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Anomalous X-ray Pulsars and Soft Gamma-ray Repeaters Sandro Mereghetti INAF - IASF Milano – PowerPoint PPT presentation

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1
Anomalous X-ray Pulsars and Soft Gamma-ray
Repeaters Sandro Mereghetti INAF - IASF Milano
2
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
3
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
4
AXP in the context Accreting pulsars
Most accreting pulsars are in massive binaries
5
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6
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
7
AXP have very soft X-ray spectra
AXP
8
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
9
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10
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)
11
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.
12
  • MAGNETIC ENERGY - field decay
    - enhanced thermal emission

13
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)
14
Magnetar model (Thompson and Duncan)
Emission powered by magnetic field decay and/or
enhanced cooling
15
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)

16
SGRs vs. GRBs
Durations
Spectra
Courtesy K. Hurley
17
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

18
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
19
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 !!
20
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21
(Kaspi et al. 2003)
SGR-like activity in the AXP 1E2259586
22
bursts have Lpeak 1036-4 1038 erg/s Change in
pulse morphology Glitch ???? 4
10-6 (Kaspi et al. 2003)
23
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
24
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
25
First evidence for significant variability in 1E
1048-59
26
The pulsed fraction decreased while the flux
increased
Spectrum did not vary BBPL kT0.6 keV ? 3
27
4U 014261 SAX (Israel et al 1999)
Power law photon index 3.9
Blackbody kT 0.4 keV
28
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
29
Are the two spectral components related to
distinct emitting regions and/or physical
processes ?
30
small energy dependence of pulsed fraction
requires ad hoc tuning of the BB and PL components
31
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
32
The pulsed fraction decreased while the flux
increased
33
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
34
  • 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

35
24 bursts from SGR 1806-20 have been detected
with the INTEGRAL Burst Alert System.
36
GCDE Bursts
37
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
38
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

39
INTEGRAL Log N- Log P (Peak Flux distribution)
INTEGRAL Log N- Log S (Fluence distribution)
40
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

41
IBIS (20-40 keV)
(INTEGRAL CP data 1 Msec, courtesy Ada Paizis)
42
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)
43
  • 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)
44
Spectral Evolution of weak bursts with INTEGRAL
15-40 keV
40-100 keV
Götz et al., 2004, AA submitted
45
Hardness-Intensity Anticorrelation with
INTEGRAL (bursts with more than 200 net counts)
Götz et al., 2004, AA submitted
46
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
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