>~200-300 km s-1, corresponding to a velocity
dispersion σV>~125-190 km
s-1. The same conclusion is reached for both a constant magnetic field
(B~10^12 G) and a magnetic field
decaying exponentially with a timescale ~10^9 yr. Such high velocities are
consistent with those derived from
radio pulsar observations. Present results, moreover, constrain the fraction
of low-velocity stars, which could
have escaped pulsar statistics, to less than 1%.
Title:
On the number of accreting and cooling isolated neutron
stars detectable with the
ROSAT All-Sky Survey
Authors:
Neuhauser, R.; Trumper, J. E.
Reference:
Astronomy and Astrophysics, v.343, p.151-156 (1999)
Abstract:
We present limits to the log N - log S curve for
isolated neutron stars, both cooling and accreting neutron
stars, which are not active as radio pulsars, as observed with the ROSAT
All-Sky Survey and compare it with
theoretical expectations. So far, only one isolated neutron star is
identified optically among ROSAT sources,
namely RXJ185635-3754 (Walter & Matthews 1997). Three more promising
candidates have been
suggested. In addition, several upper limit estimates are available on the
space density of such neutron stars
from different optical follow-up studies. We show that the log N - log S
curve according to the current
observations, including the identified neutron star, the three additional
candidates, and the upper limits, lies
between the theoretical expectations for middle-aged cooling neutron stars
and old accreting neutron stars. At
least one of the neutron star candidates found so far with ROSAT may be
cooling instead of accreting. We
suggest that the fact that more accreting isolated old neutron stars were
expected (e.g., Madau & Blaes 1994)
than observed is mostly due to the velocity distribution used in those
calculations. More recent radio
observations indicate that there are fewer slow neutron stars, ie., fewer
accreting X-ray bright old neutron
stars. At the X-ray bright end of the log N - log S curve, however, the
ROSAT observations agree well with
the theoretical expectations.
Title:
Are there any isolated old neutron stars in the ROSAT Wide
Field Camera
survey?
Authors:
Manning, R. A.; Jeffries, R. D.; Willmore, A. P.
Reference:
Monthly Notices of the Royal Astronomical Society,
Volume 278, Issue 2, pp.
577-585. (1996)
Abstract:
We present the results of Monte Carlo simulations to
predict the number of isolated old neutron stars (IONs)
that are observed in the ROSAT Wide Field Camera (WFC) survey as a result of
accretion-powered extreme
ultraviolet (EUV) emission. Magnetic field strengths of 10^9 to 10^12 G are
considered along with
different models for the local interstellar medium. The most recent
calibrations of the instrumental response
and spatial sensitivity variations are used. Our conclusions are that as
many as 10-20 IONs may have been
observed if initial magnetic fields decay quickly to values <=10^10 G, but
higher fields result in far fewer
detections. On the basis of the latest source lists and optical
identification information, which essentially rule
out the presence of more than one or two IONs in the WFC survey, we conclude
that one or all of the
following is possible. (i) The number of IONs in the Galaxy is less than
10^9 or the number of accreting
IONs may be depleted by accretion-prevention mechanisms. The latter is
considered unlikely because the
low-speed IONs that are most likely to have overcome the accretion barriers
form the majority of the detected
EUV sources. (ii) ION magnetic fields have not decayed to values <=10^10 G.
(iii) The number of
low-speed IONs may be fewer than expected, because of either dynamical
heating or uncertainties in the
birth distribution of pulsar speeds. (iv) The ION emission spectrum may be
significantly non-Planckian, with
a reduced flux in the WFC band.
Title:
The observability of old isolated neutron stars
Authors:
Treves, A.; Colpi, M.
Reference:
Astron. Astrophys. vol. 241 , p. 107 (1991)
Abstract:
The observability of old isolated neutron stars
accreting from the interstellar medium is discussed, considering
specifically the position sensitive proportional counter (PSPC) on board of
ROSAT. It is found that about
5000 old neutron stars can be detected in the All Sky Survey, and about
2/deg squared in Deep Exposures.
Key observational features of old accreting neutron stars are: (1) the weak
emission in the far UV and soft
X-ray band, (2) the absence of an optical counterpart below mv about 23, (3)
a small hydrogen column
density (N/H/ less than 10 to the 20th/sq cm), (4) a proper motion of about
0.1 arcsec/yr, and (5) the
preferred concentration along the Galactic plane observable in their
brightness distribution at the sampling
depth (about 400 pc) of the PSPC detection limit. These properties are used
to describe a selection procedure
and to assess future observational evidence. It is noted that about three
old accreting neutron stars may have
already been detected with the Einstein satellite, compatible with the
number of still unidentified objects.
Title:
The observability of old isolated neutron stars with
ROSAT. 2: Molecular
clouds and deep fields
Authors:
Colpi, M.; Campana, S.; Treves, A.
Reference:
Astron. Astrophys. vol. 278, p. 161 (1993)
Abstract:
The possibility of detecting with ROSAT old isolated
neutron stars (ONSs) accreting the interstellar medium
is re-examined. Their observability in molecular clouds is firstly
discussed. Considering in particular the
inventory of clouds by Dame (1987), it is found that a few ONSs detectable
above the ROSAT All Sky
Survey limit should be present in most of the clouds, and in some case they
should be numerous. ROSAT deep
exposures are then considered and it is shown that in the 10 square degrees
explored thus far (Hasinger 1992)
the number of detectable ONSs should be at most approximately 5. In this
case the perspective of
distinguishing ONSs from other sources seems rather arduous.
