topics in x-ray astronomy - tuebingen february 2004 s.mereghetti anomalous x-ray pulsars and soft...

Topics in X-ray astronomy - Tuebingen February 2004 S.Mereghetti

Anomalous X-ray Pulsars and

Soft Gamma-ray Repeaters

Sandro MereghettiINAF - IASF Milano


Short review of AXP and SGR properties

AXP / SGR relationship

2 recent results

XMM-Newton > Variability in AXP 1E1048

INTEGRAL > Spectral evolution in bursts from SGR 1806

OUTLINE


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 1957+20Periods from 1.5 ms - a few seconds


Spin Period (s)

Maxim

um

X-r

ay L

um

inosi

ty (

erg

/s) Most accreting

pulsars are inmassive binaries

AXP are onlyfound in the narrow periodrange 5-12 s

AXP in the context: Accreting pulsars


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 < 0.5 keV ph > 3-4

AXP PROPERTIES


AXP

AXP have very soft X-ray spectra


2 ( or 3 ? ) are in SNRs

X-ray luminosity Lx = 1034 - 1036 erg s-1

Lx > rotational energy loss for a neutron star

AXP PROPERTIES


SPIN - DOWN ENERGY LOSS

X-R

AY L

UM

INO

SIT

YROTATION-POWERED PULSARS (Possenti et al. 2002)

L x = E rot

.


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)

Operational definition of AXP


AXP census

P dP/dt (s) (10-11 s/s)

4U 0142+61 8.7 0.2 -1E 2259+586 7 0.05 CTB 1091E 1048-5937 6.4 2-3 -1E 1841-045 11.8 4 Kes 73AX J1845-03 7 - G296+0.1, Var.RXS 1708-40 11 2 -CXO J0110-72 8 2 in SMCXTE J1810-197 5.5 1.8 - Var.


MAGNETIC ENERGY - field decay - enhanced thermal emission

ACCRETION - very low mass companion - ISM / molecular clouds - disk around isolated NS

ROTATIONAL ENERGY - isolated NS - isolated WD

MODELS

?

?


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)


Magnetar model(Thompson and Duncan)

Emission powered by magnetic field decay and/or enhanced cooling

Originally developed for SGRs where evidence for high B is stronger due to large flares and bursts

Extended to AXP due to similar P and dP/dt values

Isolated NS with B > 1014 Gauss


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)


SGRs vs. GRBs

0.00 0.01 0.10 1.00 10.00 100.00 1000.00DURATION, SECONDS

0

25

50

75

100

125

150

NU

MB

ER

OF

EV

EN

TS

836 GAMMA-RAYBURSTS

0

4

8

12

16

20

NU

MB

ER

OF

EV

EN

TS42 SOFT GAMMAREPEATERS

100 101 102 103 104 105 106 107

ENERGY, keV

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

101

FLU

X, p

hoto

ns/c

m2 s

keV

GAMMA-RAY BURST

SOFT GAMMA REPEATER

Durations Spectra

Courtesy K. Hurley


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


Giant Flares

Mazets et al. 1979, Cline et al. 1980

1998 August 27 from SGR 1900+14 1979 March 5 from

SGR 0526-66 Feroci et al. 1999


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 !!


AXP


(Kaspi et al. 2003)

SGR-like activity in the AXP

1E2259+586


bursts haveLpeak: 1036-4 1038 erg/s

Change in pulse morphology

Glitch = 4 10-6

(Kaspi et al. 2003)


AXP=SGR ? 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




2 recent results



OUTLINE


(Mereghetti et al. 2004)

First evidence for significant variability in 1E 1048-59

2 EPIC

XMM-Newton

observation


2000

89 %

2003

53 %

The pulsed fraction decreased while the flux increased

Spectrumdid notvary

BB+PL

kT=0.6 keV = 3


4U 0142+61 SAX (Israel et al 1999)

Power law photon index = 3.9

+

Blackbody kT = 0.4 keV


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

4U0142+61 3.9 0.40 keV 1.8 d1 km 1.1

1E 2259+58 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


Are the two spectralcomponents relatedto distinct emittingregions and/or physical processes ?


(Oezel, Psaltis, Kaspi 2001)

small energy dependence of pulsed fraction requires ad hoc tuning of the BB and PL components


Despite the large flux variation the spectral shape did not vary

BB+PL in both observations

kT = 0.6 keVphot. Index = 3

... these are the typical parameters seen in this source


2000

89 %

2003

53 %

The pulsed fraction decreased while the flux increased

BB and PL are not physically distinct components




2 recent results



OUTLINE


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


24 bursts from SGR 1806-20 have been detected with the INTEGRAL Burst Alert System.

ToO

GCDE

INTEGRAL Julian Day

Num

ber

of b

urst

s/da

y


GC

DE

Bur

sts


JEM-X

IBIS/ISGRI

Fluence (15-100 keV)2.5×10-8 erg cm-2

Yoff = -0.97ºZoff = -2.22º

3-20 keV

100-200 keV

15-40 keV

40-100 keV


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, <kT> = 38 keV) 1 count s-1 = 1.5x10-10 erg cm-2 s-1

Spectral Analysis


INTEGRAL Log N- Log S(Fluence distribution)

INTEGRAL Log N- Log P(Peak Flux distribution)


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


IBIS (20-40 keV)

(INTEGRAL CP data ~ 1 Msec, courtesy Ada Paizis)

2-3 mCrab source6 sigma detection


Thompson & Duncan (1995)

e+e- plasma

Highly magnetized (B~1015 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

The MAGNETAR model predictions

Topics in X-ray astronomy - Tuebingen February 2004 S.Mereghetti Woods et al. (1999)

SGR 1900+14: an exceptionTwo peculiar bursts of intermediate duration (~1 s) and and with hard (kT~100 keV) spectra

Soft-to-hard evolution

• 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)

Time (seconds since trigger)

Pho

ton

Inde

x


Spectral Evolution of weak bursts with INTEGRAL

Götz et al., 2004, A&A submitted

15-40 keV

40-100 keV

Topics in X-ray astronomy - Tuebingen February 2004 S.Mereghetti Götz et al., 2004, A&A submitted

Hardness-Intensity Anticorrelation with INTEGRAL(bursts with more than 200 net counts)

Counts/s

Hard

ness

Rati

o


Conclusions

1) XMM / EPIC detected the first significant variation in the flux and pulsed fraction of the AXP 1048

the spectral invariance is a further evidence that the PL+BB 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 anticorrelation

these properties are not (yet) foreseen in the magnetar model

topics in x-ray astronomy - tuebingen february 2004 s.mereghetti anomalous x-ray pulsars and soft...

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