IUCAA Preprints
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Item Diamagnetic screening of the magnetic field in accreting neutron stars(2001-09-20) Choudhuri, Arnab Rai; Konar, SushanA possible mechanism for screening of the surface magnetic field of an accreting neutron star, by the accreted material, is investigated. In particular, we investigate the nature of the evolution of the internal field configuration in the case of a) a polar cap accretion and b) a spherical accretionItem Small-scale microwave background anisotropies arising from tangled promordial magnetic fields(2002-03-12) Subramanian, Kandaswamy; Barrow, John D.An inhomogeneous cosmologicalmagnetic field creates vortical perturbations that survive Silk damping onmuch smaller scales than compressionalmodes. This ensures that there is no sharp cut-off in anisotropy on arcminute scales. As we had pointed out earlier, tangled magnetic fields, if they exist, will then be a potentially important contributor to small-angular-scale cosmic microwave background radiation anisotropies. Several ongoing and new experiments are expected to probe the very small angular scales, corresponding tomultipoles with l >1000. In view of this observational focus, we revisit the predicted signals arising from primordial tangled magnetic fields, for different spectra and different cosmological parameters. We also identify a new regime, where the photon mean-free path exceeds the scale of the perturbation, which dominates the predicted signal at very high l. A scale-invariant spectrum of tangled fields which redshifts to a present value B0 = 3 × 10−9 G produces temperature anisotropies at the 10-µK level between l ∼1000 and 3000. Larger signals result if the universe is lambda- dominated, if the baryon density is larger, or if the spectral index ofmagnetic tangles is steeper, n >−3. The signalwill also have non-Gaussian statistics.We predict the distinctive formof the increased power expected in the microwave background at high l in the presence of significant tangled magnetic fields. We may be on the verge of detecting or ruling out the presence of tangledmagnetic fields that are strong enough to influence the formation of large-scale structure in the Universe.Item Evidence for shock acceleration and intergalactic magnetic fields in a large-scale filament of galaxies ZwC1 2341.1+0000(2002-06-19) Bagchi, Joydeep; Enßlin, Torsten A.; Miniati, Francesco; et al.We report the discovery of large-scale diffuse radio emission from what appears to be a large-scale filamentary network of galaxies in the region of cluster ZwCl 2341.1+0000, and stretching over an area of at least 6 h−1 50 Mpc in diameter. Mul- ticolour CCD observations yield photometric redshifts indicating that a significant fraction of the optical galaxies in this region is at a redshift of z=0.3. This is sup- ported by spectroscopic measurements of 4 galaxies in the Sloan Digital Sky Survey (SDSS) at a mean z =0.27. We present VLA images at λ =20 cm (NVSS) and 90 cm, showing the detailed radio structure of the filaments. Comparison with the high resolution FIRST radio survey shows that the diffuse emission is not due to known individual point sources. The diffuse radio-emission has a spectral index α . −0.5, and is most likely synchrotron emission from relativistic charged particles in an inter-galactic magnetic field. Furthermore, this optical/radio structure is detected in X-rays by the ROSAT all-sky survey. It has a 0.1–2.4 keV luminosity of about 1044 erg s−1 and shows an extended highly non-relaxed morphology. These observa- tions suggest that ZwCl 2341.1+0000 is possibly a proto-cluster of galaxies in which we are witnessing the process of structure formation. We show that the energetics of accretion shocks generated in forming large-scale structures are sufficient to pro- duce enough high energy cosmic-ray (CR) electrons required to explain the observed radio emission, provided a magnetic field of strength B & 0.3µG is present there. The latter is only a lower limit and the actual magnetic field is likely to be higher depending on the morphology of the emitting region. Finally, we show results from Preprint submitted to Elsevier Science 1 February 2008a numerical simulation of large-scale structure formation including acceleration of CR electrons at cosmological shocks and magnetic field evolution. Our results