Isolated Neutron Stars:News and Views

Sergei Popov (SAI MSU)

Good old classicsGood old classics

The pulsar in the Crab nebulaThe pulsar in the Crab nebula A binary systemA binary system

For years two main types of NSs have been discussed:radio pulsars and accreting NSs in close binary systems

The old zoo of neutron starsIn 60s the first X-ray sources have been discovered.Rocket experiments.Sco X-1.

They were neutron stars in close binary systems, BUT ... .... they were «not recognized»....

Now we know hundredsof X-ray binaries with neutron stars in the Milky Way and in other galaxies.

Discovery !!!!1967: Jocelyn Bell. Radio pulsars.

Seredipitous discovery.

The old Zoo: young pulsars & old The old Zoo: young pulsars & old accretorsaccretors

During last >10 years it became clear that neutron stars can be born very different.In particular, absolutely non-similar to the Crab pulsar.

o Compact central X-ray sources in supernova remnants. o Anomalous X-ray pulsarso Soft gamma repeaterso The Magnificent Seveno Unidentified EGRET sourceso Transient radio sources (RRATs)o Calvera ….

The new zoo of neutron stars

CCOs in SNRs Age DistanceJ232327.9+584843 Cas A 0.32 3.3–3.7 J085201.4−461753 G266.1−1.2 1–3 1–2J082157.5−430017 Pup A 1–3 1.6–3.3J121000.8−522628 G296.5+10.0 3–20 1.3–3.9J185238.6+004020 Kes 79 ~9 ~10 J171328.4−394955 G347.3−0.5 ~10 ~6

[Pavlov, Sanwal, Teter: astro-ph/0311526, de Luca: arxiv:0712.2209]

For two sources there are strong indications for large (>~100 msec) initial spin periods and low magnetic fields:1E 1207.4-5209 in PKS 1209-51/52 andPSR J1852+0040 in Kesteven 79 [see Halpern et al. arxiv:0705.0978]

Problem: small emitting area

Magnetars

dE/dt > dErot/dt By definition: The energy of the magnetic field is released

Magnetic fields 1014–1015 G

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

Spectral lines claimsAll claims were done for RXTE observations (there are few other candidates).All detections were done during bursts.

1E 1048.1-5937 Gavriil et al. (2002, 2004) 4U 0142+61 Gavriil et al. (2007)

Known magnetarsSGRs 0526-66 1627-41 1806-20 1900+14 0501+4516 – Aug.2008! 1801-23 (?)

AXPs CXO 010043.1-72 4U 0142+61 1E 1048.1-5937 CXO J1647-45 1 RXS J170849-40 XTE J1810-197 1E 1841-045 AX J1845-0258 1E 2259+586 1E 1547.0-5408

(СТВ 109) http://www.physics.mcgill.ca/~pulsar/magnetar/main.html

The newest SGRThe most recent SGR candidate was discovered in Aug. 2008 (GCN 8112 Holland et al.)

It is named SGR 0501+4516. Several reccurent bursts have been detected by several experiments (see, for example, GCN 8132 by Golenetskii et al.).

Spin period 5.769 sec. Optical and IR counterparts.

SWIFT

P=5.7620690(1) sPdot=7.4(1)E-12 s/s

Pdotdot=-4.3(1.1)E-19 s/s^2

Israel et al. ATel #1837 (11 Nov)

Extragalactic SGRs

[D. Frederiks et al. astro-ph/0609544]

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. 2006, 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 arxiv:0712.1502 andreferences therein, for example).

Burst from M31

Transient radio emission from AXP

(Camilo et al. astro-ph/0605429)

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.

ROSAT and XMM imagesan X-ray outburst happened in 2003.

