E. van der Swaluw

Superbubbles and energetic particles in the Galaxy: I. Collective effects of particle acceleration

Observations indicate that most massive stars in the Galaxy appear in groups, called OB associations, where their strong wind activity generates large structures known as superbubbles, inside which the subsequent supernovae (SNe) explode, with a tight space and time correlation. We investigate four main questions: 1) does the clustering of massive stars and SN explosions influence the particle acceleration process usually associated with SNe, and induce collective effects which would not manifest around isolated supernova remnants?; 2) does it make a difference for the general phenomenology of Galactic Cosmic Rays (GCRs), notably for their energy spectrum and composition?; 3) Can this help alleviate some of the problems encountered within the standard GCR source model?; and 4) Is the link between superbubbles and energetic particles supported by observational data, and can it be further tested and constrained? We argue for a positive answer to all these questions. Theoretical, phenomenological and observational aspects are treated in separate papers. Here, we discuss the interaction of massive stellar winds and SN shocks inside superbubbles and indicate how this leads to specific acceleration effects. We also show that due to the high SN explosion rate and low diffusion coefficient, low-energy particles experience repeated shock acceleration inside superbubbles.

An evolutionary model for pulsar-driven supernova remnants - A hydrodynamical model

We present a model of a pulsar wind nebula evolving inside its associated supernova remnant. The model uses a hydrodynamics code to simulate the evolution of this system when the pulsar has a high velocity. The simulation distinguishes four different stages of pulsar wind nebula evolution: the supersonic expansion stage, the reverse shock interaction stage, the subsonic expansion stage and ultimately the bow shock stage. The simulation bears out that, due to the high velocity of the pulsar, the position of the pulsar is off-centered with respect to its pulsar wind nebula, after the passage of the reverse shock. Subsequently the pulsar wind nebula expands subsonically until the event of the bow shock formation, when the motion of the pulsar becomes supersonic. The bow shock formation event occurs at roughly half the crossing time, when the pulsar is positioned at 0.677 times the radius of the supernova remnant blastwave, in complete agreement with analytical predictions. The crossing time is defined by the age of the supernova remnant when the pulsar overtakes the blastwave bounding the supernova remnant. The results of the model are applied to three supernova remnants: N157B, G327.1-1.1 and W44. We argue that the head of the pulsar wind nebula, containing the active pulsar, inside the first two systems is not bounded by a bow shock. However, in the case of W44 we argue for a scenario in which the pulsar wind nebula is bounded by a bow shock, due to the supersonic motion of the pulsar.

Bjerknes Compensation at High Northern Latitudes: The Ocean Forcing the Atmosphere

The mechanisms for Bjerknes compensation of heat transport variations through the atmosphere and ocean on decadal time scales are investigated, using data output from a preindustrial control run of the Third Hadley Centre Coupled Ocean–Atmosphere General Circulation Model (HadCM3). It has recently been shown that Bjerknes compensation occurs on decadal time scales in a long preindustrial control run of HadCM3. This result is elaborated on by performing lead/lag correlations of the atmospheric and oceanic heat transports. By using statistical analysis, Bjerknes compensation is observed on decadal time scales at latitudes between 50° and 80°N. A maximum compensation rate of 55% occurs at 70°N. At this latitude, the correlation rate peaks when the ocean leads the atmosphere by one year. The mechanisms by which Bjerknes compensation occurs at this latitude are investigated. Anomalies in oceanic heat transport appear to be associated with variations in the strength of the Atlantic meridional overturning circulation (MOC). The associated sea surface temperature (SST) anomalies are in general too weak to assert a significant impact on the atmosphere. At 70°N, however, such SST anomalies are a prelude to the transition from sea ice coverage to open water after which the associated changes in heat exchange with the atmosphere are strong enough to force an atmospheric response. Because of the presence of a strong MOC component in the Atlantic Ocean, this interaction is confined to the region where the northeast Atlantic and Arctic Oceans connect. The atmospheric response to increased (decreased) heating from below is a decreased (increased) poleward temperature gradient, leading to a decreased (increased) heat transport by baroclinic eddies. The anomalous thermal low that is set up by heating from the ocean is associated with anomalous advection of cold air from the Greenland landmass.