Title:
Old Isolated Accreting Neutron Stars: Contribution to the
Soft X-Ray
Background in the 0.5--2 keV Band
Authors:
Zane, Silvia; Turolla, Roberto; Zampieri, Luca; Colpi,
Monica; Treves, Aldo
Reference:
Astrophys. J. vol. 451, p. 739 (1995)
Abstract:
Title:
Can magnetic field decay explain the elusivity of old neutron
stars ?
Authors:
Treves, A.; Colpi, M.; Turolla, R.
Reference:
Astronomische Nachrichten, vol. 319, no. 1, p. 109. (1998)
Abstract:
Title:
The Elusiveness of Old Neutron Stars
Authors:
Colpi, Monica; Turolla, Roberto; Zane, Silvia; Treves,
Aldo
Reference:
Astrophysical Journal v.501, p.252 (1998)
Abstract:
Old neutron stars (ONSs) that have radiated away their
internal and rotational energy may still shine if they
are accreting the interstellar medium. Despite their large number, only two
promising candidates have been
detected so far, and rather stringent limits on their observability follow
from the analysis of ROSAT surveys.
This contrasts with optimistic theoretical estimates that predicted a large
number of sources in ROSAT fields.
We have reconsidered the issue of the observability of ONSs, accounting for
the spin and magnetic field
evolution over the neutron star lifetime. In the framework of a spin-induced
field decay model, we show that
the total number of ONSs that are at present in the accretion stage is
reduced by a factor of ~5 over previous
figures if the characteristic timescale for crustal current dissipation is
~108-109 yr. This brings theoretical
predictions much closer to observational limits. Most ONSs should be, at
present, in the propeller phase; if
subject to episodic flaring, they could be observable.
Theoretical studies of INS
Title:
The Thermal Evolution of Ultramagnetized Neutron Stars
Authors:
Heyl, Jeremy S.; Hernquist, Lars
Reference:
Astrophysical Journal Letters v.491, p.L95 (1997)
Abstract:
Using recently calculated analytic and numerical models
for the thermal structure of ultramagnetized neutron stars, we
estimate the effects that ultrastrong magnetic fields B>=10^{14} G have on
the thermal evolution of a neutron star.
Understanding this evolution is necessary to interpret models that invoke
"magnetars" to account for soft gamma-ray
emission from some repeating sources.
Title:
Old isolated neutron stars - Fire burns and cauldron
bubbles
Authors:
Treves, A.; Colpi, M.; Lipunov, V.M.
Reference:
Astron. Astrophys. vol. 269, p. 319 (1993)
Abstract:
Isolated magnetic neutron stars are considered at the
end of the phase of coherent pulsar emission when
accretion from the interstellar medium becomes important but rotation
affects the inflow dynamics. When
the centrifugal acceleration at the Alfven radius exceeds the gravitational
one, a process of piling up of matter
is expected to take place, yielding the compression of the magnetospheric
boundary. Prompt matter infall to
the neutron star surface occurs when the gravitational energy density has
increased with time to overcome the
centrifugal barrier. Recurrence times are estimated, and it is suggested
that the aging neutron star can
undergo accretion with intermittent or quasi-cycling behavior when passing
from the isolated radio pulsar
phase to quiescent steady accretion over the life time of the Galaxy. An
analogous process of build up of the
magnetodipole energy is envisaged and it is proposed that a relativistic jet
may form. Although on energetic
scales much more modest, the picture has some resemblance to the cauldron
model of SS 433. The
consequences for the detectability of old isolated neutron stars are briefly
discussed.
Title:
Period Clustering of the Anomalous X-Ray Pulsars and
Magnetic Field Decay in
Magnetars
Authors:
Colpi, Monica; Geppert, Ulrich; Page, Dany
Reference:
The Astrophysical Journal, Volume 529, Issue 1, pp.
L29-L32 (2000)
Abstract:
We confront theoretical models for the rotational,
magnetic, and thermal evolution of an ultramagnetized
neutron star, or magnetar, with available data on the anomalous X-ray
pulsars (AXPs). We argue that, if the
AXPs are interpreted as magnetars, their clustering of spin periods between
6 and 12 s (observed at present in
this class of objects), their period derivatives, their thermal X-ray
luminosities, and the association of two of
them with young supernova remnants can only be understood globally if the
magnetic field in magnetars
decays significantly on a timescale of the order of 10^4 yr.
Different types of INS:
AXPs
ROSAT dim sources
Dim sources in GCs
SGRs
Compact X-ray sources in SNRs
Geminga
Title:
Geminga: A cooling superfluid neutron star
Authors:
Page, Dany
Reference:
Astrophysical Journal vol. 428, no. 1, p. 250-260 (1994)
Abstract:
We compare the recent temperature estimate for Geminga
with neutron star cooling models. Because of its
age (approximately 3.4 x 10^5 yr), Geminga is in the photon cooling era. We
show that its surface
temperature (approximately 5.2 x 10^5 K) can be understood by both types of
neutrino cooling scenarios, i.e.,
slow neutrino cooling by the modified Urca process or fast neutrino cooling
by the direct Urca process or by
some exotic matter, and thus does not allow us to discriminate between these
two competing schemes.
However, for both types of scenarios, agreement with the observed
temperature can only be obtained if
baryon pairing is present in most, if not all, of the core of the star.