AXP has spin period 5.54 s

Another AXP detected in radio1E 1547.0-5408P= 2 secSNR G327.24-0.13

0802.0494 (see also arxiv:0711.3780 )

Pdot changed significantly on the scale of just~few monthsRotation and magnetic axis seem to be aligned

Also these AXP demostrated weakSGR-like bursts (Rea et al. 2008, GCN 8313)

Radio

X-rays

[simultaneous]

Transient radiopulsarPSR J1846-0258P=0.326 secB=5 1013 G

0802.1242, 0802.1704

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-sitehttp://hera.ph1.uni-koeln.de/~heintzma/SNR/SNR1_IV.htm

However,no radio emissiondetected. Due to beaming?

Bursts from the transient PSR

Gavriil et al. 0802.1704

Chandra: Oct 2000 June 2006

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

Unique AXP bursts?

(Woods et al. 2005)

Bursts from AXP J1810-197. Note a long exponential tail with pulsations.

Mysterious bursts of SWIFT J195509.6+261406

[Stefanescu et al. arXiv:0809.4043] See also arXiv: 0809.4231.

Optical bursts which is some sensesimilar to magnetars weak X-ray bursts.

[Kaslival et al. arXiv: 0708.0226]

Periodicity ~7 sec is suspected.

However, in our opinion, this can benot a magnetar, but a magnetic WD.Optic vs. X-ray, longer time scales,lower energies etc.

Magnificent Seven

Name Period, s

RX 1856 7.05

RX 0720 8.39

RBS 1223 10.31

RBS 1556 6.88?

RX 0806 11.37

RX 0420 3.45

RBS 1774 9.44

Radioquiet (?)Close-byThermal emissionAbsorption featuresLong periods

RX J0720.4-3125 as a variable source

[Hohle et al. 2008 arXiv:0810.5319]

Long term phase averagedspectrum variations

Phase dependent variationsduring different observations.

~10 years period: precession???

[Hohle et al. 2008]

10.711 +/-0.058 yrs

Unidentified EGRET sourcesGrenier (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.

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 the X-rays, spectrum soft and thermal, kT ~ 120 eV (Reynolds et al 2006) 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 τRRAT ≈ τPSR, βRRAT ≈ 3-5 βPSR βXDINS > 3 βPSR (Popov et al 2006) Are RRATs far away XDINSs ?

New discussion about birth rates in Keane, Kramer arXiv: 0810.1512

Calvera et al.

Rutledge et al. reported the discovery of an enigmaticNS candidated dubbed Calvera.

It can be an evolved (aged) version of Cas A source,but also it can be a M7-like object, who’s progenitor was a runaway (or, less probably, hypervelocity) star.

No radio emission was found.

The isolated neutron star candidate 2XMM J104608.7-594306

[Pires & Motch arXiv: 0710.5192 and Pires et al., in press]

A new INS candidate.

B >26, V >25.5, R >25 (at 2.5σ confidence level)

log(FX/FV) >3.1kT = 118 +/-15 eV

unabsorbed X-ray flux: Fx ~1.3 10−12 erg s−1 cm−2 in the 0.1–12 keV band.

At 2.3 kpc (Eta Carina)the luminosity is LX ~ 8.2 1032 erg s−1

R∞ ~ 5.7 km

M7-like? Yes!

Recent LIGO results

1. 0805.4758 Beating the spin-down limit on gravitational wave emission from the Crab pulsar

h095% < 3.5 10-25 ε<1.9 10-4 (single template)

2. 0708.3818 All-sky search for periodic grav. waves in LIGO S4 data 50-1000 HZ No evidence. Upper limits on isolated NSs GW emission.

3. gr-qc/0702039 Upper limits on gravitational wave emission from 78 PSRs ε< 10-6 for PSR J2124−3358 h<2.6×10−25 for PSR J1603−7202

NS birth rate

[Keane, Kramer 2008, arXiv: 0810.1512]

Too many NSs???

[Keane, Kramer 2008, arXiv: 0810.1512]

It seems, that the total birth rate is larger than the rate of CCSN.e- - capture SN cannot save the situation, as they are <~20%.

Note, that the authors do not include CCOs.

So, some estimates are wrong, or some surces evolve into another.

See also astro-ph/0603258.

