Showing papers by "M. Coleman Miller published in 2012"

General Relativistic Simulations of Magnetized Plasmas around Merging Supermassive Black Holes

Abstract: Coalescing supermassive black hole binaries are produced by the mergers of galaxies and are the most powerful sources of gravitational waves accessible to space-based gravitational observatories. Some such mergers may occur in the presence of matter and magnetic fields and hence generate an electromagnetic counterpart. In this Letter, we present the first general relativistic simulations of magnetized plasma around merging supermassive black holes using the general relativistic magnetohydrodynamic code Whisky. By considering different magnetic field strengths, going from non-magnetically dominated to magnetically dominated regimes, we explore how magnetic fields affect the dynamics of the plasma and the possible emission of electromagnetic signals. In particular, we observe a total amplification of the magnetic field of ~2 orders of magnitude, which is driven by the accretion onto the binary and that leads to much stronger electromagnetic signals, more than a factor of 104 larger than comparable calculations done in the force-free regime where such amplifications are not possible.

Psr j0007+7303 in the cta1 supernova remnant: new gamma-ray results from two years of fermi large area telescope observations

Abstract: One of the main results of the Fermi Gamma-Ray Space Telescope is the discovery of gamma-ray selected pulsars The high magnetic field pulsar, PSR J0007+7303 in CTA1, was the first ever to be discovered through its gamma-ray pulsations Based on analysis of 2 years of LAT survey data, we report on the discovery of I-ray emission in the off-pulse phase interval at the approx 6sigma level The flux from this emission in the energy range E > or =::: 100 MeV is F(sub 100) = (173+/-040) x 10(exp -8) photons/sq cm/s and is best fitted by a power law with a photon index of Gamma = 254+/-014 The pulsed gamma-ray flux in the same energy range is F(sub 100) = (395+/-007) x 10(exp -7) photons/sq cm/s and is best fitted by an exponentially-cutoff power-law spectrum with a photon index of Gamma = 141+/-023 and a cutoff energy E(sub c) = 404+/-020 GeV We find no flux variability neither at the 2009 May glitch nor in the long term behavior We model the gamma-ray light curve with two high-altitude emission models, the outer gap and slot gap, and find that the model that best fits the data depends strongly on the assumed origin of the off-pulse emission Both models favor a large angle between the magnetic axis and observer line of sight, consistent with the nondetection of radio emission being a geometrical effect Finally we discuss how the LAT results bear on the understanding of the cooling of this neutron star

An upper limit to the velocity dispersion of relaxed stellar systems without massive black holes

Abstract: Massive black holes have been discovered in all closely examined galaxies with high velocity dispersion. The case is not as clear for lower-dispersion systems such as low-mass galaxies and globular clusters. Here we suggest that above a critical velocity dispersion similar to 40 km s(-1), massive central black holes will form in relaxed stellar systems at any cosmic epoch. This is because above this dispersion primordial binaries cannot support the system against deep core collapse. If, as previous simulations show, the black holes formed in the cluster settle to produce a dense subcluster, then given the extremely high densities reached during core collapse the holes will merge with each other. For low velocity dispersions and hence low cluster escape speeds, mergers will typically kick out all or all but one of the holes due to three-body kicks or the asymmetric emission of gravitational radiation. If one hole remains, it will tidally disrupt stars at a high rate. If none remain, one is formed after runaway collisions between stars, and then it tidally disrupts stars at a high rate. The accretion rate after disruption is many orders of magnitude above Eddington. If, as several studies suggest, the hole can accept matter at that rate because the generated radiation is trapped and advected, then it will grow quickly and form a massive central black hole. (Less)

Tidal disruptions of separated binaries in galactic nuclei

Abstract: Several galaxies have exhibited X-ray ares that are consistent with the tidal disruption of a star by a central supermassive black hole. In theoretical treatments of this process it is usually assumed that the star was initially on a nearly parabolic orbit relative to the black hole. Such an assumption leads in the simplest approximation to a t 5=3 decay of the bolometric luminosity and this is indeed consistent with the relatively poorly sampled light curves of such ares. We point out that there is another regime in which the decay would be dierent: if a binary is tidally separated and the star that remains close to the hole is eventually tidally disrupted from a moderate eccentricity orbit, the decay is slower, typically t 1:2 . As a result, careful sampling of the light curves of such ares could distinguish between these processes and yield insight into the dynamics of binaries as well as single stars in galactic centres. We explore this process using three-body simulations and analytic treatments and discuss the consequences for present-day X-ray detections and future gravitational wave observations.