are in accord with the observed radio synchrotron and X-ray thermal bremsstrahlung fluxes. Thus we conclude that the reported radio detection is the first evidence of cosmic-ray particle acceleration taking place at cosmic shocks in a magnetized inter-galactic medium over scales of & 5 h−1 50 Mpc.Item Diamagnetic screening of the magnetic field in accreting neutron stars(2002-03-01) Konar, Sushan; Choudhuri, Arnab RaiA possible mechanism for screening of the surface magnetic field of an accreting neutron star, by the accreted material, is investigated. In particular, we investigate the nature of the evolution of the internal field configuration in the case of a) a polar cap accretion and b) a spherical accretion.Item Kinetic and magnetic alpha effects in nonlinear dynamo theory(2007-01-19) Sur, Sharanya; Subramanian, Kandaswamy; Brandenburg, AxelThe backreaction of the Lorentz force on the α-effect is studied in the limit of small magnetic and fluid Reynolds numbers, using the first order smoothing approximation (FOSA) to solve both the induction and momentum equations. Both steady and time dependent forcings are considered. In the low Reynolds number limit, the velocity and magnetic fields can be expressed explicitly in terms of the forcing function. The nonlinear α-effect is then shown to be expressible in several equivalent forms in agreement with formalisms that are used in various closure schemes. On the one hand, one can express α completely in terms of the helical properties of the velocity field as in traditional FOSA, or, alternatively, as the sum of two terms, a so-called kinetic α-effect and an oppositely signed term proportional to the helical part of the small scale magnetic field. These results hold for both steady and time dependent forcing at arbitrary strength of the mean field. In addition, the τ-approximation is considered in the limit of small fluid and magnetic Reynolds numbers. In this limit, the τ closure term is absent and the viscous and resistive terms must be fully included. The underlying equations are then identical to those used under FOSA, but they reveal interesting differences between the steady and time dependent forcing. For steady forcing, the correlation between the forcing function and the small-scale magnetic field turns out to contribute in a crucial manner to determine the net α-effect. However for delta-correlated time-dependent forcing, this force–field correlation vanishes, enabling one to write α exactly as the sum of kinetic and magnetic α-effects, similar to what one obtains also in the large Reynolds number regime in theτ-approximation closure hypothesis. In the limit of strong imposed fields, B0, we find α ∝ B−2 0 for delta-correlated forcing, in contrast to the well-known α ∝ B−3 0 behaviour for the case of a steady forcing. The analysis presented here is also shown to be in agreement with numerical simulations of steady as well as random helical flows.Item Primordial magnetic fields and formation of molecular hydrogen(2009-04-01) Sethi, Shiv K.; Nath, B. B.; Subramanian, Kandaswamy; et al.We study the implications of primordial magnetic fields for the thermal and ionization history of the post-recombination era. In particular we compute the effects of dissi- pation of primordial magnetic fields owing to ambipolar diffusion and decaying tur- bulence in the intergalactic medium (IGM) and the collapsing halos and compute the effects of the altered thermal and ionization history on the formation of molecular hy- drogen.We show that, for magnetic field strengths in the range 2×10−10 G < ∼ B0 < ∼ 2× 10−9 G, the molecular hydrogen fraction in IGM and collapsing halo can increase by a factor 5 to 1000 over the case with no magnetic fields. We discuss the implication of the increased molecular hydrogen fraction on the radiative transfer of UV photons and the formation of first structures in the universeItem Primordial magnetic fields and CMB anisotropies(2006-01-25) Subramanian, KandaswamyPossible signatures of primordial magnetic fields on the Cosmic Microwave Background (CMB) temperature and polarization anisotropies are reviewed. The signals that could be searched for include excess temperature anisotropies particularly at small angular scales below the Silk damping scale, B-mode polarization, and non-Gaussian statistics. A field at a few nG level produces temperature anisotropies