Interaction of a magnetized pulsar wind with its surroundings - MHD simulations of pulsar wind nebulae

Magnetohydrodynamical simulations are presented of a magnetized pulsar wind interacting directly with the interstellar medium, or, in the case of a surrounding supernova remnant, with the associated freely expanding ejecta of the progenitor star. In both cases the simulations show that the pulsar wind nebula will be elongated due to the dynamical influence of the toroidal magnetic fields, which confirm predictions from a semi-analytical model presented by Begelman & Li. The simulations follow the expansion of the pulsar wind nebula when the latter is bounded by a strong shock and show that the expansion can be modeled with a standard power-law expansion rate. By performing different simulations with different magnetization parameters, I show that the latter weakly correlates with the elongation of the pulsar wind nebula. The results from the simulations are applied to determine the nature of the expansion rate of the pulsar wind nebula 3C 58. It is shown that there is both observational and theoretical evidence which supports the scenario in which the pulsar wind nebula 3C 58 has caught up with the reverse shock of the associated (but undetected) supernova remnant.

Proper-Motion Measurements of Pulsar B1951+32 in the Supernova Remnant CTB 80

Using the Very Large Array and the Pie Town antenna, we have measured the position of the radio pulsar B1951+32 relative to nearby background radio sources at four epochs between 1989 and 2000. These data show a clear motion for the pulsar of 25 ± 4 mas yr-1 at a position angle 252° ± 7° (north through east), corresponding to a transverse velocity 240 ± 40 km s-1 for a distance to the source of 2 kpc. The measured direction of motion confirms that the pulsar is moving away from the center of its associated supernova remnant, the first time that such a result has been demonstrated. Independent of assumptions made about the pulsar birthplace, we show that the measured proper motion implies an age for the pulsar of 64 ± 18 kyr, somewhat less than its characteristic age of 107 kyr. This discrepancy can be explained if the initial spin period of the pulsar was P0 = 27 ± 6 ms.

THE X-RAY STRUCTURE OF THE PULSAR BOW SHOCK G189.22+2.90 IN THE SUPERNOVA REMNANT IC 443

Wepresentadeepobservation withtheChandraX-Ray Observatory of theneutron starbowshockG189.22+2.90 in the supernova remnant (SNR) IC 443. Our data confirm the cometary morphology and central point source seen previously, but also reveal considerable new structure. Specifically, we find that the X-ray nebula consists of two distinct components:a‘‘tongue’’of bright emissionclose to theneutron star,enveloped byalarger,fainter ‘‘tail.’’ We interpret the tongue and tail as delineating the termination shock and the postshock flow, respectively, as previously identified also in the pulsar bow shock G359.23� 0.82 (‘‘the Mouse’’). However, for G189.22+2.90 the tongue is much less elongated than for the Mouse, while the tail is much broader. These differences are consistent with the low Mach number,M P2, expected for a neutron star moving through the hot gas in a SNR’s interior, supporting the case for a physical association between G189.22+2.90 and IC 443. We resolve the standoff distance between the star and the head of the bow shock, which allows us to estimate a space velocity for the neutron star of � 230 km s � 1 , independent of distance. We detect thermal emission from the neutron star surface at a temperature of 102 � 22 eV, which is consistent with the age of SNR IC 443 for standard neutron star cooling models. We also identify two compact knots of hard emission located 1 00 Y2 00 north and south of the neutron star. Subject headingg ISM: individual (G189.22+2.90, IC 443) — pulsars: individual (CXOU J061705.3+222127) — stars: neutron — stars: winds, outflows

Magneto-rotational overstability in accretion disks

We present analytical and numerical studies of magnetorotational instabilities occuring in magnetized accretion disks. These calculations are performed for general radially stratified disks in the cylindrical limit. We elaborate on earlier analytical results and confirm and expand them with numerical computations of unstable eigenmodes of the full set of linearised compressible MHD equations. We compare these solutions with those found from approximate local dispersion equations from WKB analysis. In particular, we investigate the influence of a nonvanishing toroidal magnetic field component on the growth rate and oscillation frequency of magnetorotational instabilities in Keplerian disks. These calculations are performed for a constant axial magnetic field strength. We find the persistence of these instabilities in accretion disks close to equipartition. Our calculations show that these eigenmodes become overstable (complex eigenvalue), due to the presence of a toroidal magnetic field component, while their growth rate reduces slightly. Furthermore, we demonstrate the presence of magneto-rotational overstabilities in weakly magnetized sub-Keplerian rotating disks. We show that the growth rate scales with the rotation frequency of the disk. These eigenmodes also have a nonzero oscillation frequency, due to the presence of the dominant toroidal magnetic field component. The overstable character of the MRI increases as the rotation frequency of the disk decreases.

