Theoretical astrophysics has long relied on numerical simulations as a formidable way to improve our understanding of the dynamics of astrophysical systems. Fortunately, the mathematical framework upon which such simulations are based is nowadays developed to high levels of sophistication. The equations governing the dynamics of relativistic astrophysical systems are an intricate set of coupled, time-dependent partial differential equations, comprising the general relativistic hydrodynamics and magnetohydrodynamics equations (GRHD/GRMHD hereafter) and Einstein’s gravitational field equations. The RIT group has developed significant expertise to handle such systems over the course of the years. There are a number of long-term and ambitious projects dedicated to this area within CCRG.

**ACCRETION ONTO SUPERMASSIVE BLACK HOLE MERGERS**

Our main GRMHD project involve the accretion of magnetized gas onto one or more supermassive black holes, and to explore associated relativistic phenomena such as jets in active galactic nuclei.

Current estimates suggest that somewhere in the universe a few pairs of supermassive black holes merge every year, leaving behind a still more massive single black hole at the centers of the galaxies where this occurs. The energy release is huge, and the consequences for galactic evolution are very deep (strong correlations between galactic structure and central black hole mass indicate tig

ht feedback between black hole and galaxy growth), but no such event has ever been identified because the overwhelming majority

of the energy is given to gravitational waves, which are as yet undetectable. Nonetheless, simple estimates suggest that there should be enough gas close enough to the merging black holes that, particularly as the two approach one another, there should be photon signals of the impending event.

We are interested in performing calculations that can predict key features of the emitted radiation. Because the interaction between surrounding gas and a pair of black holes bound to one another is extremely complex and involves strongly nonlinear physics---nonlinear MHD turbulence, shock waves, dynamical spacetimes---numerical methods are the only way forward. Our principal simulation code is called HARM3d. It is a finite-volume/finite-difference code written in C by members of the group, for solving the magnetohydrodynamics (MHD) equations of motion in curved spacetime---i.e. in the context of Einstein's theory of general relativity.

**Contributors:** Manuela Campanelli, Scott Noble,Yosef Zlochower, Dennis Bowen and Brennan Ireland.

**Projects and Collaborations:**

- Collaborative Research: "Computing Supermassive Black Hole Mergers in Astrophysics" - Collaborative project among RIT and Johns Hopkins University funded by NSF CDI-Type II awards AST-1028087/1028111.
- The CCRG numerical relativity group is collaborating with the Blue Waters Team at the National Center for Supercomputing Applications (NCSA) through a series of NSF Petascale Resource Allocation (PRAC) awars OCI-0832606 and OCI-1516125. Blue Waters is one of the most powerful supercomputers in the world with a peak performance of 10 petaflops (10 quadrillion calculations every second). It can achieve a sustained performance of 1 petaflop running a range of science and engineering codes.

**COMPACT BINARIES MERGERS AND GAMMA RAY BURSTS**

Combined general relativistic and magneto-hydrodynamics (GRMHD) simulations are also used to model black hole - neutron star and neutron star - neutron star binaries. These astrophysical sources are believed to be the origin of gamma ray bursts (unexplained blasts of intense electromagnetic radiation).

**Contributors:** Manuela Campanelli, Joshua Faber, Yosef Zlochower, Zach Silberman.

**Projects and ****Collaborations:**

- The GRMHD group participates in the Einstein Toolkit Consortium to develop and support open software for relativistic astrophysics to take advantage of emerging petascale computers and advanced cyber infrastructure. This consortium is funded by NSF PHY 0903973/0903782/0904015 (CIGR), NSF PHY 0653303 (XiRel). It also received support from NASA 08-ATFP08-0093.

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