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Astronomy & Astrophysics manuscript no.
(will be inserted by hand later)
Young isolated neutron stars from the Gould Belt
Sergei B. Popov 1;5 , Monica Colpi 2 , Mikhail E. Prokhorov 1 , Aldo Treves 3 , and Roberto Turolla 4
1 Sternberg Astronomical Institute, Universitetski pr. 13, 119899 Moscow
e-mail: polar@sai.msu.ru; mike@sai.msu.ru
2 University of Milano-Bicocca, Italy
e-mail: Monica.Colpi@mib.infn.it
3 University dell'Insubria, Como, Italy
e-mail: treves@mi.infn.it
4 University of Padova, Italy
e-mail: turolla@pd.infn.it
5 Isaac Newton Institute, Moscow branch;
Abstract. We present Log N { Log S distribution for close young isolated neutron stars. On the basis of Log N
{ Log S distribution it is shown that seven ROSAT isolated neutron stars if they are young cooling objects are
genetically related to the Gould Belt. We predict, that there are about few tens unidentied close young isolated
neutron stars in the ROSAT All-Sky Survey.
1. Introduction
Among dierent types of neutron stars (NSs) there is a
small group of seven radioquiet isolated NSs discovered
by ROSAT (RINSs) (see a review in Treves et al. 2000,
and in Russian in Popov, Prokhorov 2002).
Now it is generally accepted, that at least one of these
objects (RX J1856, see the table) is a young cooling iso-
lated NS. We assume that all of them belong to this type
and make a population synthesis of close young isolated
NSs.
Our paper is motivated by the fact, that averaged pa-
rameters of the spatial density of radio pulsars in our
Galaxy cannot produce enough coolers, i.e. NSs which are
young (hot) and close enough to be observed, to explain
RINSs (Neuhauser, Trumper 1999, Popov et al. 2000b).
Previously we brie y discussed connection between
RINSs and the Gould Belt (Popov et al. 2002). Here we
address this question in more details.
2. Model
Main components of our model are: spatial distribution
of NS progenitors, NS formation rate, NS cooling histo-
ries and model of interstellar absorbtion (i.e. interstellar
medium (ISM) distribution). In addition we calculate dy-
namical evolution of NSs in the galactic potential. This
addition does not have a big in uence, as far as time scale
Send oprint requests to: S. Popov
of our calculations (cooling time scale) is short in com-
parison with a time in which a typical NS crosses the re-
gion of our calculations, and also all other components of
the model are independent on NSs velocities. But taking
into account of the dynamical evolution makes our calcu-
lations more realistic (for example, NSs in our calculations
are not produced in 50-100 pc around the Sun, but they
can enter this region if they have corresponding veloci-
ties; also dynamical evolution is very important for NSs
which are born close to the Sun). Also spatial movements
of NSs changes distribution of observed sources on the sky
in comparison with progenitor distribution.
In brief our model can be described in the following
way: NSs are born in the Galactic plane and in the Gould
Belt; at birth they recieve a kick velocity; then we follow
the evolution of NSs in the Galactic potential; while a NS
moves in the Galaxy we calculate (with some time step)
ROSAT count rate basing on cooling curves and assumed
model of interstellar absorbtion.
Typically we calculate about a thousand of NS evolu-
tionary tracks in each run of the programm (for the nal
results we perform larger runs with 10 4 tracks) and
then normalize our results (real number of young NSs in
the region we calculate is expected to be about one thou-
sand, depending on the limiting age, NS formation rate
and calculated volume).
An evolutionary track is calculated for specied initial
position and initial velocity of a NS. All tracks are used on
their whole duration and are applied to NSs of dierent
masses, i.e. for each track we have several dierent cooling

2 Sergei B. Popov et al.: Young isolated neutron stars
histories, the number of which depends on the number
of dierent masses we use in this particular run. So (as
in Popov et al. 2000a,b) each track actually represents
a population of NSs of dierent masses continuously born
at specied place with specied spatial velocity during the
whole period of calculation.
We varied parameters of our model (NS formation rate,
NSs mass spectrum, time of calculation, time step, param-
eters of the spatial distribution), in the following subsec-
tions we describe in more details parameters correspond-
ing to the results presented in Sec. 3.
2.1. Spatial distribution
We assume that NSs are born in the Galactic disk and in
the Gould Belt. NSs are considered to be born with a con-
stant rate. For NS formation rate we assume an estimate of
30 NSs births in 0.6 kpc vicinity of the Sun per Myr. This
estimate comes from direct counting of SN progenitors in
the Solar vicinity, it is not dependent on the observed SN
rate or on radio pulsar statistics, and corresponds to 2.9
10 11 massive progenitors per yr per pc 2 (Tammann et al.
