The Zoo Of Neutron Stars

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The Zoo Of Neutron Stars

• Sergei Popov
• (SAI MSU)

(www.bradcovington.com)
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Neutron stars
Superdense matter, strong gravity and superstrong
magnetic fields
Magnetospheric activity
Cooling
Accretion
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The old zoo of neutron stars
In 60s the first X-ray sources have been
discovered. They were neutron stars in close
binary systems, BUT ... .... they were not
recognized....
Now we know hundreds of X-ray binaries with
neutron stars in the Milky Way and in other
galaxies.
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The first detections in binaries
Giacconi, Gursky, Hendel (1962)
About ½ of massive stars are members of close
binary systems. Now we know hundreds of close
binary systems with neutron stars.
UHURU was launched on December 12, 1970. 2-20
keV The first sky survey. 339 sources.
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Good old classics
Radio pulsars discovery 1967 Jocelyn Bell.
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Evolution of neutron stars. I. rotation
magnetic field
Ejector ? Propeller ? Accretor ? Georotator
1 spin down 2 passage through a molecular
cloud 3 magnetic field decay
astro-ph/0101031
See the book by Lipunov (1987, 1992)
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Magnetorotational evolution of radio pulsars
Spin-down. Rotational energy is released. The
exact mechanism is still unknown.
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Evolution of NSs. II.temperature
Yakovlev et al. (1999) Physics Uspekhi
First papers on the thermal evolution appeared
already in early 60s, i.e. before the discovery
of radio pulsars.
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The old Zoo young pulsars old accretors
For years only two main types of NSs have been
discussedradio pulsars and accreting NSs in
close binary systems
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The new zoo of neutron stars

• During last 10-15 years
• it became clear that neutron stars
• can be born very different.
• In particular, absolutely
• non-similar to the Crab pulsar.
• Compact central X-ray sources
• in supernova remnants.
• Anomalous X-ray pulsars
• Soft gamma repeaters
• The Magnificent Seven
• Unidentified EGRET sources
• Transient radio sources (RRATs)
• Calvera .

All together these NSs have total birth rate
higher than normal radio pulsars(see discussion
in Popov et al. 2006, Keane, Kramer 2008)
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Compact central X-ray sources in supernova
remnants
Cas A
RCW 103
Puppis A
6.7 hour period (de Luca et al. 2006)
No pulsations, small emitting area
Vkick1500 km/s(Winkler, Petre 2006)
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CCOs in SNRs

Age Distance J232327.9584843 Cas A
0.32 3.33.7 J085201.4-461753
G266.1-1.2 13 12 J082157.5-430017 Pup A
13 1.63.3 J121000.8-522628
G296.510.0 320 1.33.9 J185238.6004020 Kes
79 9 10 J171328.4-394955
G347.3-0.5 10 6 Pavlov, Sanwal,
Teter astro-ph/0311526, de Luca
arxiv0712.2209
For two sources there are strong indications for
large (gt100 msec) initial spin periods and low
magnetic fields1E 1207.4-5209 in PKS 1209-51/52
andPSR J18520040 in Kesteven 79 see Halpern
et al. arxiv0705.0978
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Magnetars

• dE/dt gt dErot/dt
• By definition The energy of the magnetic field
  is released
• P-Pdot
• Direct measurements of the field (spectral
  lines)

Magnetic fields 10141015 G
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SGRs periods and giant flares

• 0526-66
• 1627-41
• 1806-20
• 190014
• 050145

5.7
See the review in Woods, Thompson astro-ph/0406133
and Mereghetti arXiv 0804.0250
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Historical notes

• 05 March 1979. The Konus experiment Co.
• Venera-11,12
• Events in the LMC.
• SGR 0520-66.
• Fluence about 10-3 erg/cm2

Mazets et al. 1979
N49 supernova remnant in the Large
Magellanic cloud (G.Vedrenne et al. 1979)
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Soft Gamma Repeaters main properties
Saturationof detectors

• Energetic Giant Flares (GFs, L 1045-1047
  erg/s) detected from 3 (4?) sources
• No evidence for a binary companion, association
  with a SNR at least in one case
• Persistent X-ray emitters, L 1035 - 1036 erg/s
• Pulsations discovered both in GFs tails and
  persistent emission, P 5 -10 s
• Huge spindown rates,
• ? 10-10 10-11 ss-1

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Main types of activity of SGRs

• Weak bursts. Llt1042 erg/s
• Intermediate. L10421043 erg/s
• Giant. Llt1045 erg/s
• Hyperflares. Lgt1046 erg/s

