Space Cowboys Odissey: Beyond the Gould Belt Sergei Popov (SAI MSU)

Neutron stars Superdence matter, strong gravity and superstrong magnetic fields Cooling Accretion Magnetospheric activity

Good old classics The pulsar in the Crab nebula A binary system For years two main types of NSs have been discussed: radio pulsars and accreting NSs in close binary systems The pulsar in the Crab nebula A binary system

The new zoo of neutron stars During last >10 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)

Compact central X-ray sources in supernova remnants Cas A RCW 103 Puppis A No pulsations, small emitting area 6.7 hour period (de Luca et al. 2006) Vkick=1500 km/s (Winkler, Petre 2006)

CCOs in SNRs Age Distance J232327.9+584843 Cas A 0.32 3.3–3.7 J085201.4−461753 G266.1−1.2 1–3 1–2 J082157.5−430017 Pup A 1–3 1.6–3.3 J121000.8−522628 G296.5+10.0 3–20 1.3–3.9 J185238.6+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 small initial spin periods and low magnetic fields: 1E 1207.4-5209 in PKS 1209-51/52 and PSR J1852+0040 in Kesteven 79 [see Halpern et al. arxiv:0705.0978]

Magnetars dE/dt > dErot/dt By definition: The energy of the magnetic field is released P-Pdot Direct measurements of the field (Ibrahim et al.) Magnetic fields 1014–1015 G

SGRs: periods and giant flares P, s Giant flares 8.0 6.4 7.5 5.2 5 March 1979 27 Aug 1998 27 Dec 2004 18 June 1998 (?) 0526-66 1627-41 1806-20 1900+14 0501+45 5.7 See the review in Woods, Thompson astro-ph/0406133 and Mereghetti arXiv: 0804.0250

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

Known AXPs Sources Periods, s CXO 010043-7211 8.0 4U 0142+61 8.7 6.4 1E 1547.0-5408 2.0 CXOU J164710-4552 10.6 1RXS J170849-40 11.0 XTE J1810-197 5.5 1E 1841-045 11.8 AX J1845-0258 7.0 1E 2259+586

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.

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)

Calvera et al. Recently, Rutledge et al. reported the discovery of an enigmatic NS candidated dubbed Calvera. No radio emission was found. 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.

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-by Thermal emission Absorption features Long periods

Pulsating ICoNS Quite large pulsed fractions Skewed lightcurves RX J0420.0-5022 (Haberl et al 2004) Quite large pulsed fractions Skewed lightcurves Harder spectrum at pulse minimum Phase-dependent absorption features

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)

Magnetorotational evolution of radio pulsars Spin-down. Rotational energy is released. The exact mechanism is still unknown.

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.

Population of close-by young NSs Magnificent seven Geminga and 3EG J1853+5918 Four radio pulsars with thermal emission (B0833-45; B0656+14; B1055-52; B1929+10) Seven older radio pulsars, without detected thermal emission. Where are the rest? UNCATCHABLES

Population synthesis in astrophysics A population synthesis is a method of a direct modeling of relatively large populations of weakly interacting objects with non-trivial evolution. As a rule, the evolution of the objects is followed from their birth up to the present moment. (see astro-ph/0411792 and Physics Uspekhi 2007 N11)

Why PS is necessary? No direct experiments computer experiments Long evolutionary time scales Selection effects. We see just a top of an iceberg. Expensive projects for which it is necessary to make predictions

Tasks To test and/or to determine initial and evolutionary parameters. To do it one has to compare calculated and observed popualtions. This task is related to the main pecularity of astronomy: we cannot make direct experiments under controlled conditions. To predict properties of unobserved populations. Population synthesis is actively use to define programms for future observational projects: satellites, telescopes, etc.