1994) (we assume that 90% of core collapse SNae produce
NSs).
Among these 30 NSs per Myr 20 are born in the Gould
Belt in correspondence with a rate 20{27 SN per Myr given
in (Grenier 2000), and the rest are uniformly distributed in
the disk in 0.6 kpc around the Sun. It is in correspondence
with (Torra et al. 2000), who estimate that about 2/3 of
massive stars inside 0.6 kpc are in the Gould Belt, and the
rest | in the Galactic disk. The age of the Gould Belt is
estimated (Poppel 1997) as 30{ 70 Myr, so now stars with
masses about 10 M are ending their lives, which gives
high relative number of SNae.
To take into account possible contribution of farther
NSs at low uxes (about 0.1 cts s 1 and lower) we also
calculate NSs born in the Galactic disk at distances > 0:6
kpc (but < 3 kpc) around the Sun. The NS formation rate
for this region is assumed to be the same as in the close-by
parts of the Galactic disk. In a region of 1 kpc around the
Sun it gives (together with the same xed Gould Belt con-
tribution) the rate of 20 SNae of all types per Myr per
kpc 2 in correspondence with existing estimates (Tammann
et al. 1994) (we follow that paper assuming the fraction of
core collapse SNae to be 85%). Here we underestimate the
number of far (> 1 kpc) new born NSs in the direction to
the Galactic center, but interstellar absorbtion there (see
below) is very high in the Galactic plane, and the impact
of such underestimation is not serious.
So, 20 NSs per Myr come from the Gould Belt, and
250 NSs per Myr are uniformly distributed in the Galactic
plane with a limiting distance from the Sun 3 kpc. The
closest Solar vicinity al low galactic latitudes is known
to be under populated by massive stars (Ma iz-Apellaniz
2001), so we assume, that NSs are not born in the Galactic
disk at distances < 100 pc from the Sun.
The Gould Belt is modeled as a disk with 500 pc ra-
dius and inclination to the Galactic plane equal to 18 ф .
Center of the disk is situated at 100 pc from the Sun
in the Galactic anticenter direction. In the center of the
disk there is a hole with a radius 150 pc, where NSs are
not born (see Poppel 1997 for detailed describtion of the
Gould Belt, and Torra et al. 2000 for a shorter one).
NS are assumed to be born with kick velocities with
maxwellian distribution with an average value 225 km
s 1 . Varying this parameter does not changes our results
signicantly, but it is clear, that if average velocity is
larger, then we can observe less RINSs.
For calculations of NSs evolution in the Galactic po-
tential we use the same parameters and procedure as in
Popov et al. (2000b). The Galactic potential is assumed to
be axisymmetric consisting of disk, buldge and halo parts
(see Paczynski 1990). The Sun was assumed to be at the
Galactic plane at a distance 8.5 kpc from the center of the
Galaxy.
2.2. Cooling curves and ux calculation
To calculate cooling of NSs we use the data obtained by
Sankt-Petersburg group (see Kaminker et al. 2002, and a
review in Yakovlev et al. 1999).
We use cooling curves for NSs of masses from 1:1 M
to 1:8 M with a step 0:1 M (see Fig. 1). Curves take
into account all processes of neutrino emission. EOS used
in Kaminker et al. (2002) was introduced in Prakash et al.
(1988). It is Model I of Prakash et al. for symmetry energy
and compression modulus of saturated nuclear matter, K,
is equal to 240 MeV. Maximum NS mass in that model is
1.977 M . Neutron super uidity in the crust and core is
ignored, as far as it does not in uenced the nal results
signicantly.
We start ux calculations when a NS has an age 10 000
yrs (Vela { the youngest close-by NS { has an age slightly
above this number, also NS formation rate which we use
in our calculations corresponds to a NS birth every 10
000 yrs in 1:7 kpc region around the Sun). Calculations
for each NS are truncated when its temperature falls down
to 100 000 K (it corresponds to an age 4.25 Myrs for the
lightest NSs (M = 1:1 M ) and less for more massive
stars. Such a cool NS could be detected by ROSAT PSPC
from the distance of only 10 pc as a source with count rate
5 10 3 cts s 1 .