(from Woods, Thompson 2004)
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Extragalactic SGRs
It was suggested long ago (Mazets et al.
1982) that present-day detectors could alredy
detectgiant flares from extragalactic
magnetars. However, all searches in, for
example,BATSE databse did not provide clear
candidates(Lazzati et al. 2005, Popov Stern
2006, etc.). Finally, recently several good
candidates have been proposed by different
groups (Mazets et al., Frederiks et al.,
Golenetskii et al., Ofek et al, Crider ...., see
arxiv0712.1502 andreferences therein, for
example).
Burstin M31
D. Frederiks et al. astro-ph/0609544
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Anomalous X-ray pulsars
Identified as a separate group in 1995.
(Mereghetti, Stella 1995 Van Paradijs et al.1995)

• Similar periods (5-10 sec)
• Constant spin down
• Absence of optical companions
• Relatively weak luminosity
• Constant luminosity

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Known AXPs
Sources Periods, s
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SGRs and AXPs
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Are SGRs and AXPs brothers?

• Bursts of AXPs (from 6 now)
• Spectral properties
• Quiescent periods of SGRs (0525-66 since 1983)

Gavriil et al. 2002
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Magnetic field estimates

• Spin down
• Long spin periods
• Energy to support bursts
• Field to confine a fireball (tails)
• Duration of spikes (alfven waves)
• Direct measurements of magnetic field (cyclotron
  lines)

Ibrahim et al. 2002
Gavriil et al. (2002, 2004)
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Transient radio emission from AXP
ROSAT and XMM images.The X-ray outburst
happened in 2003. AXP has spin period 5.54 s
Radio emission was detected from XTE
J1810-197during its active state. Clear
pulsations have been detected. Large radio
luminosity. Strong polarization. Precise Pdot
measurement.Important for limting models, better
distanceand coordinates determination etc.
(Camilo et al. astro-ph/0605429)
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Transient radiopulsar
PSR J1846-0258 P0.326 sec B5 1013 G
However,no radio emissiondetected. Due to
beaming?
Among all rotation poweredPSRs it has the
largest Edot.Smallest spindown age (884
yrs). The pulsar increased its luminosity in
X-rays. Increase of pulsed X-ray
flux. Magnetar-like X-ray bursts (RXTE). Timing
noise.
See additional info about this pulsar at the
web-site http//hera.ph1.unikoeln.de/heintzma/SNR
/SNR1_IV.htm
0802.1242, 0802.1704
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Twisted Magnetospheres I

• The magnetic field inside a magnetar is wound
  up
• The presence of a toroidal component induces a
  rotation of the surface layers
• The crust tensile strength resists
• A gradual (quasi-plastic ?) deformation of the
  crust
• The external field twists up
• (Thompson, Lyutikov Kulkarni 2002)

(by R. Turolla)
(Thompson Duncan 2001)
(Mereghetti arXiv 0804.0250)
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Generation of the magnetic field or fossil field?
The mechanism of the magnetic field generation
is still unknown.
a-O dynamo (Duncan,Thompson) a2 dynamo (Bonanno
et al.) or their combination
In any case, initial rotation of a protoNS is the
critical parameter.
There are reasons to suspect that the magnetic
fields of magnetars are not due to any kind of
dynamo mechanism, but just due to flux
conservation

• Study of SNRs with magnetars (Vink and Kuiper
  2006).
• 2. There are few examples of massive stars with
  field strong enough to produce magnetars
  (Ferrario and Wickramasinghe 2006)

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What is special about magnetars?
Link with massive stars There are reasons to
suspect that magnetars are connected to massive
stars (astro-ph/0611589). Link to binary
stars There is a hypothesis that magnetars are
formed in close binary systems (astro-ph/0505406)
.
AXP in Westerlund 1 most probably hasa very
massive progenitor gt40 Msolar.
The question is still on the list.
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ROSAT
ROentgen SATellite
German satellite (with participation of US and
UK).
Launched 01 June 1990. The program was
successfully ended on 12 Feb 1999.
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Close-by radioquiet NSs

• Discovery Walter et al. (1996)
• Proper motion and distance Kaplan et al.
• No pulsations
• Thermal spectrum
• Later on six brothers

RX J1856.5-3754
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Magnificent Seven
Radioquiet (?) Close-by Thermal
emission Absorption features Long periods
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Pulsating ICoNS

• Quite large pulsed fractions
• Skewed lightcurves
• Harder spectrum at pulse minimum
• Phase-dependent absorption features

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The Optical Excess

• In the four sources with a confirmed optical
  counterpart Fopt ? 5-10 x B?(TBB,X)
• Fopt ? ?2 ?
• Deviations from a Rayleigh-Jeans continuum in RX
  J0720 (Kaplan et al 2003) and RX J1605 (Motch et
  al 2005). A non-thermal power law ?

