PowerPoint-Pr

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Anomalous X-ray Pulsars and Soft Gamma-ray
Repeaters Sandro Mereghetti INAF - IASF Milano
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OUTLINE
Short review of AXP and SGR properties AXP /
SGR relationship 2 recent results XMM-Newton
gt Variability in AXP 1E1048 INTEGRAL
gt Spectral evolution in bursts from SGR 1806
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NORMAL x-ray pulsars are rotating magnetized
neutron stars
1) In binary systems powered by accretion from a
companion star e.g. Vela X-1, Cen X-3
Periods from 60 ms to a few 1000 s
2) rotation powered radio pulsars e.g. Crab
, Geminga, PSR 195720 Periods from 1.5 ms -
a few seconds
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AXP in the context Accreting pulsars
Most accreting pulsars are in massive binaries
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AXP PROPERTIES
No evidence for massive companion stars
limits on ax sin i from timing limits on
Fx/Fopt from optical/IR observations Period of
a few seconds (6-12 s) Almost steady spin
down Very soft X-ray spectrum kTBB lt 0.5
keV ? ph gt 3-4
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AXP have very soft X-ray spectra
AXP
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AXP PROPERTIES
2 ( or 3 ? ) are in SNRs X-ray
luminosity Lx 1034 - 1036 erg s-1 Lx
gt rotational energy loss for a neutron star
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Operational definition of AXP
a spinning down pulsar, with a soft X-ray
spectrum, apparently not powered by accretion
from a (massive) companion star, and with
luminosity larger than the rotational energy loss
(assuming a neutron star)
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AXP census
P dP/dt (s) (10-11 s/s) 4U
014261 8.7 0.2 - 1E 2259586 7 0.05 CTB
109 1E 1048-5937 6.4 2-3 - 1E
1841-045 11.8 4 Kes 73 AX J1845-03 7 - G296
0.1, Var. RXS 1708-40 11 2 - CXO
J0110-72 8 2 in SMC XTE J1810-197 5.5 1.8 -
Var.
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• MAGNETIC ENERGY - field decay
  - enhanced thermal emission

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Isolated NS accretion disk Thorne-Zytkow
object (Van Paradijs et al.
1995, Ghosh et al. 1997) fall back after SN
explosion (Chatterjiee et al. 2000, Alpar 2001)
capture of SNR ejecta by fast moving
NS (Marsden et al. 2000, 2001)
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Magnetar model (Thompson and Duncan)
Emission powered by magnetic field decay and/or
enhanced cooling
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Soft Gamma-ray Repeaters

• Initially discovered as a peculiar class of
  Gamma-Ray Bursts
• soft
• repeating
• About 5 currently known (1 in the LMC)
• Not always active (long quiescent periods)

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SGRs vs. GRBs
Durations
Spectra
Courtesy K. Hurley
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Energetics of SGRs

• Short Bursts
• Peak Luminosity 1038-1042 erg s-1
• Total Energy 1039-1042 erg
• Giant Flares
• Peak Luminosity 4 x 1044 erg s-1
• Total Energy 0.7-2 x 1044 erg

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Giant Flares
1998 August 27 from SGR 190014
1979 March 5 from SGR 0526-66
Feroci et al. 1999
Mazets et al. 1979, Cline et al. 1980
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Persistent X-ray emission from SGRs

• Lx 1035 -1036 erg /s (1-10 keV)
• Pulsations with periods 5 - 10 s
• secular spin-down at 10-11 s/s
• power law ( blackbody) spectra