Within the slow neutrino cooling scenario,
early neutrino cooling is not sufficient to explain the observed low
temperature, and extensive pairing in the
core is necessary to reduce the specific heat and increase the cooling rate
in the present photon cooling era.
Within all the fast neutrino cooling scenarios, pairing is necessary
throughout the whole core to control the
enormous early neutrino emission which, without pairing suppression, would
result in a surface temperature at
the present time much lower than observed. We also comment on the recent
temperature estimates for PSR
0656+14 and PSR 1055-52, which pertain to the same photon cooling era. If
one assumes that all neutron
stars undergo fast neutrino cooling, then these two objects also provide
evidence for extensive baryon pairing
in their core; but observational uncertainties also permit a more
conservative interpretation, with slow neutrino
emission and no pairing at all. We argue though that observational evidence
for the slow neutrino cooling
model (the 'standard' model) is in fact very dim and that the interpretation
of the surface temperature of all
neutron stars could be done with a reasonable theoretical a priori within
the fast neutrino cooling scenarios
only. In this case, Geminga, PSR 0656+14, and PSR 1055-52 all show evidence
of baryon pairing down to
their very centers.
Related subjects (reviews and selected papers)
Field decay
Title:
Period Clustering of the Anomalous X-Ray Pulsars and
Magnetic Field Decay in
Magnetars
Authors:
Colpi, Monica; Geppert, Ulrich; Page, Dany
Reference:
The Astrophysical Journal, Volume 529, Issue 1, pp.
L29-L32 (2000)
Abstract:
We confront theoretical models for the rotational,
magnetic, and thermal evolution of an ultramagnetized
neutron star, or magnetar, with available data on the anomalous X-ray
pulsars (AXPs). We argue that, if the
AXPs are interpreted as magnetars, their clustering of spin periods between
6 and 12 s (observed at present in
this class of objects), their period derivatives, their thermal X-ray
luminosities, and the association of two of
them with young supernova remnants can only be understood globally if the
magnetic field in magnetars
decays significantly on a timescale of the order of 10^4 yr.
Title:
ROSAT X-ray sources and exponential field decay in
isolated neutron stars
Authors:
Popov, S. B.; Prokhorov, M. E.
Reference:
Astronomy and Astrophysics, v.357, p.164-168 (2000)
Abstract:
The influence of exponential magnetic field decay on
the spin evolution of isolated neutron stars is studied.
ROSAT observations have revealed several X-ray sources which may be
accreting old isolated neutron stars.
Assuming that this interpretation is correct, the observations can be used
to constrain parameters of the
exponential field decay.
We show that the range of minimum value of magnetic moment, mu_b , and the
characteristic decay time,
t_d, ~ 10^29.5>= mu_b >= 10^28 G cm3, ~ 10^8>= t_d >= 10^7 yrs are excluded
assuming the standard initial
magnetic momentum, mu_0 =10^30 G cm3. For these parameters, neutron stars
would never reach the stage
of accretion from the interstellar medium even for a low space velocity of
the stars and a density of the
ambient plasma. The range of excluded parameters increases for lower values
of mu_0 .
Cooling of NSs
Title:
Cooling neutron stars and superfluidity in their
interiors
Authors:
D.G. Yakovlev, K.P. Levenfish, Yu.A. Shibanov
Reference:
Phys. Usp. 42, 737 (1999)
astro-ph
9906456
Abstract:
We study the heat capacity and neutrino emission
reactions (direct and modified Urca processes,
nucleon-nucleon bremsstrahlung, Cooper pairing of nucleons) in matter
of supranuclear density of the
neutron star cores with superfluid neutrons and protons. Various
superfluidity types are analysed
(singlet-state pairing and two types of triplet-state pairing, without
and with nodes of the gap at a nucleon
Fermi surface). The results are used for cooling simulations of
isolated neutron stars. Both, the standard
cooling and the cooling enhanced by the direct Urca process, are
strongly affected by nucleon
superfluidity. Comparison of cooling theory of isolated neutron stars
with observations of their thermal
radiation may give stringent constraints on the critical temperatures
of the neutron and proton
superfluidities in the neutron star cores.
Title:
Neutrino Emission from Neutron Stars
Authors:
D. G. Yakovlev, A. D. Kaminker, O. Y. Gnedin, P.
Haensel
Reference:
Physics Reports (2001)
astro-ph
0012122
Abstract:
We review the main neutrino emission mechanisms in
neutron star crusts and cores. Among them are the
well-known reactions such as the electron-positron annihilation,
plasmon decay, neutrino bremsstrahlung of
electrons colliding with atomic nuclei in the crust, as well as the
Urca processes and neutrino
bremsstrahlung in nucleon-nucleon collisions in the core. We emphasize
recent theoretical achievements,
for instance, band structure effects in neutrino emission due to
scattering of electrons in Coulomb crystals
of atomic nuclei. We consider the standard composition of matter
(neutrons, protons, electrons, muons,
hyperons) in the core, and also the case of exotic constituents such as
the pion or kaon condensates and
quark matter. We discuss the reduction of the neutrino emissivities by
nucleon superfluidity, as well as the
specific neutrino emission produced by Cooper pairing of the superfluid
particles. We also analyze the
effects of strong magnetic fields on some reactions, such as the direct
Urca process and the neutrino
synchrotron emission of electrons. The results are presented in the
form convenient for practical use. We
illustrate the effects of various neutrino reactions on the cooling of
neutron stars. In particular, the neutrino
emission in the crust is critical in setting the initial thermal
relaxation between the core and the crust.