RX J1605 multiwavelength SED (Motch et al 2005)
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Period Evolution

• RX J0720.4-3125 bounds on derived by Zane et
  al. (2002) and Kaplan et al (2002)
• Timing solution by Cropper et al (2004), further
  improved by Kaplan Van Kerkwijk (2005)
• 7x10-14 s/s, B 2x1013 G
• RX J1308.62127 timing solution by Kaplan Van
  Kerkwijk (2005a),
• 10-13 s/s, B 3x1013 G
• Spin-down values of B in agreement with
  absorption features being proton cyclotron lines

B 1013 -1014 G
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Featureless ? No Thanks !

• RX J1856.5-3754 is convincingly featureless
  (Chandra 500 ks DDT Drake et al 2002 Burwitz et
  al 2003)
• A broad absorption feature detected in all other
  ICoNS (Haberl et al 2003, 2004, 2004a Van
  Kerkwijk et al 2004 Zane et al 2005)
• Eline 300-700 eV evidence for two lines with
  E1 2E2 in RBS 1223 (Schwope et al 2006)
• Proton cyclotron lines ? H/He transitions at high
  B ?

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Long Term Variations in RX J0720.4-3125

• A gradual, long term change in the shape of the
  X-ray spectrum AND the pulse profile (De Vries et
  al 2004 Vink et al 2004)
• Steady increase of TBB and of the absorption
  feature EW (faster during 2003)
• Evidence for a reversal of the evolution in 2005
  (Vink et al 2005)

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Unidentified EGRET sources
Grenier (2000), Gehrels et al. (2000)
Unidentified sources are divided into several
groups. One of them has sky distribution similar
to the Gould Belt objects. It is suggested that
GLAST (and, probably, AGILE) Can help to solve
this problem. Actively studied subject (see for
example papers by Harding, Gonthier)
no radio pulsars in 56 EGRET error boxes
(Crawford et al. 2006) However, Keith et al.
(0807.2088) found a PSR at high frequency.
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Discovery of RRATs

• 11 sources detected in the
• Parkes Multibeam survey
• (McLaughlin et al 2006)
• Burst duration 2-30 ms, interval 4 min-3 hr
• Periods in the range 0.4-7 s
• Period derivative measured in 3 sources
• B 1012-1014 G, age 0.1-3 Myr
• RRAT J1819-1458 detected in X-rays, spectrum soft
  and thermal,
• kT 120 eV (Reynolds et al 2006)

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RRATs

• P, B, ages and X-ray properties of RRATs very
  similar to those of XDINSs
• Estimated number of RRATs
• 3-5 times that of PSRs
• If tRRAT tPSR,
• ßRRAT 3-5 ßPSR
• ßXDINS gt 3 ßPSR (Popov et al 2006)
• Are RRATs far away XDINSs ?

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RRAT in X-rays
X-ray pulses overlaped onradio data of RRAT
J1819-1458. Thermally emitting NS kT 120
eV (Reynolds et al 2006)
(arXiv 0710.2056)
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Calvera et al.
Recently, Rutledge et al. reported the discovery
of an enigmatic NS candidated dubbed Calvera. It
can be an evolved (aged) version of Cas A
source, but also it can be a M7-like object,
whos progenitor was a runaway (or, less
probably, hypervelocity) star. No radio emission
was found.
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CCO vs. M7. New population?
Gotthelf Halpern (arXiv0704.2255) recently
suggested that 1E 1207.4-5209 and PSR J18520040
(in Kes 79) can beprototypes of a different
subpopulation of NSs born withlow magnetic field
(lt few 1011 G) and relatively long spin periods
(few tenths of a second). These NSs are
relatively hot, and probably not very
rare. Surprisingly, we do not see objects of this
type in our vicinity. In the solar neighbourhood
we meet a different class of object. This can be
related to accreted envelopes (see, for example,
Kaminker et al. 2006). Sources in CCOs have them,
so they look hotter,but when these envelopes
disappear, they are colderthan NSs which have no
envelopes from the very beginning.So, we do not
see such sources among close-by NSs.
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M7 and CCOs
Both CCOs and M7 seem to bethe hottest at their
ages (103 and 106 yrs). However, the former
cannot evolve to become the latter ones!

• Accreted envelopes (presented in CCOs,
  absent in the M7)
• Heating by decaying magnetic field in the
  case of the M7

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Accreted envelopes, B or heating?
(Yakovlev Pethick 2004)
It is necessary to make population synthesis
studies to test all these possibilities.

