Isolated Neutron Stars
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Population synthesis and related studies
Theoretical studies of INS
Different types of INS:
AXPs
ROSAT dim sources
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SGRs
Compact X-ray sources in SNRs
Geminga
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Field decay
Cooling of NSs
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NS atmospheres
Internal structure of NSs
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Propeller stage
Magneto-rotational evolution
Fall-back
Selected objects:
RX J185635-3754
RX J0720.4-3125
RBS 1223
RBS 1556 (RX J1605.3+3249)
RCW 103
Important Web-pages
Future conferences (updated)
Ostriker J.P., Rees M.J., and Silk J.
Astrophys. Lett. vol. 6, p.179 (1970)
Isolated Neutron Stars: Accretors and Coolers
Treves, Aldo; Turolla, Roberto; Zane, Silvia; Colpi, Monica
The Publications of the Astronomical Society of the Pacific, Volume 112, Issue 769, pp. 297-314. (2000)
As many as 10^9 neutron stars populate the Galaxy, but only ~10^3 are directly observed as pulsars or as accreting sources in X-ray binaries. In principle, also the accretion of the interstellar medium may make isolated neutron stars shine, and their weak luminosity could be detected in soft X-rays. Recent ROSAT observations have convincingly shown that neutron stars accreting from the interstellar medium are extremely rare, if observed at all, in contrast with earlier theoretical predictions. Until now two possible explanations for their elusiveness have been proposed: their velocity distribution may peak at ~200-400 km s-1, as inferred from pulsar statistics, and this would severely choke accretion; the magnetic field may decay on timescales ~10^8-10^9 yr, preventing a large fraction of neutron stars from entering the accretor stage. The search for accreting neutron stars has produced up to now a handful of promising candidates. While little doubt is left that these objects are indeed isolated neutron stars, the nature of their emission is still controversial. In particular, accreting objects can be confused with much younger, cooling neutron stars. However, a combination of observations and theoretical modeling may help in discriminating between the two classes.
Astrophysics of neutron stars
Lipunov V.M.
Springer, Berlin (1992)
Black Holes, White dwarfs and Neutron stars
Shapiro S.L., Teukolsky S.A.
Wiley, New York 1983
Compact stars
Glendenning N.
Theory of neutron star magnotosphere
F. Curtis Michel
The University of Chicago Press; Chicago and London (1991)
High-Energy Radiation from magnetized neutron stars
Meszaros P.
Chicago University Press; Chicago (1992)
Accretion power in astrophysics
Frank J., King A.R., Raine D.J.
Cambridge University Press, Cambridge (1985)
The LOG N-LOG S Distributions of Accreting and Cooling Isolated Neutron Stars
Popov, S. B.; Colpi, M.; Prokhorov, M. E.; Treves, A.; Turolla, R.
The Astrophysical Journal, Volume 544, Issue 1, pp. L53-L56 (2000)
We model populations of isolated neutron stars in the Galaxy following their orbital and magnetorotational evolution. It is shown that accretors become more abundant than coolers at fluxes below ~10^-13 ergs cm-2 s-1, and one can predict that about one accreting neutron star per square degree should be observed at the Chandra and Newton flux limits of ~10^-16 ergs cm-2 s-1. The soft ROSAT sources associated with isolated neutron stars can be relatively young cooling objects only if the neutron star birthrate in the solar vicinity during the last ~10^6 yr is higher than that inferred from radio pulsar observations.
The Neutron Star Census
Popov, S. B.; Colpi, M.; Treves, A.; Turolla, R.; Lipunov, V. M.; Prokhorov, M. E.
