Z. Frei

Finding the electromagnetic counterparts of cosmological standard sirens

The gravitational waves (GWs) emitted during the coalescence of supermassive black holes (SMBHs) in the mass range ~104-107 M?/(1 + z) will be detectable out to high redshifts with the future Laser Interferometer Space Antenna (LISA). The distance and direction to these standard sirens can be inferred directly from the GW signal, with a precision that depends on the masses, spins, and geometry of the merging system. In a given cosmology, the LISA-measured luminosity distance translates into a redshift shell. We calculate the size and shape of the corresponding three-dimensional error volume in which an electromagnetic counterpart to a LISA event could be found, taking into account errors in the background cosmology (as expected by the time LISA flies), weak gravitational lensing (de)magnification due to inhomogeneities along the line of sight, and potential source-peculiar velocities. Weak-lensing errors largely exceed other sources of uncertainties (by a factor of ~7 for typical sources at z = 1). Under the plausible assumption that SMBH-SMBH mergers are accompanied by gas accretion leading to Eddington-limited quasar activity, we then compute the number of quasars that would be found in a typical three-dimensional LISA error volume, as a function of BH mass and event redshift. Low redshifts offer the best opportunities to identify quasar counterparts to cosmological standard sirens. For mergers of ~4 ? (105-107) M? SMBHs, the LISA error volume will typically contain a single near-Eddington quasar at z ~ 1. If SMBHs are spinning rapidly, the error volume is smaller and may contain a unique quasar out to redshift z ~ 3. This will allow a straightforward test of the hypothesis that GW events are accompanied by bright quasar activity and, if the hypothesis proves correct, will guarantee the identification of a unique quasar counterpart to a LISA event, with a B-band luminosity of LB ~ (1010-1011) L?. Robust counterpart identifications would allow unprecedented tests of the physics of SMBH accretion, such as precision measurements of the Eddington ratio. They would clarify the role of gas as a catalyst in SMBH coalescences and would also offer an alternative method to constrain cosmological parameters.

Prompt Shocks in the Gas Disk Around a Recoiling Supermassive Black Hole Binary

Supermassive black hole binaries (BHBs) produced in galaxy mergers recoil at the time of their coalescence due to the emission of gravitational waves (GWs). We simulate the response of a thin, two-dimensional disk of collisionless particles, initially on circular orbits a round a 10 6 MBHB, to kicks that are either parallel or perpendicular to the initial orbital plane. Typical kick velocities (vkick) can exceed the sound speed in a circumbinary gas disk. While the inner disk is strongly boun d to the recoiling binary, the outer disk is only weakly bound or unbound. This leads to differential motions in the disturbed disk that increase with radius and can become supersonic at � 700 Schwarzschild radii for vkick = 500 km s -1 , implying that shocks form beyond this radius. We indeed find that kicks in the disk plane lead to immediate strong density enhancements (within weeks) in a tightly wound spiral caustic, propagati ng outward at the speedv kick. Concentric density enhancements are also observed for kicks perpendicular to t he disk, but are weaker and develop into caustics only after a long delay (> one year). Unless both BH spins are low or precisely aligned with the orbital angular momentum, a significant fraction ( � > several %) of kicks are sufficiently large and well aligned wi th the orbital plane for strong shocks to be produced. The shocks could result in an afterglow whose characteristic photon energy increases with time, from the UV (� 10eV) to the soft X-ray (� 100eV) range, between one month and one year after the merger. This could help identify EM counterparts to GW sources discovered by LISA. Subject headings:black hole physics - galaxies: nuclei - gravitational waves

Detection of a supervoid aligned with the cold spot of the cosmic microwave background

We use the WISE-2MASS infrared galaxy catalogue matched with Pan-STARRS1 (PS1) galaxies to search for a supervoid in the direction of the cosmic microwave background (CMB) cold spot (CS). Our imaging catalogue has median redshift z ≃ 0.14, and we obtain photometric redshifts from PS1 optical colours to create a tomographic map of the galaxy distribution. The radial profile centred on the CS shows a large low-density region, extending over tens of degrees. Motivated by previous CMB results, we test for underdensities within two angular radii, 5°, and 15°. The counts in photometric redshift bins show significantly low densities at high detection significance, ≳5σ and ≳6σ, respectively, for the two fiducial radii. The line-of-sight position of the deepest region of the void is z ≃ 0.15–0.25. Our data, combined with an earlier measurement by Granett, Szapudi & Neyrinck, are consistent with a large Rvoid = (220 ± 50) h−1 Mpc supervoid with δm ≃ −0.14 ± 0.04 centred at z = 0.22 ± 0.03. Such a supervoid, constituting at least a ≃3.3σ fluctuation in a Gaussian distribution of the Λ cold dark matter model, is a plausible cause for the CS.

Premerger localization of gravitational-wave standard sirens with LISA: Harmonic mode decomposition

The continuous improvement in localization errors (sky position and distance) in real time as LISA observes the gradual inspiral of a supermassive black hole binary can be of great help in identifying any prompt electromagnetic counterpart associated with the merger. We develop a new method, based on a Fourier decomposition of the time-dependent, LISA-modulated gravitational-wave signal, to study this intricate problem. The method is faster than standard Monte Carlo simulations by orders of magnitude. By surveying the parameter space of potential LISA sources, we find that counterparts to supermassive black hole binary mergers with total mass

Cross-correlation of WMAP7 and the WISE full data release

M\ensuremath{\sim}{10}^{5}\char21{}{10}^{7}{\mathrm{M}}_{\ensuremath{\bigodot}}

3D visualization for Intelligent Space: Time-delay compensation in a remote controlled environment

and redshifts

Eccentric Black Hole Gravitational-wave Capture Sources in Galactic Nuclei: Distribution of Binary Parameters

z\ensuremath{\lesssim}3

Supervoid origin of the cold spot in the cosmic microwave background

can be localized to within the field of view of astronomical instruments (

A statistical method to search for recoiling supermassive black holes in active galactic nuclei

\ensuremath{\sim}{\mathrm{deg}}^{2}

Phase statistics of the WMAP 7 year data

) typically hours to weeks prior to coalescence. This will allow a triggered search for variable electromagnetic counterparts as the merger proceeds, as well as monitoring of the most energetic coalescence phase. A rich set of astrophysical and cosmological applications would emerge from the identification of electromagnetic counterparts to these gravitational-wave standard sirens.