• Related to e-capture SN?
• low-mass objects
• low kicks
• 10 of all NSs

However, small emitting area remains
unexplained.Accretion???
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M7 and RRATs
Similar periods and Pdots In one case similar
thermal properties Similar birth rate?
(arXiv 0710.2056)
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M7 and RRATs pro et contra
Based on similarities between M7 and RRATs it was
proposed that they can bedifferent
manifestations of the same type of INSs
(astro-ph/0603258).To verify it a very deep
search for radio emission (including RRAT-like
bursts)was peformed on GBT (Kondratiev et
al.).In addition, objects have been observed
with GMRT (B.C.Joshi, M. Burgay et al.). In both
studies only upper limits were derived. Still,
the zero result can be just due to unfavorable
orientations(at long periods NSs have very
narrow beams).It is necessary to increase
statistics.
(Kondratiev et al, in press, see also arXiv
0710.1648)
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M7 and high-B PSRs
Strong limits on radio emission from the M7are
established (Kondratiev et al. 2008). However,
observationally it is still possible thatthe M7
are just misaligned high-B PSRs.
Are there any other considerations to verify a
link between thesetwo popualtions of NSs?
In most of population synthesis studies of
PSRsthe magnetic field distribution is described
as agaussian, so that high-B PSRs appear to be
notvery numerous.On the other hand, population
synthesis of thelocal population of young NSs
demonstrate thatthe M7 are as numerous as
normal-B PSRs.
So, for standard assumptionsit is much more
probable, thathigh-B PSRs and the M7 are not
related.
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Magnetars, field decay, heating
A model based on field-dependent decay of the
magnetic moment of NSscan provide an
evolutionary link between different populations.
Magnetic fields of NSs are expected to decay
due to decay ofcurrents which support them.
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Period evolution with field decay
An evolutionary track of a NS isvery different
in the case of decaying magnetic field. The
most important feature isslow-down of
spin-down. Finally, a NS can nearly freezeat
some value of spin period. Several episodes of
relativelyrapid field decay can happen. Number
of isolated accretors can be both decreased or
increasedin different models of field decay. But
in any case their average periods become shorter
and temperatures lower.
astro-ph/9707318
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Magnetic field decay vs. thermal evolution
Magnetic field decay can be an important source
of NS heating.
Heat is carried by electrons. It is easier to
transport heat along field lines. So, poles are
hotter. (for light elements envelope
thesituation can be different).
Ohm and Hall decay
arxiv0710.0854 (Aguilera et al.)
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Joule heating for everybody?
It is important to understandthe role of heating
by thefield decay for different typesof INS.
In the model by Pons et al.the effect is more
importantfor NSs with larger initial B. Note,
that the characteristicage estimates (P/2
Pdot)are different in the case ofdecaying
field!
arXiv 0710.4914 (Aguilera et al.)
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Magnetic field vs. temperature
The line marks balancebetween heating due to the
field decay and cooling.It is expected by the
authors(Pons et al.) that a NSevolves downwards
till itreaches the line, then theevolution
proceeds along the line. Selection effects
are notwell studied here.A kind of
populationsynthesis modeling iswelcomed.
Teff Bd1/2
(astro-ph/0607583)
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Log N Log S with heating

• Log N Log S for 4 different magnetic fields.
• No heating (lt1013 G) 3. 1014 G
• 5 1013 G 4. 2 1014 G

Different magnetic field distributions.
Popov, Pons, work in progress the code used in
Posselt et al. AA (2008) with modifications
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Log N Log L
Two magnetic field distributionswith and
without magnetars(i.e. different magnetic
fielddistributions are used). 6 values of inital
magnetic field, 8 masses of NSs. SNR 1/30
yrs-1. Without magnetars meansno NSs with
B0gt1013 G.
Popov, Pons, work in progress
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Populations, new candidates ....
Birthrate of magnetars is uncertain due to
discovery of transient sources. Just from
standard SGR statistics it is only 10, then,
for example,the M7 cannot be aged magnetars
with decayed fields, but if there are many
transient AXPs and SGRs then the situation is
different. Limits, like the one by Muno et al.,
on the number of AXPs from asearch for
periodicity (lt540) are very important and have
to be improved(a task for eROSITA?).
Lxgt 3 1033 erg s-1
Muno et al. 2007
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Conclusions

• There are several types of sources CCOs, M7,
• SGRs, AXPs, RRATs ...
• Significant fraction of all newborn NSs
• Unsolved problems
• 1. Are there links?
• 2. Reasons for diversity

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Dorothea Rockburne
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Some reviews on isolated neutron stars

• NS basics physics/0503245
• 
  astro-ph/0405262
• SGRs AXPs astro-ph/0406133
  
  arXiv0804.0250
• CCOs
  astro-ph/0311526
  arxiv0712.2209
• Quark stars arxiv0809.4228
• The Magnificent Seven astro-ph/0609066
• 
  arxiv0801.1143
• RRATs arXiv0710.2056
  
• Cooling of NSs astro-ph/0508056
• 
  astro-ph/0402143
• NS structure arXiv0705.2708
• EoS
  astro-ph/0612440
• NS atmospheres astro-ph/0206025
• NS magnetic fields arxiv0711.3650
  
  arxiv0802.2227