Finally, we discuss the prospects of exploring the properties of
supernuclear matter by confronting cooling
simulations with observations of the thermal radiation from isolated
neutron stars.
Title:
General relativistic treatment of the thermal, magnetic
and rotational evolution of
isolated neutron stars with crustal magnetic fields
Authors:
Page, D.; Geppert, U.; Zannias, T.
Reference:
Astronomy and Astrophysics, Vol. 360, p.1052-1066 (2000)
Abstract:
We investigate the thermal, magnetic and rotational
evolution of isolated neutron stars assuming that the
dipolar magnetic field is confined to the crust. Our treatment, for the
first time, uses a fully general
relativistic formalism not only for the thermal but also for the magnetic
part, and includes partial general
relativistic effects in the rotational part. Due to the fact that the
combined evolution depends crucially upon
the compactness of the star, three different equations of state have been
employed in the calculations. In the
absence of general relativistic effects, while upon increasing compactness a
decrease of the crust thickness
takes place leading into an accelerating field decay, the inclusion of
general relativistic effects intend to
"decelerate this acceleration". As a consequence we find that, within the
crustal field hypothesis, a given
equation of state is compatible with the observed distribution of pulsar
periods P and period derivative
&mathaccent "705Frelax dot; provided the initial field strength and current
location as well as the
magnitude of the impurity content are appropriately constrained.
Finally, we access the flexibility of the soft, medium and stiff classes of
equations of state as candidates in
describing the state of the matter in the neutron star interiors. The
comparison of our model calculations with
observations, together with the consideration of independent information
about neutron star evolution,
suggests that a not too soft equation of state describes neutron star
interiors and its cooling proceeds along the
tandard' scenario.
Title:
The cooling of neutron stars by the direct URCA process
Authors:
Page, Dany; Applegate, James H.
Reference:
Astrophysical Journal vol. 394, p. L17-L20 (1992)
Abstract:
Title:
Fast Cooling of Neutron Stars: Superfluidity versus
Heating and Accreted
Envelope
Authors:
Page, Dany
Reference:
Astrophysical Journal Letters v.479, p.L43 (1997)
Abstract:
It is generally considered that the neutron star
cooling scenarios involving fast neutrino emission, from a kaon
or pion condensate, quark matter, or the direct Urca process, require the
presence of baryon pairing in the
central core of the star to control the strong neutrino emission and to
produce surface temperatures compatible
with observations. I show here that within the kaon condensate scenario,
pairing is not necessary if (1) the
equation of state is stiff enough for the star to have a thick crust in
which sufficient friction can occur to heat
the star and (2) a thin layer, of mass Delta M larger than ~10-12 Msolar, of
light elements (H and He) is
present at the stellar surface. Both the occurrence of heating and the
presence of H and/or He at the surface
(deposited during the late postsupernova accretion) can possibly be
confirmed or refuted by future observations.
SNRs
PSRs
SN rate
Title:
Galactic and extragalactic supernova rates
Authors:
van den Bergh, Sidney; Tammann, Gustav A.
Reference:
Annual review of astronomy and astrophysics. Vol. 29
p. 363-407 (1991)
Abstract:
Recent research on Galactic and extragalactic supernova
(SN) rates is reviewed. SN frequencies, luminosities,
and discovery probabilities are addressed. The inclination and radial
distance effect on the likelihood of
discovery is examined, and SN rates are considered as a function of parent
galaxy luminosity. Relative SN
frequencies in different types of galaxies and the transformation of such
frequencies into absolute ones are
discussed. The determination of Galactic SN rates from radio SNRs and the
mass spectrum of star formation is
examined. SN rates in Local Group galaxies is addressed.
Title:
On relative supernova rates and nucleosynthesis roles
Authors:
Arnett, W. David; Schramm, David N.; Truran, James W.
Reference:
Astrophysical Journal vol. 339, p.L25 (1989)
Abstract:
It is shown that the Ni-56-Fe-56 observed in SN 1987A
argues that core collapse supernovae may be
responsible for more than 50 percent of the iron in the galaxy. Furthermore
it is argued that the time
averaged rate of thermonuclear driven Type I supernovae may be at least an
order of magnitude lower than
the average rate of core collapse supernovae. The present low rate of Type
II supernovae (below their time
averaged rate of approx. 1/10 yr) is either because the past rate was much
higher because many core collapse
supernovae are dim like SN 1987A. However, even in this latter case they are
only an order of magnitude
dimmer that normal Type II's due to the contribution of Ni-56 decay to the
light curve.
Kick velocity
NS atmospheres
Internal structure of NSs
SN explosions
Disc accretion
Propeller stage
Magneto-rotational evolution
Title:
Magnetic and Spin History of Very Young Pulsars
Authors:
Muslimov, Alexander; Page, Dany
Reference:
Astrophysical Journal v.458, p.347 (1996)
Abstract:
Fall-back
Title:
Supernova Fallback and the Emergence of a Black Hole
Authors:
Zampieri, Luca; Colpi, Monica; Shapiro, Stuart L.;
Wasserman, Ira
Reference:
The Astrophysical Journal, Volume 505, Issue 2, pp.
