We study evolution of isolated neutron stars on long time scale and calculate distribution of these sources in the main evolutionary stages: Ejector, Propeller, Accretor, and Georotator. We compare different initial magnetic field distributions taking into account a possibility of magnetic field decay, and include in our calculations the stage of subsonic Propeller. It is shown that though the subsonic propeller stage can be relatively long, initially highly magnetized neutron stars ($B_0\ga 10^{13}$ G) reach the accretion regime within the Galactic lifetime if their kick velocities are not too large. The fact that in previous studies made $>$10 years ago, such objects were not considered results in a slight increase of the Accretor fraction in comparison with earlier conclusions. Most of the neutron stars similar to the Magnificent seven are expected to become accreting from the interstellar medium after few billion years of their evolution. They are the main predecestors of accreting isolated neutron stars.
Deep Dive into Evolution of isolated neutron stars till accretion. The role of initial magnetic field.
We study evolution of isolated neutron stars on long time scale and calculate distribution of these sources in the main evolutionary stages: Ejector, Propeller, Accretor, and Georotator. We compare different initial magnetic field distributions taking into account a possibility of magnetic field decay, and include in our calculations the stage of subsonic Propeller. It is shown that though the subsonic propeller stage can be relatively long, initially highly magnetized neutron stars ($B_0\ga 10^{13}$ G) reach the accretion regime within the Galactic lifetime if their kick velocities are not too large. The fact that in previous studies made $>$10 years ago, such objects were not considered results in a slight increase of the Accretor fraction in comparison with earlier conclusions. Most of the neutron stars similar to the Magnificent seven are expected to become accreting from the interstellar medium after few billion years of their evolution. They are the main predecestors of accreting
Accreting isolated neutron stars (AINS) were predicted 40 years ago by Shvartsman (1971) and independently by Ostriker et al. (1970). In early 90s there was some enthusiasm due to the launch of the ROSAT satellite, which was expected to find many sources of this kind (Treves & Colpi 1991). Several populational studies have been made (Blaes et al. 1990;Blaes & Rajagopal 1991;Blaes & Madau 1993;Madau & Blaes 1994;Blaes et al. 1995;Manning et al. 1996). However, it came out that AINS, if they exist, are very elusive (Colpi ⋆ E-mail: polar@sai.msu.ru (SBP) et al. 1998). The main reason is that initial (kick) velocities of NSs appeared to be significantly larger, than it have been thought before (Lyne & Lorimer 1994). Initial guess that the number of Accretors is small due to low luminosity of high-velocity NSs was shown to be wrong. In a detailed study by Popov et al. (2000) (hereafter Paper I) it was shown that INS with Crab-like initial parameters and constant magnetic fields spend all their lives as Ejectors (we follow the classification summarized in Lipunov 1992) if their initial velocities are > ∼ 100 km s -1 . Then, the fraction of Accretors was mainly determined by the fraction of low-velocity NSs.
Up to the very end of 90s, it was believed that the wast majority of NSs are born simc 0000 RAS ilar to the Crab pulsar. I.e., that they have short initial spin periods (from milliseconds to few tens of millisecond) and magnetic fields B ∼ 10 12 G. Now it is believed, that about one half of NSs have different initial properties (Popov et al. 2006;Keane & Kramer 2008). There are at least three groups of sources with distinct parameters: compact central objects (CCOs) in supernova remnants (SNR), magnetars (anomalous X-ray pulsars -AXPs, and soft gamma-ray repeaters -SGRs), and cooling radioquiet NSs dubbed the Magnificent seven (M7) (Popov 2008). CCOs have low initial fields ∼ 10 11 G (Halpern et al. 2007;Gotthelf & Halpern 2009) and relatively long spin periods (hundreds of millisecond). AXPs and SGRs have large fields ∼ 10 14 G (see a review in Mereghetti 2008). The Magnificent seven-like NSs have fields slightly above 10 13 G (Haberl 2007;Kaplan 2008). Probably, some of rotating radio transients (RRATs, McLaughlin et al. 2006) are similar to the M7. This variety in initial properties deserves new studies of evolving NSs using the population synthesis technique (see a review in Popov & Prokhorov 2007). In this paper we present the first step.
We describe two models. At first, we discuss a simple semianalytical approach, which is used to illustrate the main features of the scenario. In this model velocities and ambient densities are not changing. Then we present a detailed numerical model, which takes into account spatial movements of NSs in the Galactic potential and realistic 3D distribution of the interstellar medium (ISM). Our main results are based on this model.
In the next section we present basic concepts used in both models, and describe each of them. Then, in Sec. 3, we present results. Discussion is given in Sec.4. In the last section we present our conclusions.
In this section we describe our models. We start with explanation of some basic processes and parameters of magneto-rotational evolution used in both models. Then we discuss the semianalytical and the full numerical model, consequently.
Here we describe some aspects of magnetorotational evolution implemented in both semianalytical and numerical models.
Here we mainly follow the approach described in Lipunov (1992). We consider a NS being born as an Ejector. At this stage a relativistic wind and Poynting flux are so strong that they prevent incoming matter to penetrate inside neither gravitational capture radius, RG, nor inside the light cylinder radius, R l . RG represent the typical scale at which the ISM is captured by the NS gravity:
where M is a NS mass and v rel is a relative velocity of the NS and the ISM. The light cylinder radius is defined as:
where ω is the spin frequency of a NS. The ejected matter creates a cavern in the ISM within the distance of the Shvartsman radius, R sh , at which the magneto-dipole pressure, P ∼ µ 2 /R 4 l R 2 Sh , is equal to the ram pressure of the ISM, P ∼ ρv 2 rel . At this stage a young NS can be visible as a radiopulsar (PSR), and we assume that it losses energy via relativistic wind and Poynting flux according to the magneto-dipole formula:
Here I = 10 45 g cm 2 is a moment of inertia of a NS, µ is a magnetic dipole moment, χ is an angle between rotational and magnetic axis, which is assumed to be π/2 everywhere below. The Ejector stage finish when the Shvartsman radius, R sh , becomes less than RG. The regime changes because after matter appear inside RG, its pressure start to grow Pmatter ∼ r -5/2 . This more rapidly than the relativistic wind pressure growth: P wind ∼ r -2 . So, the condition Pmatter > P wind is reached and the pulsar wind cannot stop the incoming flow.
Another reason t
…(Full text truncated)…
This content is AI-processed based on ArXiv data.