The Astrophysical Journal, Volume 530, Issue 2, pp. 896-903 (2000)
The paucity of old isolated accreting neutron stars in ROSAT observations is used to derive a lower limit on the mean velocity of neutron stars at birth. The secular evolution of the population is simulated following the paths of a statistical sample of stars for different values of the initial kick velocity, drawn from an isotropic Gaussian distribution with mean velocity 0<=<=550 km s-1. The spin-down, induced by dipole losses and by the interaction with the ambient medium, is tracked together with the dynamical evolution in the Galactic potential, allowing for the determination of the fraction of stars which are, at present, in each of the four possible stages: ejector, propeller, accretor, and georotator. Taking from the ROSAT All Sky Survey an upper limit of ~10 accreting neutron stars within ~140 pc from the Sun, we infer a lower bound for the mean kick velocity, >~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%.
On the number of accreting and cooling isolated neutron stars detectable with the ROSAT All-Sky Survey
Neuhauser, R.; Trumper, J. E.
Astronomy and Astrophysics, v.343, p.151-156 (1999)
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.
Are there any isolated old neutron stars in the ROSAT Wide Field Camera survey?
Manning, R. A.; Jeffries, R. D.; Willmore, A. P.
Monthly Notices of the Royal Astronomical Society, Volume 278, Issue 2, pp. 577-585. (1996)
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.
The observability of old isolated neutron stars
Treves, A.; Colpi, M.
Astron. Astrophys. vol. 241 , p. 107 (1991)
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.
The observability of old isolated neutron stars with ROSAT. 2: Molecular clouds and deep fields
Colpi, M.; Campana, S.; Treves, A.
Astron. Astrophys. vol. 278, p. 161 (1993)
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.
Old Isolated Accreting Neutron Stars: Contribution to the Soft X-Ray Background in the 0.5--2 keV Band
Zane, Silvia; Turolla, Roberto; Zampieri, Luca; Colpi, Monica; Treves, Aldo
Astrophys. J. vol. 451, p. 739 (1995)
Can magnetic field decay explain the elusivity of old neutron stars ?
Treves, A.; Colpi, M.; Turolla, R.
Astronomische Nachrichten, vol. 319, no. 1, p. 109. (1998)
The Elusiveness of Old Neutron Stars
Colpi, Monica; Turolla, Roberto; Zane, Silvia; Treves, Aldo
Astrophysical Journal v.501, p.252 (1998)
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.
The Thermal Evolution of Ultramagnetized Neutron Stars
Heyl, Jeremy S.; Hernquist, Lars
Astrophysical Journal Letters v.491, p.L95 (1997)
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.
Old isolated neutron stars - Fire burns and cauldron bubbles
Treves, A.; Colpi, M.; Lipunov, V.M.
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.
Period Clustering of the Anomalous X-Ray Pulsars and Magnetic Field Decay in Magnetars
Colpi, Monica; Geppert, Ulrich; Page, Dany
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.
Geminga: A cooling superfluid neutron star
Page, Dany
Astrophysical Journal vol. 428, no. 1, p. 250-260 (1994)
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.
ROSAT X-ray sources and exponential field decay in isolated neutron stars
Popov, S. B.; Prokhorov, M. E.
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 neutron stars and superfluidity in their interiors
D.G. Yakovlev, K.P. Levenfish, Yu.A. Shibanov
Phys. Usp. 42, 737 (1999)
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.
Neutrino Emission from Neutron Stars
D. G. Yakovlev, A. D. Kaminker, O. Y. Gnedin, P. Haensel
Physics Reports (2001)
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.
General relativistic treatment of the thermal, magnetic and rotational evolution of isolated neutron stars with crustal magnetic fields
Page, D.; Geppert, U.; Zannias, T.
Astronomy and Astrophysics, Vol. 360, p.1052-1066 (2000)
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.
The cooling of neutron stars by the direct URCA process
Page, Dany; Applegate, James H.
Astrophysical Journal vol. 394, p. L17-L20 (1992)
Fast Cooling of Neutron Stars: Superfluidity versus Heating and Accreted Envelope
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.
Galactic and extragalactic supernova rates
van den Bergh, Sidney; Tammann, Gustav A.