876-896 (1998)
Abstract:
We present the first fully relativistic investigation
of matter fallback in a supernova. We investigate
spherically symmetric supernova fallback using a relativistic radiation
hydrodynamics Lagrangian code that
handles radiation transport in all regimes. Our goal is to answer the
fundamental questions: did SN 1987A
produce a black hole and, if so, when will the hole become detectable? We
compute the light curve, assuming
that a black hole has been formed during the explosion, and compare it with
the observations. Our
preliminary calculations lack radioactive energy input and adopt a very
simple chemical composition (pure
hydrogen). As a result, our computed models cannot fit the observed data of
SN 1987A in detail.
Nevertheless, we can show that, during the first hours, the accretion flow
is self-regulated and the accretion
luminosity stays very close to the Eddington limit. The light curve is
completely dominated, during the first
few weeks, by the emission of the stellar envelope thermal energy and
resembles that obtained in `standard''
supernova theory. Only long after hydrogen recombination takes place is
there even a chance to actually
detect radiation from the accreting black hole above the emission of the
expanding envelope. The presence of
a black hole is thus not inconsistent with observations to date. Because of
the exponential decay of the ^44Ti
radioactive heating rate, the date of the emergence of the black hole is not
very sensitive to the actual
parameters of the models and turns out to be about 1000 yr. The bulk of the
emission then is expected to be in
the visible band but will be unobservable with present instrumentation. We
discuss the implications of our
results in connection with the possible emergence of black holes in other
supernovae.
Selected objects:
RX J185635-3754
Title:
On the X-ray position and deep optical imaging of the
neutron star candidate RX
J1856.5-3754
Authors:
Neuhaeuser, R.; Thomas, H.-C.; Danner, R.; Peschke, S.;
Walter, F. M.
Reference:
Astronomy and Astrophysics, v.318, p.L43-L46 (1997)
Abstract:
We present X-ray and optical studies of the ROSAT
source RXJ1856.5-3754, a neutron star candidate. With
recent optical observations of a potential optical counterpart to another
X-ray source in the field and correcting
the X-ray positions for two effects not taken into account before, we
determine the ROSAT X-ray source
position and its error, both different from previous work. Also, we present
deep optical imaging in B and V
obtained with the ESO-MPI 2.2m telescope on La Silla and in B and R obtained
with the Keck telescope.
Title:
The optical counterpart of the isolated neutron star RX
J185635-3754
Authors:
Walter, F. M.; Matthews, L. D.
Reference:
Nature, 389, 358-360 (1997)
Abstract:
RX J0720.4-3125
Title:
Constraints on optical emission from the isolated neutron
star candidate
RXJ0720.4-3125
Authors:
Motch, C.; Haberl, F.
Reference:
Astronomy and Astrophysics, v.333, p.L59-L62 (1998)
Abstract:
Deep optical B band images of the ROSAT HRI error
region of RX J0720.4-3125 \ reveal the presence of
two faint stellar-like objects with B = 26.1 $phi m 0.25 and B = 26.5 phi m
0.30. Exposures obtained
through U, V and I filters are not sensitive enough to detect the two
candidates and provide upper limits of U
= 24.9, V = 23.2 and I = 21.9. These new observations virtually establish
that RX J0720.4-3125 \ is a
slowly rotating, probably completely isolated neutron star. The absence of
an optical counterpart brighter than
B = 26.1 seems incompatible with a neutron star atmosphere having a chemical
composition dominated by
Hydrogen or Helium. UBI photometry of field stars shows astonishingly little
interstellar reddening in the
direction of the X-ray source. Together with the small column density
detected by the ROSAT PSPC, this
suggests a mean particle density in the range of n = 0.1-0.4cm^{-3}. Such
average densities would imply
very low velocities relative to interstellar medium (v_{rel} \leq 10km
s^{-1}$) if the source were powered
by accretion. These stringent constraints may be relaxed if the neutron star
is presently crossing a small size
structure of higher density or if the effective temperature of the heated
atmosphere is overestimated by the
blackbody approximation. Alternatively, RX J0720.4-3125 \ could be a young
and highly magnetized
cooling neutron star.
Based on observations obtained at the European Southern Observatory, La
Silla (Chile) with the NTT and
ESO-Dutch telescopes
Title:
RXJ0720.4-3125: strong evidence for an isolated pulsating
neutron star
Authors:
Haberl, F.; Motch, C.; Buckley, D. A. H.; Zickgraf,
F.-J.; Pietsch, W.
Reference:
Astronomy and Astrophysics, v.326, p.662-668
Abstract:
We report the discovery of RX J0720.4-3125, a soft
X-ray source showing 8.391s pulsations, from the
ROSAT all-sky survey. The relative constant long-term X-ray intensity, the
blackbody-like spectrum with
little photo-electric absorption and the limit for f_x_/f_v_ of more than
500 are consistent with an isolated
neutron star accreting from the interstellar medium (all very similar to
RXJ1856.5-3754, so far the best
candidate). We estimate the magnetic field strength of the neutron star to
be less than 10^10^G. RX
J0720.4-3125 may have emerged from common envelope evolution of a high mass
X-ray binary. The final
neutron star is expected to be close the galactic plane, have a low space
velocity and a low magnetic field,
accreting interstellar matter very effectively. In this case RX J0720.4-3125
would not belong to the
expected large group of old neutron stars evolved from single stars. The low
derived magnetic field strength
and the pulse period are however also compatible with an old (10^9^yr)
neutron star in which case we see a
neutron star from the very low end of the velocity distribution. The low
number of isolated neutron star
candidates from the ROSAT all-sky survey remains to be explained.