Pulsars, positrons, PAMELA

[Dan Hooper et al. 2008 arXiv: 0810.1527]

[O. Adriani et al.] arXiv:0810.4995

Geminga, PSR B0656+14, and all PSRs

Magnetars, field decay, heatingA model based on field-dependent decay of the magnetic moment of NSscan provide an evolutionary link between different populations.

P

Pdo

t

PSRs

M7

B=const

Magnetars

Magnetic field decayMagnetic fields of NSs are expected to decay due to decay of currents which support them.

Crustal field of core field?

It is easy to decay in the crust.

In the core the filed is in the formof superconducting vortices.They can decay only when they aremoved into the crust (during spin-down).

Still, in most of models strong fields decay.

Period evolution with field decay

astro-ph/9707318

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.

Magnetic field decay vs. thermal evolution

arxiv:0710.0854 (Aguilera et al.)

Magnetic field decay can be an important source of NS heating.

Ohm and Hall decay

Heat is carried by electrons.It is easier to transport heat along field lines. So, poles are hotter.(for light elements envelope the

situation can be different).

Joule heating for everybody?

arXiv: 0710.4914 (Aguilera et al.)

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!

Magnetic field vs. temperature

(astro-ph/0607583)

The line marks balancebetween heating due tothe 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

Log N – Log S with heating

[Popov, Pons, work in progress; the code used in Posselt et al. A&A (2008) with modifications]

Log N – Log S for 4 different magnetic fields.1. No heating (<1013 G) 3. 1014 G2. 5 1013 G 4. 2 1014 G

Different magnetic field distributions.

Log N – Log L

[Popov, Pons, work in progress]

Two magnetic field distributions:with 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” means“no NSs with B0>1013 G”.

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 aremany 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 (<540) are very important and have to be improved(a task for eROSITA?).

[Muno et al. 2007]

Lx> 3 1033 erg s-1

What is special about magnetars?Link withLink with massive starsmassive starsThere are reasons to suspect that magnetars are connected to massive stars (astro-ph/0611589). Recently, two massive magnetized stars have been found [Petit et al. arxiv:0803.2691]

Link to binary starsThere is a hypothesis that magnetars are formed in close binary systems (astro-ph/0505406).

The question is still on the list.

AXP in Westerlund 1 most probably hasa very massive progenitor >40 Msolar.

A question:

• 5-10 % of NSs are expected to be binary (for moderate and small kicks)

• All known magnetars (or candidates) are single objects.

• At the moment from the statistical point of view it is not a miracle, however, it’s time to ask this question.

Why do all magnetars are isolated?

Two possible explanations

• Large kick velocities

• Particular evolutionary path

Magnetars origin • Probably, magnetars are isolated due to

their origin

• Fast rotation is necessary (Thompson, Duncan)

• Two possibilities to spin-up during evolution in a binary

1) Spin-up of a progenitor star in a binary via accretion or synchronization

2) Coalescence

Rem: Now there are claims (Vink et al., Ferrario et al.) that magnetars can be born slowly rotating, so the field is fossil. We do not discuss this ideas here.

The code

We use the “Scenario Machine” code.Developed in SAI (Moscow) since 1983by Lipunov, Postnov, Prokhorov et al.(http://xray.sai.msu.ru/~mystery/articles/review/ )

We run the population synthesis of binaries to estimate the fraction of NS progenitors with enhanced rotation.

The model

Among all possible evolutionary paths that result in formation of NSs we select those that lead to angular momentum increase of progenitors.

• Coalescence prior to a NS formation.

• Roche lobe overflow by a primary without a common envelope.

• Roche lobe overflow by a primary with a common envelope.

• Roche lobe overflow by a secondary without a common envelope.

• Roche lobe overflow by a secondary with a common envelope.

Results of calculations-2

Most of “magnetars” appear after coalescences or from secondary companions after RLO by primaries.

They are mostly isolated.

• We made population synthesis of binary systems to derive the relative number of NSs originated from progenitors with enhanced rotation -``magnetars''.

