Young isolated neutron stars:Young isolated neutron stars:magnetic field decay and magnetic field decay and

evolutionary linksevolutionary links

Sergei PopovSergei Popov

(SAI MSU)(SAI MSU)

Diversity of young neutron starsDiversity of young neutron stars

Young isolated neutron starscan appear in many flavours:

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 ….

All together these NSs have total birth rate higher than normal radio pulsars(see discussion in Popov et al. 2006, Keane, Kramer 2008)

We need more sources to have better statistics!Estimates show that eROSITA can find ~ few dozensof NSs like the M7.

The isolated neutron star candidate The isolated neutron star candidate 2XMM J104608.7-5943062XMM J104608.7-594306

[Pires & Motch arXiv: 0710.5192 and Pires et al., arXiv: 0812.4151]

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!

NS birth rateNS birth rate

[Keane, Kramer 2008, arXiv: 0810.1512]

Too many NSs???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 sources evolve into another.

See also astro-ph/0603258.

CCO vs. M7CCO vs. M7

Gotthelf and Halpern (2007) presented evidence in favor of hypothesis that among CCOs there is a population of NSs born with long spin periods (few tenths of a second) and small magnetic fields (<1011 G).These sources are hot. The M7 sources are hot, too, but they seemto belong to different populations.

It is necessary to make a general population synthesis, which would include all types of isolated NSs.

M7 and CCOsM7 and CCOs

Both CCOs and M7 seem to be the hottest at their ages (103 and 106 yrs).However, the former cannot evolve to 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?Accreted envelopes, B or heating?

(Yako

vle

v &

Peth

ick

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???

M7 and RRATsM7 and RRATsSimilar periods and PdotsIn one case similar thermal propertiesSimilar birth rate?

(arXiv: 0710.2056)

M7 and RRATs: pro et contraM7 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 be different 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 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 - to have more sources.

M7 and high-B PSRsM7 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 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.

Magnetars, field decay, Magnetars, field decay, heatingheating

A model based on field-dependent decay of the magnetic moment of NSscan provide an evolutionary link between different populations (Pons et al.).

P

Pdot

PSRs

M7

B=const

Magnetars

Magnetic field decayMagnetic 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 decayPeriod 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. Magnetic field decay vs. thermal evolutionthermal 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?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. temperatureMagnetic field vs. temperature

(astro-ph/0607583)

The line marks balancebetween heating due tothe field decay and cooling.It is expected 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 heatingLog N – Log S with heating

[Popov, Pons et al. 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.

Populations and constraintsPopulations and constraints

Birthrate of magnetars is uncertain due to discovery of transient sources.Just from “standard” SGR statistics it is just 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 are very important and have to be improved(the task for eROSITA?).

Such limits can be also re-scaledto put constraints on the number ofthe M7-like NSs and the number ofisolated accretors with decayed field.

[Muno et al. 2007]

Lx> 3 1033 erg s-1

Log N – Log LLog 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”.

Non-thermal contribution is nottaken into account.

no magnetars

magnetars

Muno et al.

P-Pdot diagram and field P-Pdot diagram and field decaydecay

(Popov, Pons in prep.)

τOhm=106 yrsτHall=104/(B0/1015 G) yrs

Decay parameters and P-PdotDecay parameters and P-Pdot

τOhm=107 yrsτHall =102/(B0/1015 G)

τOhm=106 yrsτHall =103/(B0/1015 G)

τOhm=106 yrsτHall =104/(B0/1015 G)

Longer time scale for the Hall field decay is favoured.

It is interesting to look at HMXBs to see if it is possibleto derive the effect of field decay and convergence.

Realistic tracksRealistic tracks

(Popov, Pons in prep.)

Using the model by Pons et al.(arXiv: 0812.3018) we plotrealistic tracks for NS withmasses 1.4 Msolar.

Initial fields are: 3 1012, 1013, 3 1013, 1014, 3 1014, 1015, 3 1015 G

Color on the track encodessurface temperature.

Tracks start at 103 years,and end at 2 106 years.

ConclusionsConclusions Total birth rate of INSs seems to be too Total birth rate of INSs seems to be too

largelarge Without any doubts some subpopulations Without any doubts some subpopulations

arearedefinitely different from the very beginningdefinitely different from the very beginning

Some can be linked to each otherSome can be linked to each other Magnetic field decay can help to linkMagnetic field decay can help to link

AXPs, SGRs, RRATs and M7,AXPs, SGRs, RRATs and M7,but we need better statistics and morebut we need better statistics and morepopulation synthesis studiespopulation synthesis studies

eROSITA can contribute a lot to the fieldeROSITA can contribute a lot to the field