In our calculations we assume, that NSs have at dis-
tribution in mass spectrum between 1.1 and 1.8 solar
masses. With the numbers above (4.25 Myrs and 10 000
yrs) we can say, that each track in our calculation repre-
sents (425 8) NSs of eight dierent masses born with a
time step 10 4 yrs at the same place with the same initial
velocity in a period 4.25-0.01 Myrs ago.
We assume that emission comes from the whole sur-
face of a NS. It is a reasonable assumption as far as pulsed
fractions for RINSs are very low or there are only upper

Sergei B. Popov et al.: Young isolated neutron stars 3
Log Time, yrs
0 2 4 6
4.5
5
5.5
6
6.5
Fig. 1. Cooling curves for dierent masses (here we use calcu-
lations presented in Kaminker et al. 2002). Curves from top to
bottom corresponds to masses 1.1; 1.2; ...; 1.8 M . On curves
and everywhere in the text we refer to the eective surface red-
shifted (i.e. observed at innity) temperatures in the case of a
pure black body emission (no possible atmospheric eects are
included)
limits (see Haberl, Zavlin 2002). We do not take into ac-
count any eects of NS's atmosphere.
As one can see from the Fig. 1 the main contribution
to Log N - Log S distribution is given by NSs with masses
< 1:5 M (young isolated NSs which are observed in sev-
eral SN remnants as compact X-ray sources due to their
thermal radiation also should be relatively low massive
objects, see Kaminker et al. 2002). Observations of binary
radio pulsars suggest, that NSs masses are strongly peaked
between 1.3 and 1.4 M . Such distribution was also tested
in our calculations. It does not change our result signi-
cantly (numbers of RINSs are increased by 30 %).
2.3. ISM and absorbtion
As far as we expect our NSs to emit most of their lumi-
nosity at soft energies 20 200 eV (which corresponds
to temperatures about 10 5 {10 6 K) we have to take into
account interstellar absorbtion. Absorbtion is a very im-
portant feature of our model. Any attempts to estimate
the amount of observable cooling isolated NSs using un-
absorbed ux greatly overestimate this number.
For ISM distribution we use the same model as in
Popov, Prokhorov (1998). The Local Bubble is modelled
as a sphere with 140 pc radius and ISM number density
0.1 cm 3 . Typical column densities, NH , for observable
RINSs are about 10 19 {10 21 cm 2 .
After a column density is calculated for a current posi-
tion of a NSs we calculate unabsorbed ux basing on cur-
rent temperature, radius of a NS and its distance from the
Sun, and apply starndard procedure to calculate ROSAT
counts.
Outside the Local Bubble in directions close to the
Galactic plane absorbtion starts to play a crucial role.
That is why regions closer to the Galactic center (< 7
kpc), where NS formation rate should be higher than in
the Galactic disk in the Solar vicinity, cannot add many
sources to our sample (at NH =3 10 21 cm 2 even young
and hot low mass NSs with kT 0:1 keV can not be
detected from the distance 1 kpc in the ROSAT Bright
Survey (count rate > 0.2 cts s 1 )).
Our model does not take into account small irregulari-
ties of the ISM distribution (except the Local Bubble, but
even it we treat in a simplyed way). They can be impor-
tant if one makes an attempt to produce a realistic map
of RINSs distribution on the celestial sphere. But for the
case of calculation of averaged over the sky Log N { Log
S distribution our approximation is valid.
3. Results
Our aim is to calculate Log N { Log S distribution for
young close isolated NSs born at the Gould Belt and in
the Galactic disk, and to compare this distribution with
known INSs observed by ROSAT due to their thermal
emission.
Main results are presented in the Fig. 2. We present
a curve for NSs born in the Gould Belt and the Galactic
disk and a curve only for the disk to show the relative
importance of the Gould Belt. All curves are refering for
the whole sky.
We put observed points basing on ROSAT data for
the \magnicent seventh" (seven RINSs) and six other
close-by young isolated NSs. Note, that for the later we
use total count rate, where non-thermal component can be
signicant, it is especially important for the Vela pulsar
and for 1929+10.
It was shown before (Neuhauser, Trumper 1999, Popov
et al. 2000b) that cooling NSs with spatial density of nor-
mal radiopulsars cannot explain the observed Log N {
Log S distribution for RINSs. In Popov et al. (2002) we
suggested, that the Gould Belt is responsible for that in-
creased spatial density of young NSs, Here we show by
population synthesis calculations, that increase in density
is easily explained by the local feature | the Gould Belt,
and the curve excellently ts not only RINSs, but also
other isolated NSs observed by ROSAT.