at the 5µK level, and B-mode polarization anisotropies 10 times smaller, and is therefore potentially detectable via the CMB anisotropies. An even smaller field, with B0 < 0.1 nG, could lead to structure formation at high redshift z > 15, and hence naturally explain an early re-ionization of the Universe.Item Origin and evolution of cluster magnetism(2006-04-02) Shukurov, A.; Subramanian, Kandaswamy; Haugen, N. E. L.Random motions can occur in the intergalactic gas of galaxy clusters at all stages of their evolution. Depending on the poorly known value of the Reynolds number, these motions can or cannot become turbulent, but in any case they can generate random magnetic fields via dynamo action. We argue that magnetic fields inferred observationally for the intracluster medium require dynamo action, and then estimate parameters of random flows and magnetic fields at various stages of the cluster evolution. Polarization in cluster radio halos predicted by the model would be detectable with the SKA.Item Galactic dynamo and helicity losses through fountain flow(2006-01-19) Shukurov, A.; Sokoloff, Dmitry; Subramanian, KandaswamyNonlinear behaviour of galactic dynamos is studied, allowing for magnetic helicity removal by the galactic fountain flow. Methods. A suitable advection speed is estimated, and a one-dimensional mean-field dynamo model with dynamic α-effect is explored. Results. It is shown that the galactic fountain flow is efficient in removing magnetic helicity from galactic discs. This alleviates the constraint on the galactic mean-field dynamo resulting from magnetic helicity conservation and thereby allows the mean magnetic field to saturate at a strength comparable to equipartition with the turbulent kinetic energy.Item Evolving turbulence and magnetic fields in galaxy clusters(2006-01-10) Subramanian, Kandaswamy; Shukurov, A.; Haugen, N. E. L.We discuss, using simple analytical models and MHD simulations, the origin and parameters of turbulence and magnetic fields in galaxy clusters. Any pre-existing tangled magnetic field must decay in a few hundred million years by generating gas motions even if the electric conductivity of the intracluster gas is high. We argue that tur- bulent motions can be maintained in the intracluster gas and its dynamo action can prevent such a decay and amplify a random seed magnetic field by a net factor typically 10⁴ in 5Gyr. Three physically distinct regimes can be identified in the evolution of turbulence and magnetic field in galaxy clusters. Firstly, the fluctuation dynamo will produce microgauss-strong, random magnetic fields during the epoch of cluster formation and major mergers. At this stage pervasive turbulent flows with r.m.s. velocity of about 300 kms−ᶥ can be maintained at scales 100–200 kpc. The magnetic field is intermittent, has a smaller scale of 20–30 kpc and average strength of 2 G. Secondly, turbulence will decay after the end of the major merger epoch; we discuss the dynamics of the decaying turbulence and the behavior of magnetic field in it. Magnetic field and turbulent speed undergo a power-law decay, decreasing by a factor of two during this stage, whereas their scales increase by about the same factor. Thirdly, smaller-mass subclusters and cluster galaxies will produce turbulent wakes where magnetic fields will be generated as well. Although the wakes plausibly occupy only a small fraction of the cluster volume, we show that their area covering factor can be close to unity, and thus they can produce some of the signatures of turbulence along virtually all lines of sight. The latter could potentially allow one to reconcile the possibility of turbulence with ordered filamentary gas structures, as in the Perseus cluster. The turbulent speeds and magnetic fields in the wakes are estimated to be of order 300 kms−ᶥ and 2 G, respectively, whereas the turbulent scales are of order 200 kpc for wakes behind subclusters of a mass 3 × 10ᶥᶟM⊙ and about 10 kpc in the galactic wakes. Magnetic field in the wakes is intermittent and has the scale of about 30 kpc and 1 kpc in the subcluster and galactic wakes, respectively. Random Faraday rotation measure is estimated to be typically 100–200 radm−², in agreement with observations. We predict detectable polarization of synchrotron emission from cluster radio halos at wavelengths 3–6 cm, if observed at sufficiently high resolution.