• With an inclusion of single stars (with the totalnumber equal to the total number of binaries) the fraction of ``magnetars'‘ is ~8-14%.

• Most of these NSs are isolated due to coalescences of components prior to NS formation, or due to a system disruption after a SN explosion.

• The fraction of ``magnetars'' in survived binaries is about 1% or lower.

• The most numerous companions of ``magnetars'' are BHs.

MNRAS vol. 367, p. 732 (2006)

New calculations

[Bogomazov, Popov in press]

Here we study a lessoptimistic scenario,when we require thata stellar core has largeangular momentum justprior to collapse, becauseif a star is spun-up earlier,than it very probable thatit looses momentumbefore it forms a NS.

Maxwellian kick

Random kick or aligned kick

Velocity distribution is adelta-function.

1 and 2 – random kick3 and 4 – kick alogn spin

1 and 3 – weak stellar wind2 and 4 – strong wind

Spin-dependent kick

V=V0 ∙ 0.001/Pmsec

1 – weak stellar wind2 – strong wind

In a more conservative scenario it is not that easyto obtain small binaryfraction.

Are there magnetors in binaries?

Magnetor

At the moment all known SGRs and AXPs are isolated objects.About 10% of NSs are expected to be in binaries.The fact that all known magnetars are isolated can be relatedto their origin, but this is unclear.

If a magnetar appears in a very close binary system, thenan analogue of a polar can be formed.The secondary star is insidethe huge magnetosphere of a magnetar.This can lead to interestingobservational manifestations.

Binaries with magnetars - magnetors

Can RCW 103 be a prototype?

6.7 hour period (de Luca et al. 2006)

RCW 103

Possible explanations:1. Magnetar, spun-down by disc2. Double NS system3. Low-mass companion + magnetar= magnetor

arXiv:0803.1373

SuperGiant Fast X-Ray Transients

[Bozzo et al. 2008 arXiv: 0811.0995]

HMXBs.

Transition from Propeller to Accretordue to fluctuations in the mass loss ratefrom a supergiant.

Burst: L~1036-1037 erg/sQuiescent: L~1031-1033 erg/sBurst duration: ~minutes - ~hours

New population?

Gotthelf & Halpern (arXiv:0704.2255) recently suggested that 1E 1207.4-5209 and PSR J1852+0040 (in Kes 79) can beprototypes of a different subpopulation of NSs born withlow magnetic field (< 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.

M 7 and CCOsBoth CCOs and M7 seem to bethe hottest at their ages (103 and 106 yrs).However, the former cannot evolveto become the latter ones!

Age

Tem

pera

ture

CCOs

M7

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

Accreted envelopes, B or heating?

(Yakovl

ev

& P

eth

ick 2

004)

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???

M7 and RRATsSimilar periods and PdotsIn one case similar thermal propertiesSimilar birth rate?

(arXiv: 0710.2056)

M7 and RRATs: pro et contra

(Kondratiev et al, in press, see also arXiv: 0710.1648)

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.

M7 and high-B PSRsStrong limits on radio emission from the M7are established (Kondratiev et al. 2008: 0710.1648 ).However, observationally it is still possible thatthe M7 are just misaligned high-B PSRs.

Are there any other considerations to verify a link between these

two 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, that

high-B PSRs and the M7 are not related.

Some reviews on isolated neutron stars• NS basics: physics/0503245 astro-ph/0405262• SGRs & AXPs: astro-ph/0406133 arXiv:0804.0250 • CCOs: astro-ph/0311526 arxiv:0712.2209 • Quark stars: arxiv:0809.4228 • The Magnificent Seven: astro-ph/0609066 arxiv:0801.1143 • RRATs: arXiv:0710.2056 • Cooling of NSs: astro-ph/0508056 astro-ph/0402143• NS structure arXiv:0705.2708 • EoS astro-ph/0612440• NS atmospheres astro-ph/0206025 • NS magnetic fields arxiv:0711.3650 arxiv:0802.2227