As can be seen the Gould Belt alone can explain all
observed points. As soon as velocity vectors for RINSs will
be know it will be possible to calculate their exact birth
places in the Belt (or, possibly, in the disk). Addition of
the disk contribution gives room for few more sources at
high (> 0.1 cts s 1 ) uxes, and for tens | at low (< 0.03
cts s 1 ). At lower uxes ((< 0.01 cts s 1 ) the Galactic disk
starts to dominate over the Gould Belt. The disk curve

4 Sergei B. Popov et al.: Young isolated neutron stars
2 1 0 1
1
0
1
2
RBS
RX 1856
Vela
0659+14
RX 0720
RBS 1556
Geminga
105752
RX 0806
RBS 1223
RBS 1774
RX 0420
3EG 1835
1920+10
disk+GB
disk
Fig. 2. All-sky Log N - Log S distribution. Black trian-
gles | seven RINSs. Crosses | Geminga, \three muske-
teers", 1929+10 and 3EG J1835. We also show the RBS limit
(Schwope et al. 1999). Upper curve: NSs born in the Gould
Belt and in the Galactic disk (r disk = 3 kpc, total birth rate
270 Myr 1 ). Lower curve: only disk (r disk = 3 kpc, birth rate
250 Myr 1 )
alone is always below observed points (except the faintest
ones | 3EG 1835 and 1929+10) and below the RBS limit
(note, that RBS itself was devoted to the sources outside
the galactic plane).
Absorbtion and at geometry of NSs distribution nat-
uraly explain very at (< 1) Log N { Log S. In this
work we do not try to t exactly the observed data, but
note, that potentially one can vary mass spectrum of NSs,
NS formation rate and atmosphere model (we use simple
black body) to improve the Log N { Log S distribution to
get better correspondence with observations. In our opin-
ion we make simple realistic assumption, and are able to
show that even in that simple picture it is possible to ex-
plain data on isolated NSs observed by ROSAT.
Decit of very bright (> few cts s 1 ) objects in ex-
plained by poor statistics (for example the youngest NS
in our sample | Vela | could be born closer, or the
brightest (the closest?) object | RX J1856 | could be
younger).
Our calculations show, that there are about few dozens
of unidentied close isolated NSs in ROSAT All-Sky
Survey (> 0.015 cts s 1 ) depending on parameters of
the model, also there can be few unidentied RINSs and
uxes > 0:1 cts s 1 at low Galactic latitudes (see also
Schwope et al. 1999). Most of objects should be observed
at 20 ф from the Galactic plane towards the directions
of lower absorbtion. Some of them can have counterparts
among unidentied gamma-ray sources (also connected
with the Gould Belt, see Grenier 2000). Identication of
these objects can be important for choosing a correct cool-
ing model and for determination of mass spectrum of NSs.
4. Discussion
In this part of the paper we brie y discuss some topics
connected with properties of RINSs and other types of
isolated NSs.
4.1. Local population of young NSs
Here we discuss near-by (r < 1 kpc) young (age< 4:25
Myrs) isolated NSs.
At the present moment it is known about 20 NSs satis-
fying the above criteria (see Table). It includes: \magni-
cent seventh" (seven RINSs), Geminga and Geminga-like
object 3EG J1835, \three musketeers" (Vela, 0656+14,
1055-52), 1929+10 and seven radio pulsars, which are not
detected in X-rays.
The later objects are relatively old (we mention a
strong \jump" in ages in between PSR 1055-52 and PSR
J0056+4756; ux of PSR 1929+10 may be mainly due
to non-thermal radiation). At these distances even if NSs
have very low masses they are too cold now to be detected
by ROSAT.
If we limit ourselves with a maximum age 1 Myr,
then we see nearly all bright (means also low massive)
cooling isolated NSs in our vicinity (see Fig. 1), the num-
ber of RINSs above 0:1 cts s 1 can be maximaly dou-
bled.
We see, that among local young isolated NSs we have
normal radio pulsars, pulsars beams of which do not
pass the Earth (Geminga and probably 3EG J1835), and
RINSs. Also we can expect, that around us (inside 1 kpc)
there are about one hundred of isolated NSs younger than
4 Myr (some of NSs born inside 1 kpc of course can
leave this volume in several Myrs). These NSs are not de-
tected as radio pulsars, but tens of them can be identied
in ROSAT data as dim sources. The beaming eect can be
resposible only for part of these undetected (as radiopul-
sars) young NSs (about 50-70% of young pulsars are not
visible from Earth (Tauris, Manchester 1998)), and most
of RINSs should be really radio silent (it is diфcult to
construct a model in which one observes X-ray pulsations,
pure black body spectrum and no signs of radio emission
from a close o-beam pulsar, also one has to take into ac-
count long periods of four RINSs). It gives strong support
to the arguments by Gotthelf and Vasisht (2000), that
\at least half of the observed young neutron stars follow
an evolutionary path quite distinct from that of the Crab
pulsar".
