXPs
have hard spectra extending above ~10 keV. The details of the hard
tails are presented by Kuiper et al. (ApJ accepted,
astro-ph/603467,
see also the ESA Science News Release:
"INTEGRAL catches stellar 'corpses' by the tail").
The figure (
Goetz et al. 2006, A&A, 449, L31; astro-ph/0602359)
shows the broad band spectra of five magnetars (SGRs and AXPs)
observed with INTEGRAL. IBIS data are shown in red with the best fit
power law model in dashed blue. The black line represents the total
X-ray spectrum observed below 10 keV, which can be decomposed in a
black body component (light blue) and a power law (green).
While the AXPs spectra are characterized by soft spectra at low energy
and a hardening above 10-20 keV, the SGRs show the opposite behaviour.
The difference between the two SGRs, with SGR 1900+14 much softer than
SGR 1806-20, could be due to the different activity level of the two
sources, with the former being in quiescence from November 2002 up to
March 2006 and the latter in a moderately active state.
