Young isolated neutron stars: magnetic field decay and evolutionary links

1. Young isolated neutron stars:magnetic field decay and evolutionary links Sergei Popov (SAI MSU)

2. Diversity of young neutron stars • Young isolated neutron starscan appear in many flavours: • 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) We need more sources to have better statistics! Estimates show that eROSITA can find ~ few dozensof NSs like the M7.

3. The isolated neutron star candidate 2XMM J104608.7-594306 A new INS candidate. B >26, V >25.5, R >25 (at 2.5σ confidence level) log(FX/FV) >3.1 kT = 118 +/-15 eV unabsorbed X-ray flux: Fx~1.310−12 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 [Pires & Motch arXiv: 0710.5192 and Pires et al., arXiv: 0812.4151] M7-like? Yes!

4. NS birth rate [Keane, Kramer 2008, arXiv: 0810.1512]

5. Too many NSs??? 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. [Keane, Kramer 2008, arXiv: 0810.1512]

6. CCO 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.

7. CCOs Temperature M7 Age M7 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! • Accreted envelopes (presented in CCOs, absent in the M7) • Heating by decaying magnetic field in the case of the M7

8. 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???

9. M7 and RRATs Similar periods and Pdots In one case similar thermal properties Similar birth rate? (arXiv: 0710.2056)

10. M7 and RRATs: pro et contra 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. (Kondratiev et al, in press, see also arXiv: 0710.1648)

11. 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.

12. Magnetars Pdot B=const M7 PSRs P Magnetars, field decay, heating A model based on field-dependent decay of the magnetic moment of NSscan provide an evolutionary link between different populations (Pons et al.).

13. Magnetic field decay Magnetic 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.

14. 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

15. 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 arxiv:0710.0854 (Aguilera et al.)

16. 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.)

17. Magnetic field vs. temperature The line marks balancebetween heating due to the 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 (astro-ph/0607583)

18. Log N – Log S with heating • Log N – Log S for 4 different magnetic fields. • No heating (<1013 G) 3. 1014 G • 5 1013 G 4. 2 1014 G Different magnetic field distributions. [Popov, Pons et al. work in progress; the code used in Posselt et al. A&A (2008) with modifications]

19. Populations 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 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 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. Lx> 3 1033 erg s-1 [Muno et al. 2007]

20. Log N – Log L 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. magnetars no magnetars Muno et al. [Popov, Pons, work in progress]

21. P-Pdot diagram and field decay τOhm=106 yrs τHall=104/(B0/1015 G) yrs (Popov, Pons in prep.)

22. Decay parameters and P-Pdot τOhm=107 yrs τHall =102/(B0/1015 G) τOhm=106 yrs τHall =103/(B0/1015 G) τOhm=106 yrs τHall =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.

23. Realistic tracks 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. (Popov, Pons in prep.)

24. Conclusions • Total birth rate of INSs seems to be too large • Without any doubts some subpopulations aredefinitely different from the very beginning • Some can be linked to each other • Magnetic field decay can help to linkAXPs, SGRs, RRATs and M7,but we need better statistics and morepopulation synthesis studies • eROSITA can contribute a lot to the field