VERY SIMILAR TO AXPs !!
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(Kaspi et al. 2003)
SGR-like activity in the AXP 1E2259586
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bursts have Lpeak 1036-4 1038 erg/s Change in
pulse morphology Glitch ???? 4
10-6 (Kaspi et al. 2003)
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AXPSGR ?
Only observational selection effects introduced a
distinction between these sources belonging to
the same class of objects in AXP the quiescent,
pulsating emission was discovered first SGR
were discovered through their bursts
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OUTLINE
Short review of AXP and SGR properties AXP /
SGR relationship 2 recent results XMM-Newton
gt Variability in AXP 1E1048 INTEGRAL
gt Spectral evolution in bursts from SGR 1806
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First evidence for significant variability in 1E
1048-59
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The pulsed fraction decreased while the flux
increased
Spectrum did not vary BBPL kT0.6 keV ? 3
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4U 014261 SAX (Israel et al 1999)
Power law photon index 3.9
Blackbody kT 0.4 keV
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Most AXP require 2 component model PL BB
Phot.index kTBB RBB NH 1022 1E
1048-59 2.9 0.63 keV 0.4 d3 km
1.0 4U014261 3.9 0.40 keV 1.8 d1 km
1.1 1E 225958 3.6 0.41 keV 2.6 d4 km
0.9 RXS1708-40 2.6 0.46 keV 7.9 d8 km
1.4 AXJ1845-00 - 0.64 keV 3.9 d15 km
6 1E 1841-0045 3.0 - - 2
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Are the two spectral components related to
distinct emitting regions and/or physical
processes ?
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small energy dependence of pulsed fraction
requires ad hoc tuning of the BB and PL components
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Despite the large flux variation the spectral
shape did not vary BBPL in both observations
kT 0.6 keV phot. Index 3 ... these are
the typical parameters seen in this source
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The pulsed fraction decreased while the flux
increased
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OUTLINE
Short review of AXP and SGR properties AXP /
SGR relationship 2 recent results XMM-Newton
gt Variability in AXP 1E1048 INTEGRAL
gt Spectral evolution in bursts from SGR 1806
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• SGR1806-20 entered a new period of activity in
  July 2003
• An INTEGRAL ToO observation started on 3
  September 2003, while the source was still active
  
• INTEGRAL continued to observe SGR 1806-20 (l
  9.99 deg, b -0.24 deg) during the Galactic
  Center Deep Exposre (GCDE) until mid October
• 24 bursts were detected by IBIS in real time by
  the INTEGRAL Burst Alert System (IBAS) and
  confirmed later by off-line analysis

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24 bursts from SGR 1806-20 have been detected
with the INTEGRAL Burst Alert System.
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GCDE Bursts
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3-20 keV
JEM-X
Yoff -0.97º Zoff -2.22º
15-40 keV
40-100 keV
IBIS/ISGRI
Fluence (15-100 keV) 2.510-8 erg cm-2
100-200 keV
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Spectral Analysis

• 15-100 keV IBIS/ISGRI spectra of the bursts with
  more than 500 net counts
• Optically Thin Thermal Bremsstrahlung model
  provides good fits (power-law, blackbody, Band
  GRB model are ruled out)
• kT 32-42 keV
• Conversion factor (15-100 keV, ltkTgt 38 keV) 1
  count s-1 1.5x10-10 erg cm-2 s-1

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INTEGRAL Log N- Log P (Peak Flux distribution)
INTEGRAL Log N- Log S (Fluence distribution)
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THE SGR BURSTS OBSERVED BY IBIS ARE NORMAL IN
MOST RESPECTS

• Durations, energy spectra are typical
• However, the fluences are very low,1.5x10-8
  erg/cm2 , 25-100 keV
• These are the among the weakest bursts seen from
  this SGR thanks to imaging, we are certain that
  the source is indeed SGR1806-20

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IBIS (20-40 keV)
(INTEGRAL CP data 1 Msec, courtesy Ada Paizis)
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The MAGNETAR model predictions

• Highly magnetized (B1015 G), slowly rotating
  (P 5-8 s) neutron stars
• Bursts are triggered by a sudden shift in the
  magnetospheric footpoints driven by a fracture in
  the neutron star crust
• The radiation originates from the cooling of an
  optically thick pair-photon plasma

ee- plasma
Thompson Duncan (1995)
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• For typical (0.1 s long) bursts
• No signifcant spectral evolution predicted and in
  general NOT observed up to now (e.g. Fenimore et
  al. 1994, Kouveliotou et al. 1987)

SGR 190014 an exception Two peculiar bursts of
intermediate duration (1 s) and and with hard
(kT100 keV) spectra

• Soft-to-hard evolution

Woods et al. (1999)
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Spectral Evolution of weak bursts with INTEGRAL
15-40 keV
40-100 keV
Götz et al., 2004, AA submitted
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Hardness-Intensity Anticorrelation with
INTEGRAL (bursts with more than 200 net counts)
Götz et al., 2004, AA submitted
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Conclusions

• 1) XMM / EPIC detected the first significant
  variation in the flux and pulsed fraction of the
  AXP 1048the spectral invariance is a further
  evidence that the PLBB spectral decomposition
  does not have a physical meaning
• 2) INTEGRAL / IBIS detected the first evidence
  for spectral evolution of fain SGR bursts as well
  as a hardness intensity anticorrelationthese
  properties are not (yet) foreseen in the
  magnetar model