Log N – Log S Log of the number of sources -3/2 sphere: number ~ r3 flux ~ r-2 -1 disc: number ~ r2 calculations Log of the number of sources brighter than the given flux Log of flux (or number counts)

Population synthesis: ingredients Birth rate of NSs Initial spatial distribution Spatial velocity (kick) Mass spectrum Thermal evolution Interstellar absorption Detector properties

Population synthesis – I. Gould Belt : 20 NS Myr-1 Gal. Disk (3kpc) : 250 NS Myr-1 Arzoumanian et al. 2002 ROSAT Cooling curves by Blaschke et al. Mass spectrum 18° Gould Belt

New version of the code Recently we finished an advanced version of the code. Spatial distribution Interstellar distribution Interstellar absorption Response matrix Mass spectrum B. Posselt, S. Popov, F. Haberl, R. Neuhauser, J. Truemper, R. Turolla A&A 482, 617 (2008)

The Gould Belt Poppel (1997) R=300 – 500 pc Age 30-50 Myrs Center at 150 pc from the Sun Inclined respect to the galactic plane at 20 degrees 2/3 massive stars in 600 pc belong to the Belt

Population synthesis – II. recent improvements 1. Spatial distribution of progenitor stars We use the same normalization for NS formation rate inside 3 kpc: 270 per Myr. Most of NSs are born in OB associations. For stars <500 pc we even try to take into account if they belong to OB assoc. with known age. a) Hipparcos stars up to 500 pc [Age: spectral type & cluster age (OB ass)] b) 49 OB associations: birth rate ~ Nstar c) Field stars in the disc up to 3 kpc

Effects of the new spatial distribution on Log N – Log S There are no significant effects on the Log N – Log S distribution due to more clumpy initial distribution of NSs. But, as we’ll see below, the effect is strong for sky distribution. Solid – new initial XYZ Dashed – Rbelt = 500 pc Dotted – Rbelt = 300 pc

Mass spectrum of NSs Mass spectrum of local young NSs can be different from the general one (in the Galaxy) Hipparcos data on near-by massive stars Progenitor vs NS mass: Timmes et al. (1996); Woosley et al. (2002) astro-ph/0305599

Population synthesis – II. recent improvements 2. New cross sections & abundances and new mass spectrum Low mass progenitors for the dotted mass spectrum are treated following astro-ph/0409422. The new spectrum looks more “natural”. But the effect is ....

Effects of the new mass spectrum and abundances on the Log N – Log S ... Effect is negligible We also introduced new abundances, and calculated count rate more accurately than before. Still, the effect is small. Solid – new abundances, old mass Dotted – old abundances, old mass Dashed – new abundances, new mass

Population synthesis – II. recent improvements 3. Spatial distribution of ISM (NH) instead of : NH inside 1 kpc (see astro-ph/0609275 for details) now : Modification of the old one Hakkila

Effects of the new ISM distribution Again, the effect is not very significant for Log N – Log S, but it is strong for the sky distribution (see below). Dot-dashed and dot-dot-dashed lines Represent two new models of the ISM distribution.

First results: new maps b= +90° b= -90° Popov et al. 2005 Count rate > 0.05 cts/s Ori Sco OB Cep?Per? PSRs+ Geminga+ M7 PSRs- Clearly several rich OB associations start to dominate in the spatial distribution

INSs and local surrounding Massive star population in the Solar vicinity (up to 2 kpc) is dominated by OB associations. Inside 300-400 pc the Gould Belt is mostly important. Motch et al. 2006 De Zeeuw et al. 1999

50 000 tracks, new ISM model Candidates: Agueros Chieregato radiopulsars Magn. 7

Age and distance distributions 0.01 < cts/s < 0.1 0.1 < cts/s < 1 1 < cts/s < 10 Age New cands. Distance

Different models: age distributions Bars with vertical lines: old model for Rbelt=500 pc White bars: new initial dist Black bars: new ISM (analyt.) and new initial distribution Diagonal lines: new ISM (Hakkila) and new initial distribution

Different models: distance distr.

Where to search for more cowboys? We do not expect to find much more candidates at fluxes >0.1 cts/s. Most of new candidates should be at fluxes 0.01< f < 0.1 cts/s. So, they are expected to be young NSs (<few 100 Mys) just outside the Belt. I.e., they should be in nearby OB associations and clusters. Most probable candidates are Cyg OB7, Cam OB1, Cep OB2 and Cep OB3. Orion region can also be promising. Name           l-      l+      b-    b+      Dist., pc Cyg OB7     84      96     -5     9       600-700 Cep OB2     96     108    -1   12       700 Cep OB3    108    113     1     7       700-900 Cam OB1   130    153    -3     8       800-900 130 L=110 90 10 -10 (ads.gsfc.nasa.gov/mw/)

Gamma-ray selected sources Recently Crawford et al. (astro-ph/0608225) presented a study of 56 well-identified EGRET error boxes. The idea was to find radio pulsars. Nothing was found. Obviously, they can be geminga-like sources, or represent some other subpopulation of cooling NSs. However, Keith et al. (0807.2088) found a PSR at high frequency in one of EGRET error boxes. We are wainting for results from GLAST-Fermi. Gamma-ray selected INSs.

