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Register a new userRadio polarimetry of Galactic centre pulsars
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Added by
Dominic Schnitzeler
Publication date
2016
fields
Physics
and research's language is
English
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To study the strength and structure of the magnetic field in the Galactic centre (GC) we measured Faraday rotation of the radio emission of pulsars which are seen towards the GC. Three of these pulsars have the largest rotation measures (RMs) observed in any Galactic object with the exception of Sgr A*. Their large dispersion measures, RMs and the large RM variation between these pulsars and other known objects in the GC implies that the pulsars lie in the GC and are not merely seen in projection towards the GC. The large RMs of these pulsars indicate large line-of-sight magnetic field components between ~ 16-33 microgauss; combined with recent model predictions for the strength of the magnetic field in the GC this implies that the large-scale magnetic field has a very small inclination angle with respect to the plane of the sky (~ 12 degrees). Foreground objects like the Radio Arc or possibly an ablated, ionized halo around the molecular cloud G0.11-0.11 could contribute to the large RMs of two of the pulsars. If these pulsars lie behind the Radio Arc or G0.11-0.11 then this proves that low-scattering corridors with lengths >~ 100 pc must exist in the GC. This also suggests that future, sensitive observations will be able to detect additional pulsars in the GC. Finally, we show that the GC component in our most accurate electron density model oversimplifies structure in the GC.
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Globular clusters (GCs) are the ideal environment for the formation of neutron stars (NSs) and millisecond pulsars (MSPs). NSs origin and evolution provide a useful information on stellar dynamics and evolution in star clusters, and are among the most interesting astrophysical objects, being precursors of several high-energy phenomena such as gravitational waves and gamma-ray bursts. Due to a large velocity kick that they receive at birth, most of the NSs escape the local field, affecting the evolution and dynamics of their parent cluster. In this paper, we study the origin and dynamical evolution of NSs within GCs with different initial masses, metallicities and primordial binary fractions. We find that the radial profile of NSs is shaped by the BH content of the cluster, which partially quenches the NS segregation until most of the BHs are ejected from the system. Independently on the cluster mass and initial configuration, the NSs map the average stellar population, as their average radial distance is $approx 60-80%$ of the cluster half-mass radius. Finally, by assuming a recycling fraction of $f_mathrm{rec}=0.1$ and an average MSP gamma-ray emission of $L_gamma=2times 10^{33}$ erg s$^{-1}$, we show that the typical gamma-ray emission from our GCs agrees with observations and supports the MSP origin of the gamma-ray excess signal observed by the Fermi-LAT telescope in the Galactic Centre.
In this contribution, we summarize our results concerning the observational constraints on the electric charge associated with the Galactic centre black hole - Sgr A*. According to the no-hair theorem, every astrophysical black hole, including supermassive black holes, is characterized by at most three classical, externally observable parameters - mass, spin, and the electric charge. While the mass and the spin have routinely been measured by several methods, the electric charge has usually been neglected, based on the arguments of efficient discharge in astrophysical plasmas. From a theoretical point of view, the black hole can attain charge due to the mass imbalance between protons and electrons in fully ionized plasmas, which yields about $sim 10^8,{rm C}$ for Sgr A*. The second, induction mechanism concerns rotating Kerr black holes embedded in an external magnetic field, which leads to electric field generation due to the twisting of magnetic field lines. This electric field can be associated with the induced Wald charge, for which we calculate the upper limit of $sim 10^{15},{rm C}$ for Sgr A*. Although the maximum theoretical limit of $sim 10^{15},{rm C}$ is still 12 orders of magnitude smaller than the extremal charge of Sgr A*, we analyse a few astrophysical consequences of having a black hole with a small charge in the Galactic centre. Two most prominent ones are the effect on the X-ray bremsstrahlung profile and the effect on the position of the innermost stable circular orbit.
