Transcript Close-by young isolated neutron stars (and black holes)

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
2
Good old classics
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
3
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.
o Compact central X-ray sources
in supernova remnants.
o Anomalous X-ray pulsars
o Soft gamma repeaters
o The Magnificent Seven
o Unidentified EGRET sources
o 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)
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Compact central X-ray sources in
supernova remnants
Cas A
No pulsations,
small emitting area
RCW 103
6.7 hour period
(de Luca et al. 2006)
Puppis A
Vkick=1500 km/s
(Winkler, Petre 2006)
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CCOs in SNRs
J232327.9+584843
J085201.4−461753
J082157.5−430017
J121000.8−522628
J185238.6+004020
J171328.4−394955
Age
Cas A
0.32
G266.1−1.2 1–3
Pup A
1–3
G296.5+10.0 3–20
Kes 79
~9
G347.3−0.5 ~10
Distance
3.3–3.7
1–2
1.6–3.3
1.3–3.9
~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]
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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
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SGRs: periods and giant flares
0526-66
 1627-41
 1806-20
 1900+14
 0501+45

P, s
Giant flares
8.0
5 March 1979
6.4
18 June 1998 (?)
7.5
27 Dec 2004
5.2
27 Aug 1998
5.7
See the review in
Woods, Thompson
astro-ph/0406133
and Mereghetti
arXiv: 0804.0250
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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
CXO 010043-7211
8.0
4U 0142+61
8.7
1E 1048.1-5937
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
7.0
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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

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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)
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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.
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Magnificent Seven
Name
RX 1856
RX 0720
RBS 1223
RBS 1556
RX 0806
RX 0420
RBS 1774
Period, s
7.05
8.39
10.31
6.88?
11.37
3.45
9.44
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

RX J0420.0-5022 (Haberl et al 2004)
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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
First papers on the thermal
evolution appeared already
in early 60s, i.e. before
the discovery of radio pulsars.
(Yakovlev et al. (1999)
Physics Uspekhi)
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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
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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)
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Why PS is necessary?
1. No direct experiments
computer experiments
2. Long evolutionary time scales
3. Selection effects. We see just a top of an iceberg.
4. Expensive projects for which it is necessary to make predictions
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Tasks
1. 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.
2. To predict properties of unobserved populations.
Population synthesis is actively use to define programms for future
observational projects: satellites, telescopes, etc.
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Log of the number of sources
brighter than the given flux
Log N – Log S
calculations
-3/2 sphere:
number ~ r3
flux
~ r-2
-1 disc:
number ~ r2
flux
~ r-2
Log of flux (or number counts)
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Population synthesis: ingredients
 Birth rate of NSs
 Initial spatial distribution
 Spatial velocity (kick)
 Mass spectrum
 Thermal evolution
 Interstellar absorption
 Detector properties
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Population synthesis – I.
Gould Belt : 20 NS Myr-1
Gal. Disk (3kpc) : 250 NS Myr-1
• Cooling curves by
• Blaschke et al.
• Mass spectrum
ROSAT
18°
Gould Belt
Arzoumanian et al. 2002
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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)
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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

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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.
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
For stars <500 pc we even
try to take into account
if they belong to OB assoc.
with known age.
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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
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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
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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 ....
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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
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Population synthesis – II.
recent improvements
3. Spatial distribution of ISM (NH)
instead of :
now :
Modification of the old one
NH inside 1 kpc
(see astro-ph/0609275 for details)
Hakkila
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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.
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First results: new maps
Popov et al. 2005
Count rate > 0.05 cts/s
b= +90°
Cep?Per?
Sco OB
Ori
b= -90°
PSRs+
Geminga+
M7
Clearly several rich
OB associations start
to dominate in the
spatial distribution
PSRs35
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.
De Zeeuw et al. 1999
Motch et al. 2006
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50 000 tracks, new ISM model
Candidates:
Agueros
Chieregato
radiopulsars
Magn. 7
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Age and distance distributions
0.01 < cts/s < 0.1
0.1 < cts/s < 1
1 < cts/s < 10
Age
New cands.
Distance
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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
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Different models: distance distr.
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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+
Cyg OB7
84
96
-5
9
Cep OB2
96
108
-1 12
700
Cep OB3
108
113
1
700-900
7
Dist., pc 130
90
L=110
10
600-700
0
-10
Cam OB1 130
153
-3
8
800-900
(ads.gsfc.nasa.gov/mw/)
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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.
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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
Optical star
bh
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.
(for BHs done in astro-ph/0511224)
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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.
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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!
Temperature
CCOs
M7
Age
• Accreted envelopes
(presented in CCOs,
absent in the M7)
• Heating by decaying
magnetic field
in the case of the M7
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(Yakovlev & Pethick 2004)
Accreted envelopes, B or heating?
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?
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(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)
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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.
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Magnetars, field decay, heating
Pdot
A model based on field-dependent decay of the magnetic moment of NSs
can provide an evolutionary link between different populations (Pons et al.).
Magnetars
M7
PSRs
P
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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.
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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
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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 the
situation can be different).
Ohm and Hall decay
arxiv:0710.0854 (Aguilera et al.)
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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.)
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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.
Teff ~ Bd1/2
Selection effects are not
well studied here.
A kind of population
synthesis modeling is
welcomed.
(astro-ph/0607583)
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Log N – Log S with heating
Log N – Log S for 4 different
magnetic fields.
1. No heating (<1013 G) 3. 1014 G
2. 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]
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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]
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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]
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Resume on the
pop. synthesis of INSs

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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.
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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!
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