OB runaway stars Another possibility to find new ICoNSs is to search for (un)bound compact companions of OB runaway stars. More than one hundred OB runaway stars are known in 1 kpc around the Sun (astro-ph/9809227). Unbounded NSs Bounded NSs Sayer et al. 1996 and Philp et al. 1996 looked for radio pulsars as companions of runaway stars. It is reasonable to look for M7-like companions around young OB stars. Optical star bh (for BHs done in astro-ph/0511224)

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 (<1012 G). These sources are hot. The M7 sources are hot, too, but they seem to belong to different populations. This can be explained by accreted envelopes in CCOs (Kaminker et al. 2006). It is necessary to make a general population synthesis, which would include all types of isolated NSs.

M 7 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 Temperature 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? (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???

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

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, 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)

M7 and high-B PSRs Strong limits on radio emission from the M7 are established (Kondratiev et al. 2008: 0710.1648). However, observationally it is still possible that the 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 PSRs the magnetic field distribution is described as a gaussian, so that high-B PSRs appear to be not very numerous. On the other hand, population synthesis of the local population of young NSs demonstrate that the M7 are as numerous as normal-B PSRs. So, for standard assumptions it is much more probable, that high-B PSRs and the M7 are not related.

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

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 form of superconducting vortices. They can decay only when they are moved into the crust (during spin-down). Still, in most of models strong fields decay.

Period evolution with field decay An evolutionary track of a NS is very different in the case of decaying magnetic field. The most important feature is slow-down of spin-down. Finally, a NS can nearly freeze at some value of spin period. Several episodes of relatively rapid field decay can happen. Number of isolated accretors can be both decreased or increased in different models of field decay. But in any case their average periods become shorter and temperatures lower. astro-ph/9707318

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 the situation can be different). Ohm and Hall decay arxiv:0710.0854 (Aguilera et al.)

Joule heating for everybody? It is important to understand the role of heating by the field decay for different types of INS. In the model by Pons et al. the effect is more important for NSs with larger initial B. Note, that the characteristic age estimates (P/2 Pdot) are different in the case of decaying field! arXiv: 0710.4914 (Aguilera et al.)

Magnetic field vs. temperature The line marks balance between heating due to the field decay and cooling. It is expected by the authors (Pons et al.) that a NS evolves downwards till it reaches the line, then the evolution proceeds along the line. Selection effects are not well studied here. A kind of population synthesis modeling is welcomed. Teff ~ Bd1/2 Pons et al. There is an extra free parameter b. It is the ratio B^2/B_d^2. B - field in the crust due to surrents, B_d – dipole field in the crust. (astro-ph/0607583)

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

Log N – Log L Two magnetic field distributions: with and without magnetars (i.e. different magnetic field distributions 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”. [Popov, Pons, work in progress]

Populations .... 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 a search for periodicity are very important and have to be improved (a task for eROSITA?). Lx> 3 1033 erg s-1 [Muno et al. 2007]

Resume on the pop. synthesis of INSs New more detailed population synthesis model for local population of isolated NS is made New results provide a hint to search for new coolers. We predict that new objects can be identified at 0.01<cts/s<0.1 behind the Gould Belt in the directions of close-by rich OB associations, in particular Cep OB2. These objects are expected to be younger and hotter than the M7. New ways to find candidates can be discussed. The M7 can be related to RRATs (and even magnetars), but CCOs are different. If the magnetic field decay is important or not is still unclear for the M7. Still, it is possible to explain the local population of NSs with field decay.

The Magnificent Seven Vs. Uncatchables Born in the Gould Belt. Bright. Middle-aged. Already observed. Born behind the Belt. Dimmer. Younger. Wanted. That’s all!