An upper limit to the velocity dispersion of relaxed stellar systems without massive black holes

Abstract: Massive black holes have been discovered in all closely examined galaxies with high velocity dispersion. The case is not as clear for lower-dispersion systems such as low-mass galaxies and globular clusters. Here we suggest that above a critical velocity dispersion of roughly 40 km/s, massive central black holes will form in relaxed stellar systems at any cosmic epoch. This is because above this dispersion primordial binaries cannot support the system against deep core collapse. If, as previous simulations show, the black holes formed in the cluster settle to produce a dense subcluster, then given the extremely high densities reached during core collapse the holes will merge with each other. For low velocity dispersions and hence low cluster escape speeds, mergers will typically kick out all or all but one of the holes due to three-body kicks or the asymmetric emission of gravitational radiation. If one hole remains, it will tidally disrupt stars at a high rate. If none remain, one is formed after runaway collisions between stars, then it tidally disrupts stars at a high rate. The accretion rate after disruption is many orders of magnitude above Eddington. If, as several studies suggest, the hole can accept matter at that rate because the generated radiation is trapped and advected, then it will grow quickly and form a massive central black hole.

Sowing the seeds of massive black holes in small galaxies: Young clusters as the building blocks of Ultra-Compact-Dwarf Galaxies

Abstract: Interacting galaxies often have complexes of hundreds of young stellar clusters of individual masses $\sim 10^{4-6}~M_\odot$ in regions that are a few hundred parsecs across. These cluster complexes interact dynamically, and their coalescence is a candidate for the origin of some ultracompact dwarf galaxies (UCDs). Individual clusters with short relaxation times are candidates for the production of intermediate-mass black holes of a few hundred solar masses, via runaway stellar collisions prior to the first supernovae in a cluster. It is therefore possible that a cluster complex hosts multiple intermediate-mass black holes that may be ejected from their individual clusters due to mergers or binary processes, but bound to the complex as a whole. Here we explore the dynamical interaction between initially free-flying massive black holes and clusters in an evolving cluster complex. We find that, after hitting some clusters, it is plausible that the massive black hole will be captured in an ultracompact dwarf forming near the center of the complex. In the process, the hole typically triggers electromagnetic flares via stellar disruptions, and is also likely to be a prominent source of gravitational radiation for the advanced ground-based detectors LIGO and VIRGO. We also discuss other implications of this scenario, notably that the central black hole could be considerably larger than expected in other formation scenarios for ultracompact dwarfs.

Constraining neutron star masses and radii using thermonuclear X-ray bursts

Abstract: Precisely measured neutron star masses and especially radii would provide unique constraints on the properties of cold matter at several times nuclear density. Observations using the Rossi X-ray Timing Explorer suggest that such measurements might be possible using thermonuclear X-ray bursts. Here we discuss the prospects for mass and radius constraints, with a particular focus on potential systematic errors.

The Be/X-ray Binary Swift J1626.6-5156 as a Variable Cyclotron Line Source

Abstract: Swift J1626.6-5156 is a Be/X-ray binary that was in outburst from December 2005 until November 2008. We have examined RXTE/PCA and HEXTE spectra of three long observations of this source taken early in its outburst, when the PCA 2-20 keV count rate was >70 counts/s/PCU, as well as several combined observations from different stages of the outburst. The spectra are best fit with an absorbed cutoff power law with a ~6.4 keV iron emission line and a Gaussian optical depth absorption line at ~10 keV. We present strong evidence that this absorption-like feature is a cyclotron resonance scattering feature, making Swift J1626.6-5156 a new candidate cyclotron line source. The redshifted energy of ~10 keV implies a magnetic field strength of ~8.6(1+z) x 10^11 G in the region of the accretion column close to the magnetic poles where the cyclotron line is produced. Analysis of phase averaged spectra spanning the duration of the outburst suggests a possible positive correlation between the fundamental cyclotron energy and source luminosity. Phase resolved spectroscopy from a long observation reveals a variable cyclotron line energy, with phase dependence similar to a variety of other pulsars, as well as the first harmonic of the fundamental cyclotron line.