The duck redux: An improved proper-motion upper limit for the pulsar B1757-24 near the supernova remnant G5.4-1.2

‘‘The Duck’’ is a complicated nonthermal radio system, consisting of the energetic radio pulsar B1757� 24, its surrounding pulsar wind nebula G5.27Y0.90, and the adjacent supernova remnant (SNR) G5.4Y1.2. PSR B1757� 24 was originally claimed to be a young (� 15,000 yr) and extreme-velocity (k1500 km s � 1 ) pulsar, which had penetratedandemergedfromtheshelloftheassociatedSNRG5.4Y1.2;butrecentupperlimitsonthepulsar’smotion have raised serious difficulties with this interpretation. We here present 8.5 GHz interferometric observations of the nebula G5.27Y0.90 over a 12 yr baseline, doubling the time span of previous measurements. These data correspondingly allowus to halve theprevious upperlimit onthe nebula’swestward motion to14 masyr � 1 (5 � ), allowing a substantive reevaluation of this puzzling object. We rule out the possibility that the pulsar and SNR were formed from a common supernova explosion � 15,000 yr ago, as implied by the pulsar’s characteristic age, but conclude that an old (k70,000 yr) pulsar/SNR association, or a situation in which the pulsar and SNR are physically unrelated, are both still viable explanations. Subject headingg ISM: individual (G5.4Y1.2) — pulsars: individual (B1757� 24) — radio continuum: ISM — stars: neutron — supernova remnants

Convective magneto-rotational instabilities in accretion disks

We present a study of instabilities occuring in thick magnetized accretion disks. We calculate the growth rates of these instabilities and characterise precisely the contribution of the magneto-rotational and convective mechanism. All our calculations are performed in radially stratified disks in the cylindrical limit. The numerical calculations are performed using the appropriate local dispersion equation solver discussed in Blokland et al. (2005, A&A, 444, 337). A comparison with recent results by Narayan et al. (2002, ApJ, 577, 295) shows excellent agreement with their approximate growth rates only if the disks are weakly magnetized. However, for disks close to equipartition, the dispersion equation from Narayan et al. (2002) loses its validity. Our calculations allow for quantitative determination of the increase in growth rate due to the magneto-rotational mechanism. We find that the increase of the growth rate for long wavelength convective modes caused by this mechanism is almost neglible. On the other hand, the growth rate of short wavelength instabilities can be significantly increased by this mechanism, reaching values up to 60%.

Do we really observe a bow shock in N157B...

Abstract I present a model of a pulsar wind interacting with its associated supernova remnant. I will use the model to argue that one can explain the morphology of the pulsar wind nebula inside N157B, a supernova remnant in the Large Magellanic Cloud, without the need for a bow shock interpretation. The model uses a hydrodynamics code which simulates the evolution of a pulsar wind nebula, when the pulsar is moving at a high velocity (1000 km/s) through the expanding supernova remnant. The evolution of the pulsar wind nebula can roughly be divided into three stages. In the first stage the pulsar wind nebula is expanding supersonically through the freely expanding ejecta of the progenitor star (∼1000 years). In the next stage the expansion of the pulsar wind nebula is not steady, due to the interaction with the reverse shock of the supernova remnant; the pulsar wind nebula oscillates violently between contraction and expansion, but will ultimately relax towards a steady subsonic expansion (∼1000–10,000 years). The last stage occurs when the head of the pulsar wind nebula, containing the active pulsar, deforms into a bow shock (>10,000 years), due to the motion of the pulsar becoming supersonic. Ultimately it is this bow shock structure bounding the pulsar, which directly interacts with the shell structure of the supernova remnant, just before the pulsar breaks out of the supernova remnant. I will argue that the pulsar wind nebula inside N157B is currently in the second stage of its evolution, i.e., the expansion of the pulsar wind nebula is subsonic and there is no bow shock around the pulsar wind bubble. The strongly off-centered position of the pulsar with respect to its pulsar wind nebula is naturally explained by the result of the interaction of the reverse shock with the pulsar wind nebula, as the simulation bears out.