4.2. Alignment
An interesting feature of RINSs population is an existence
of periods about 5-20 seconds (typical for magnetars) for
four objects and their absence for the rest three.

Sergei B. Popov et al.: Young isolated neutron stars 5
Table 1. Local (r < 1 kpc) population of young (age < 4:25 Myrs) isolated neutron stars
Object name Period, Count rate, _
P Distance, Age a , Ref.
s ROSAT cts/s =10 15 kpc Myrs
RX J185635-3754 | 3.64 | 0.117 d 0:5 [1,2]
RX J0720.4-3125 8.37 1.69 30 60 | | [1,3]
1RXS J130848.6+212708 (RBS1223) 5.15 0.29 10 4 | | [1.4]
RX J1605.3+3249 (RBS1556) | 0.88 | | | [1]
RX J0806.4-4123 11.37 0.38 | | | [1,5]
RX J0420.0-5022 22.7 0.11 | | | [1]
1RXS J214303.7+065419 (RBS1774) | 0.18 | | | [6]
PSR B0633+17 (Geminga) 0.237 0.54 c 10.97 0.16 d 0.34 [7]
RX J1836.2+5925 (3EG J1835+5918) | 0.015 | | | [8]
PSR B0833-45 (Vela) 0.089 3.4 c 124.88 0.294 d 0.01 [7,9,10]
PSR B0656+14 0.385 1.92 c 55.01 0.762 e 0.11 [7,10]
PSR B1055-52 0.197 0.35 c 5.83 1 b 0.54 [7,10]
PSR B1929+10 0.227 0.012 c 1.16 0.33 d 3.1 [7,10]
PSR J0056+4756 0.472 | 3.57 0.998 e 2.1 [10]
PSR J0454+5543 0.341 | 2.37 0.793 e 2.3 [10]
PSR J1918+1541 0.371 | 2.54 0.684 e 2.3 [10]
PSR J2048-1616 1.962 | 10.96 0.639 e 2.8 [10]
PSR J1848-1952 4.308 | 23.31 0.956 e 2.9 [10]
PSR J0837+0610 1.274 | 6.8 0.722 e 3.0 [10]
PSR J1908+0734 0.212 | 0.82 0.584 e 4.1 [10]
a ) Ages for pulsars are estimated as P=(2 _
P ),
for RX J1856 estimate of an age comes from kinematical considerations.
b ) Distance to J1057-52 is uncertain ( 0.9-1.5 kpc)
c ) Total count rate (black body + non-thermal)
d ) Distances determined through parallactic measurements
e ) Distances determined with dispersion measure
(1) Treves et al. (2000)
(2) Kaplan et al. (2002a)
(3) Zane et al. (2002)
(4) Hambaryan et al. (2001)
(5) Haberl, Zavlin (2002)
(6) Zampieri et al. (2001)
(7) Becker, Trumper (1997)
(8) Mirabal, Halpern (2001)
(9) Pavlov et al. (2001)
(10) ATNF Pulsar Catalogue (http://wwwatnf.atnf.csiro.au/research/pulsar/catalogue/)
One explanation can be that four RINSs are magne-
tars. Magnetar periods are explained by magnetodipole
spin-down and eld decay (\period freezing", see Colpi
et al. 2000). But recent observations (Zane et al. 2002,
Kaplan et al. 2002b) suggest, that RX J0720 is not (and
was not) a magnetar.
The fact of small pulsation fractions or there absence
in RINSs also can be explained by the light bending (see
discussion in Haberl, Zavlin 2002). But this mechanism
works better for massive NSs, and we suspect, that RINSs
are vice versa the lightest NSs, as far as they are relatively
hot. So, it is better to nd another way to explain this
situation.
Vasily Beskin (2001, private communication) sug-
gested, that it can happen due to alignment of magnetic
and spin axis (see for example Tauris, Manchester 1998 for
a recent discussion on alignment). Alignment is a process
which also leads to a \period freezing" and low pulsation
fraction (see Haberl, Zavlin 2002 for the data on pulsation
fraction in RINSs).