The role of starburst winds versus active galactic nuclei (AGN) jets/winds in the formation of the kiloparsec scale radio emission seen in Seyferts is not yet well understood. In order to be able to disentangle the role of various components, we have observed a sample of Seyfert galaxies exhibiting kpc-scale radio emission suggesting outflows, along with a comparison sample of starburst galaxies, with the EVLA B-array in polarimetric mode at 1.4 GHz and 5~GHz. Polarization is clearly detected in three Seyfert galaxies and one starburst galaxy. The Seyfert galaxy NGC,2639, shows highly polarized secondary radio lobes, not observed before, which are aligned perpendicular to the known pair of radio lobes. The additional pair of lobes represent an older epoch of emission. A multi-epoch multi-frequency study of the starburst-Seyfert composite galaxy NGC,3079, reveals that the jet together with the starburst superwind and the galactic magnetic fields might be responsible for the well-known 8-shaped radio lobes observed in this galaxy. We find that many of the Seyfert galaxies in our sample show bubble-shaped lobes, which are absent in the starburst galaxies that do not host an AGN.
The Galactic centre - as the closest galactic nucleus - holds both intrinsic interest and possibly represents a useful analogue to star-burst nuclei which we can observe with orders of magnitude finer detail than these external systems. The environmental conditions in the GC - here taken to mean the inner 200 pc in diameter of the Milky Way - are extreme with respect to those typically encountered in the Galactic disk. The energy densities of the various GC ISM components are typically ~two orders of magnitude larger than those found locally and the star-formation rate density ~three orders of magnitude larger. Unusually within the Galaxy, the Galactic centre exhibits hard-spectrum, diffuse TeV (=10^12 eV) gamma-ray emission spatially coincident with the regions molecular gas. Recently the nuclei of local star-burst galaxies NGC 253 and M82 have also been detected in gamma-rays of such energies. We have embarked on an extended campaign of modelling the broadband (radio continuum to TeV gamma-ray), non- thermal signals received from the inner 200 pc of the Galaxy. On the basis of this modelling we find that star-formation and associated supernova activity is the ultimate driver of the regions non-thermal activity. This activity drives a large-scale wind of hot plasma and cosmic rays out of the GC. The wind advects the locally-accelerated cosmic rays quickly, before they can lose much energy in situ or penetrate into the densest molecular gas cores where star-formation occurs. The cosmic rays can, however, heat/ionize the lower density/warm H2 phase enveloping the cores. On very large scales (~10 kpc) the non-thermal signature of the escaping GC cosmic rays has probably been detected recently as the spectacular Fermi bubbles and corresponding WMAP haze.
The so-called S2 star reached its closest approach to the massive black hole (BH) at around 1500 $R_mathrm{s}$ in May 2018. It has been proposed that the interaction of its stellar wind with the high-density accretion flow at this distance from Sgr A* will lead to a detectable, month-long X-ray flare. Our goal is to verify whether or not the S2 star wind can be used as a diagnostic tool to infer the properties of the accretion flow towards Sgr A* at its pericentre (an unprobed distance regime), putting important constraints on BH accretion flow models. We run a series of three-dimensional adaptive mesh refinement simulations with the help of the Ramses code which include the realistic treatment of the interaction of S2s stellar wind with the accretion flow along its orbit and - apart from hydrodynamical and thermodynamical effects - include the tidal interaction with the massive BH. These are post-processed to derive the X-ray emission in the observable 2-10 keV window. No significant excess of X-ray emission from Sgr A* is found for typical accretion flow models. A measurable excess is produced for a significantly increased density of the accretion flow. This can, however, be ruled out for standard power-law accretion flow models as in this case the thermal X-ray emission without the S2 wind interaction would already exceed the observed quiescent luminosity. Only a significant change of the wind parameters (increased mass loss rate and decreased wind velocity) might lead to an (marginally) observable X-ray flaring event. Even the detection of an (month-long) X-ray flare during the pericentre passage of the S2 star would not allow for strict constraints to be put on the accretion flow around Sgr A* due to the degeneracy caused by the dependence on multiple parameters (of the accretion flow model as well as the stellar wind).
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