Title:
RX J0720.4--3125 as a Possible Example of the Magnetic Field
Decay of Neutron Stars
Authors:
Konenkov, D. Yu.; Popov, S. B.
Reference:
Astronomy Letters, Volume 23, Issue 4,
pp.498-503 (1997)
astro-ph
9707318
Abstract:
We studied possible evolution of the rotational period
and the magnetic field of the X-ray source RX
J0720.4-3125 assuming this source to be an isolated neutron star
accreting interstellar medium. Magnetic
field of the source is estimated to be $10^6 - 10^9$ G, and it is
difficult to explain observed rotational
period 8.38 s without invoking hypothesis of the magnetic field decay.
We used the model of ohmic decay
of the crustal magnetic field. The estimates of accretion rate
($10^{-14} - 10^{-16} M_\odot/yr$),
velocity of the source relative to interstellar medium ($10 - 50 $
km/s), neutron star age ($2\cdot 10^9 -
10^{10}$ yrs) are obtained.
Title:
Evidence for Magnetic Field Decay in RX J0720.4-3125
Authors:
Wang, John C. L.
Reference:
Astrophysical Journal Letters v.486, p.L119 (1997)
Abstract:
The unidentified X-ray source RX J0720.4-3125 is a
candidate isolated neutron star showing evidence for
pulsed emission with an 8.39 s period and a spectrum consistent with a
blackbody at kT = 80 eV. We show
that this source is most likely an old isolated neutron star accreting from
surrounding media. We then argue
that unless it was born with a long spin period (Pi >~ 0.5 s) and a weak
field (Bi <~ 10^10 G), the magnetic
field on this star must have decayed. With Bi ~ 10^12 G, we find decay
timescales of >~10^7 yr for
power-law decay and >~10^8 yr for exponential decay. A measured period
derivative P dot <~ 10^-16 s s-1
would be consistent with an old accreting isolated neutron star. Both
power-law and exponential decay models
can give a P dot ~ 10^-16 s s-1, though a P dot substantially less than this
would be indicative of exponential
field decay.
Title:
RX J0720.4-3125: Implications for neutron star cooling and
the dense matter
equation of state
Authors:
Wang, John C. L.; Link, Bennett; van Riper, Kenneth;
Arnaud, Keith A.;
Miralles, Juan A.
Reference:
Astronomy and Astrophysics, v.345, p.869-883
(1999)
Abstract:
The soft X-ray source RX J0720.4-3125 appears to be a
clean example of a cooling neutron star. Its X-ray
emission is well-fit by a 79+/-4 eV blackbody and displays a periodic
modulation with period P=8.391 sec and
semi-amplitude ~ 10%. The observational upper limit to the period derivative
implies a minimum spindown
age of t_{0}=1.7x 10(5) yr if the star was born rapidly rotating. The
absence of a visible supernova remnant
independently suggests an age for this source of gtrsim 10(5) yr. With the
interpretation of this source as a
cooling-driven, magnetized, rotating neutron star, we explore the
implications for the dense matter equation
of state (EOS), the mode of energy loss (modified URCA vs. direct URCA), and
the rate of internal heating
due to superfluid friction. For the standard (modified URCA) cooling models,
we study two types of stars:
those born slowly rotating, with relatively small spin-down rates and
conventional dipole magnetic fields ( ~
10(12) G), and those born rapidly rotating with large spin-down rates and
magnetar-scale dipole fields ( ~
10(14) G). We find that standard cooling with a stiff or moderately stiff
EOS is consistent with the
observations of RX J0720.4-3125 provided the star's age is <=sssim 3
t_0=5*E(5) yr. If the EOS is very
stiff, the star must be born with a short rotation period and significant
internal heating by superfluid friction
is required. More moderate heating suffices only if the star is very massive
( ~ 2 M_sun) and has an age ~
t_0. Stars with M ~ 1.4 M_sun and a moderately stiff EOS give modulations
about a factor of five below
that observed. However, the inclusion of atmospheric effects or more complex
field geometries could increase
the modulation to a level consistent with the observations. Stars with a
stiff EOS give modulations close to
that observed. As an illustration of the effects of accelerated cooling
processes, we consider direct quark URCA
cooling. We find that these models cool too fast and are cleanly ruled out
for this source. Hence, exotic matter
is an insignificant component in the stellar core, or does not participate
in accelerated cooling. Direct URCA
reactions in nucleonic matter are similarly ruled out. A measure of RX
J0720.4-3125's spin-down age would
afford crucial tests of our conclusions.
Title:
What is the nature of RX J0720.4-3125?
Authors:
Heyl, Jeremy S.; Hernquist, Lars
Reference:
Monthly Notices of the Royal Astronomical Society,
Volume 297, Issue 3, pp. L69-L70 (1998)
Abstract:
RX J0720.4-3125 has recently been identified as a
pulsating soft X-ray source in the ROSAT all-sky survey with a
period of 8.391 s. Its spectrum is well characterized by a blackbody with a
temperature of 8x10^5 K. We propose that
the radiation from this object is thermal emission from a cooling neutron
star. For this blackbody temperature we can
obtain a robust estimate of the object's age of ~3x10^5 yr, yielding a polar
field ~10^14 G for magnetic dipole
spin-down and a value of P compatible with current observations.