6 Sergei B. Popov et al.: Young isolated neutron stars
However in the case of coolers alignment should op-
erate on short timescale | timescale of NS cooling ( 1
Myr). As far as for radiopulsars timescale of alignment
is 10 7 yrs (Tauris, Manchester 1998) or longer, we
think, that it is unlikely that this mechanism is responsi-
ble for RINSs distribution over pulsation fraction, other-
wise one has to assume that RINSs form a population
separate from normal radiopulsars. For example, if we
assume, that alignment timescale is
/(
2
0 cos 2 0 B 2
0 ) 1
(it comes from magnetodipole spin-down and condition

0 cos 0
=
cos; also there can be other assumptions
about alignment process, but at the moment there is no
generally accepted model), then we have to assume that
RINSs have dierent (from radiopulsars) distribution in
B 0 or/and 0 , and the observed situation is just a selec-
tion efect. For example they can be a sample selected by
relatively high surface temperatures, which for young NSs
means by low masses.
4.3. Possible correlation between magnetic eld and
mass
If any part of RINSs (with or without observed pulsations)
is explained by high magnetic eld, then one has to explain
why the fraction of magnetars is so high ( 50%). It can be
explained as a selection eect if NSs with higher magnetic
elds are hotter. It can be so, if they have (on average)
lower masses (below some value, which is about 1.3 M
but depends on the equation of state, direct URCA process
are not allowed, and NS cooling is much slower).
The possible correlation can be explained if more mas-
sive NSs get their additional masses in the process of fall-
back. In that case their magnetic eld can be signicantly
supressed (Page et al. 1998), so more massive NSs should
have lower initial magnetic elds.
For example, if we calculate so called accretion period,
which separates accretor and propeller stages (Lipunov
1992), then we have:
PA = 2 5=14 (GM) 5=7 ( 2 = _
M) 3=7 ;
here { magnetic moment, _
M { accretion rate. If we
substitute = 10 32 G cm 3 (typical for magnetars) and
_
M = 1 M yr 1 (typical for fall-back process), then we
have:
PA 20(M=M ) 5=7 ( 2
32 =( _
M=(M yr 1 ))) 3=7 ms:
This is a typical value of a spin period for a newborn NS.
Of course, there is a question if the matter is swept out,
or it is accumulated close to the magnetospheric boundary,
and then accreted by the compact object. But numerical
estimates (Toropin et al. 1999) suggest, that signicant
part of matter is swept from the propelling NS.
Vice versa, strong initial magnetic eld together with
fast rotation can prevent strong fall-back (it is espe-
cially possible if magneto-rotational mechanism of super-
nova explosion is valid, see Bisnovatyi-Kogan 1970 and
Prokhorov, Postnov 2001), and may be it can also lead to
long initial spin periods.
Dierent crust thickness and temperature of NSs of
dierent masses can give additional correlations between
mass and magnetic eld, the same can be said about dif-
ferent properties at cores of NSs of dierent masses (for
example, deconnement in central parts of massive NSs,
Sedrakian, Blaschke 2002).
To summarize, we can expect correlations between
magnetic eld and masses of NSs, and observations of
RINSs can be of great importance here. Future determi-
nation of RINSs parallaxies and proper motions can help
to reconstruct their kinematical history and derive their
ages. It can give a clue to their mass determination basing
on cooling curves (see Kaminker et al. 2002). Our results
suggest, that the fraction of low massive NSs (M <1.3-
1.4 M ) can not be small, but on the other hand there
should be a room for massive NSs, because other wise we
overpredict the number of bright objects.
5. Conslusions and nal remarks
In this paper we show, that RINSs can be naturaly ex-
plained as cooling NSs originated in the Gould Belt, so
relatively high local spatial density of young NSs is due to
large number of massive progenitors. As far as there are
few young radio pulsars in the Solar vicinity many (about
1/2 or more) of isolated NSs should be radio quiet.
Our results show, that the fraction of NSs with masses
below 1.4 M is not small, but on the other hand it is
unlikely that they form the most part of NSs population.
We nd that in ROSAT All-Sky Survey it can be about
few tens of unidentied RINSs. Also there can be few
unidentied RINSs at uxes > 0:1 cts s 1 at low Galactic
latitudes.
Acknowledgements. We want to thank D.G. Yakovlev for the
data on cooling curves and comments on them, and V.S.
Beskin, Matteo Chieregato and Andrea Possenti for discus-
sions.
The work of S.P. was supported by the RFBR grant 02-02-
06663.
S.P. thanks Universities of Como, Milano-Bicocca and
Padova for hospitality.
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