Annual review of astronomy and astrophysics. Vol. 29 p. 363-407 (1991)
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.
On relative supernova rates and nucleosynthesis roles
Arnett, W. David; Schramm, David N.; Truran, James W.
Astrophysical Journal vol. 339, p.L25 (1989)
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.
Magnetic and Spin History of Very Young Pulsars
Muslimov, Alexander; Page, Dany
Supernova Fallback and the Emergence of a Black Hole
Zampieri, Luca; Colpi, Monica; Shapiro, Stuart L.; Wasserman, Ira
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.
On the X-ray position and deep optical imaging of the neutron star candidate RX J1856.5-3754
Neuhaeuser, R.; Thomas, H.-C.; Danner, R.; Peschke, S.; Walter, F. M.
Astronomy and Astrophysics, v.318, p.L43-L46 (1997)
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.
The optical counterpart of the isolated neutron star RX J185635-3754
Walter, F. M.; Matthews, L. D.
Nature, 389, 358-360 (1997)
Constraints on optical emission from the isolated neutron star candidate RXJ0720.4-3125
Motch, C.; Haberl, F.
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
RXJ0720.4-3125: strong evidence for an isolated pulsating neutron star
Haberl, F.; Motch, C.; Buckley, D. A. H.; Zickgraf, F.-J.; Pietsch, W.
Astronomy and Astrophysics, v.326, p.662-668
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.
RX J0720.4--3125 as a Possible Example of the Magnetic Field Decay of Neutron Stars
Konenkov, D. Yu.; Popov, S. B.
Astronomy Letters, Volume 23, Issue 4, pp.498-503 (1997)
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.
Evidence for Magnetic Field Decay in RX J0720.4-3125
Wang, John C. L.
Astrophysical Journal Letters v.486, p.L119 (1997)
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.
RX J0720.4-3125: Implications for neutron star cooling and the dense matter equation of state
Wang, John C. L.; Link, Bennett; van Riper, Kenneth; Arnaud, Keith A.; Miralles, Juan A.
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.
What is the nature of RX J0720.4-3125?
Monthly Notices of the Royal Astronomical Society, Volume 297, Issue 3, pp. L69-L70 (1998)
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: an isolated neutron star candidate with evidence for 22.7 s X-ray pulsations
Haberl, F.; Pietsch, W.; Motch, C.
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)
The isolated neutron star candidate RBS1223 (1RXS J130848.6+212708)
Schwope, A. D.; Hasinger, G.; Schwarz, R.; Haberl, F.; Schmidt, M.
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.
The isolated neutron star candidate RX J1605.3+3249
Motch, C.; Haberl, F.; Zickgraf, F.-J.; Hasinger, G.; Schwope, A. D.
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.
Discovery of a 69 Millisecond X-Ray Pulsar: A Compact Source in the Vicinity of the Supernova Remnant RCW 103
Torii, K.; Kinugasa, K.; Toneri, T.; Asanuma, T.; Tsunemi, H.; Dotani, T.; Mitsuda, K.; Gotthelf, E. V.; Petre, R.
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.
X-Ray Variability from the Compact Source in the Supernova Remnant RCW 103
Gotthelf, E. V.; Petre, R.; Vasisht, G.
The Astrophysical Journal, Volume 514, Issue 2, pp. L107-L110 (1999)
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.
The Nature of the Radio-quiet Compact X-Ray Source in SNR RCW 103
Gotthelf, E. V.; Petre, R.; Hwang, U.
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.
RCW 103 - revisiting a cooling neutron star
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.
Nature of the X-ray Source in Supernova Remnant RCW103
Popov, Sergei B.
Astrophysics and Space Science, v. 274, Issue 1/2, p. 285-290 (2000)
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.
On the nature of the compact X-ray source inside RCW 103
Astron. Astrophys. Transact., v. 17, p. 35-40 (1998)
astro-ph/
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.
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