RX J0420.0-5022
Title:
RX J0420.0-5022: an isolated neutron star candidate with
evidence for 22.7 s
X-ray pulsations
Authors:
Haberl, F.; Pietsch, W.; Motch, C.
Reference:
Astronomy and Astrophysics, v.351, p.L53-L57 (1999)
Abstract:
We report the discovery of a new isolated neutron star
candidate, RX J0420.0-5022, showing evidence
(4sigma ) for 22.7 s X-ray pulsations in ROSAT data. NTT observations of the
field around the soft X-ray
source do not reveal any likely optical counterpart brighter than B = 25.25
implying an X-ray to optical flux
ratio of >103.3 and ruling out other possible kinds of X-ray emitters. The
X-ray spectrum of RX
J0420.0-5022 can be described with blackbody emission with temperature kT of
~ 57 eV and four ROSAT
detections are consistent with no flux variations on time scales of years.
The X-ray pulsations, if confirmed,
make RX J0420.0-5022 the second long-period isolated neutron star candidate
after RX J0720.4-3125. As
for this latter source similar conclusions about the magnetic field strength
of the neutron star can be drawn
depending on evolutionary scenarios.
Partly based on NTT observations performed at the European Southern
Observatory, La Silla, Chile
RBS 1223 (RX
J130848.6+212708)
Title:
The isolated neutron star candidate RBS1223 (1RXS
J130848.6+212708)
Authors:
Schwope, A. D.; Hasinger, G.; Schwarz, R.; Haberl, F.;
Schmidt, M.
Reference:
Astronomy and Astrophysics, v.341, p.L51-L54 (1999)
Abstract:
In the ROSAT Bright Survey (RBS) we have almost
completely optically identified the brightest ~ 2000
high-galactic latitude sources from the ROSAT All-Sky Survey Bright Source
Catalogue (1RXS). A small
number of sources has empty X-ray error circles on optical images. ROSAT HRI
follow-up observations of
RBS1223 (=1RXS J130848.6+212708), a soft object with extreme X-ray to
optical flux ratio, have
confirmed a relatively bright X-ray source, whose position could be
determined to an accuracy of 1.6 arcsec
(90%) due to the presence of a nearby, X-ray detected bright star. Deep Keck
R- and B-band images of the
field were taken, but the refined X-ray error circle remains empty to a
limiting magnitude B ~ 26(m) .
With an X-ray to optical flux ratio of log (f_X/f_opt)>4.1 this object is
almost certainly an isolated neutron
star, similar to the two so far best-known examples RX J1856.4-3754 and RX
J0720.4-3125. We discuss
limits on the number of similar objects in the RBS catalogue.
RBS 1556 (RX J1605.3+3249)
Title:
The isolated neutron star candidate RX J1605.3+3249
Authors:
Motch, C.; Haberl, F.; Zickgraf, F.-J.; Hasinger, G.;
Schwope, A. D.
Reference:
Astronomy and Astrophysics, v.351, p.177-184 (1999)
Abstract:
We report on X-ray and optical observations of a ROSAT
X-ray source, RX J1605.3+3249, selected from the
all-sky survey on the basis of its spectral softness and lack of bright
optical counterpart. The ROSAT PSPC
energy distribution is well fitted by a blackbody with kT = 92$phi m6 eV and
N_H = 1.1phi m0.4 x
10^{20} cm^{-2}$. X-ray observations spanning 6.5 years fail to reveal any
flux or spectral variability on
any time scales. The ROSAT HRI error circle only contains a R = 23.25 M star
which is unlikely to be
associated with the X-ray source. We conclude that RX J1605.3+3249 is a
probable nearby isolated neutron
star detected from its thermal emission. The present data do not allow to
unambiguously determine the X-ray
powering mechanism, cooling from a young neutron star or heating by
accretion from the interstellar
medium onto an old neutron star. However, the long term stability of the
X-ray flux favours the young
neutron star hypothesis.
RCW 103
Title:
Discovery of a 69 Millisecond X-Ray Pulsar: A Compact Source
in the Vicinity
of the Supernova Remnant RCW 103
Authors:
Torii, K.; Kinugasa, K.; Toneri, T.; Asanuma, T.;
Tsunemi, H.; Dotani, T.;
Mitsuda, K.; Gotthelf, E. V.; Petre, R.
Reference:
Astrophysical Journal Letters v.494, p.L207 (1998)
Abstract:
We report a rare discovery of a fast (69 ms) pulsar
using X-ray data acquired with the Advanced Satellite for
Cosmology and Astrophysics (ASCA). The highly significant detection arises
from the serendipitous ASCA
X-ray source AXS J161730-505505, located near the young Galactic supernova
remnant (SNR) RCW
103. The epoch-folded light curve displays a single asymmetric pulse profile
with a pulse fraction of ~50%.
Spectral fits to the high-energy (i.e., greater than 3.5 keV) source data
using a simple absorbed power-law
model, assuming the hydrogen column density to the SNR, gives a photon index
of gamma
=1.6^{+0.2}_{-0.3} and an unabsorbed flux of ~6.4x10^{-12}{ergs cm}^{-2}
{s}^{-1} . The extracted
source spectrum below 3.5 keV is contaminated by mirror-scattered soft
thermal emission from the ~9'
diameter RCW 103, whose projected center is located just ~7' away. If the
pulsar is associated with the
remnant, the implied neutron star velocity is at the high end of the pulsar
velocity distribution for the
distance estimates to the remnant but still plausible. We suggest that AXS
J161730-505505 is likely a
young rotation-powered pulsar with a characteristic spin-down age of ~8000
yr. The physical association of
the pulsar with RCW 103 and its central source, 1E 161348-5055, remains
intriguing.
Title:
X-Ray Variability from the Compact Source in the Supernova
Remnant RCW
103
Authors:
Gotthelf, E. V.; Petre, R.; Vasisht, G.
Reference:
The Astrophysical Journal, Volume 514, Issue 2, pp.
L107-L110 (1999)
Abstract:
A new ASCA observation of 1E 161348-5055, the central
compact X-ray source in the supernova remnant
RCW 103, reveals an order-of-magnitude decrease in its 3-10 keV flux since
the previous ASCA
measurement 4 yr earlier. This result is difficult to reconcile with
suggestions that the bulk of the emission is
simple quasi-blackbody, cooling radiation from an isolated neutron star.
Furthermore, archived Einstein and
ROSAT data sets spanning 18 yr confirm that this source manifests long-term
variability, to a lesser degree.
This provides a natural explanation for difficulties encountered in
reproducing the original Einstein detection
of 1E 161348-5055. Spectra from the new data are consistent with no
significant spectral change despite the
decline in luminosity. We find no evidence for a pulsed component in any of
the data sets, with a best upper
limit on the pulsed modulation of 13%. We discuss the phenomenology of this
remarkable source.
Title:
The Nature of the Radio-quiet Compact X-Ray Source in SNR
RCW 103
Authors:
Gotthelf, E. V.; Petre, R.; Hwang, U.
Reference:
Astrophysical Journal Letters v.487, p.L175 (1997)
Abstract:
We consider the nature of the elusive neutron star
candidate 1E 161348-5055 using X-ray observations
obtained with ASCA. The compact X-ray source is centered on the shell-type
Galactic supernova remnant
RCW 103 and has been interpreted as a cooling neutron star associated with
the remnant. The X-ray
spectrum of the remnant shell can be characterized by a nonequilibrium
ionization (NEI) thermal model for a
shocked plasma of temperature kT~0.3 keV. The spectrum falls off rapidly
above 3 keV to reveal a point
source in the spectrally resolved images, at the location of 1E 161348-5055.
A blackbody model fit to the
source spectrum yields a temperature kT=0.6 keV, with an unabsorbed 0.5--10
keV luminosity of
L_{{X}}~10^{34} ergs s ^{-1} (for an assumed distance of 3.3 kpc), both of
which are at least a factor of 2
higher than predicted by cooling neutron star models. Alternatively, a
power-law model for the source
continuum gives a steep photon index of alpha ~3.2 , similar to that of
other radio-quiet, hard X-ray point
sources associated with supernova remnants. 1E 161348-5055 may be
prototypical of a growing class of
radio-quiet neutron stars revealed by ASCA; we suggest that these objects
account for previously hidden
neutron stars associated with supernova remnants.
Title:
RCW 103 - revisiting a cooling neutron star
Authors:
Heyl, Jeremy S.; Hernquist, Lars
Reference:
Monthly Notices of the Royal Astronomical Society,
Volume 298, Issue 1, pp. L17-L20 (1998)
Abstract:
Recent observations of the compact source embedded
within the supernova remnant RCW 103 rekindle interest in the
origin of this object's emission. We contrast several models in which
neutron-star cooling powers RCW 103. Specifically,
either the presence of an accreted envelope or a sufficiently intense
magnetic field can account for the X-ray emission
from this object.
Title:
Nature of the X-ray Source in Supernova Remnant RCW103
Authors:
Popov, Sergei B.
Reference:
Astrophysics and Space Science, v. 274, Issue 1/2, p.
285-290 (2000)
Abstract:
I discuss the nature of the compact X-ray source in the
center of the supernova remnant RCW 103. Several
models, based on the accretion onto a compact object are briefly discussed.
I show that it is more likely that the
central X-ray source is an accreting neutron star than an accreting black
hole. I also argue that models of a
disrupted binary system consisting of an old accreting neutron star and a
new one observed as a 69-ms pulsar
are most favored.
Title:
On the nature of the compact X-ray source inside RCW 103
Authors:
Popov, Sergei B.
Reference:
Astron. Astrophys. Transact., v. 17, p. 35-40 (1998)
astro-ph/
9708044
Abstract:
I discuss the nature of the compact X-ray source inside
the supernova remnant RCW 103. Several models,
based on the accretion onto a compact object such as a neutron star or
a black hole (isolated or binary), are
analyzed. I show that it is more likely that the X-ray source is an
accreting neutron star than an accreting
black hole. I also argue that models of a binary system with an old
accreting neutron star are most favored.
Important Web-pages
Fred Walter's
Web-page
Neutron star atmosphere models
Princetone Pulsar Group
Vicky Kaspi's Web-page
Neutron star
theory group at UNAM
Pulsar
newsletter
US/Russia
collaboration in Plasma Astrophysics
The Scenario Machine: Binary Star Population Synthesis (a review)
Future conferences
ADS
and
astro-ph
data systems were used to prepare this page
Page was designed by Sergei
Popov
Started 03.01.2001. Now - under construction (sorry, if your paper
or Web-page is not
included yet, you can send me an e-mail: polar@sai.msu.ru)
Draft of the
Review (in Russian; 03.08.2001)