Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: GRay: a Massively Parallel GPU-Based Code for Ray Tracing in
  Relativistic Spacetimes   We introduce GRay, a massively parallel integrator designed to trace the
trajectories of billions of photons in a curved spacetime. This GPU-based
integrator employs the stream processing paradigm, is implemented in CUDA
C/C++, and runs on nVidia graphics cards. The peak performance of GRay using
single precision floating-point arithmetic on a single GPU exceeds 300 GFLOP
(or 1 nanosecond per photon per time step). For a realistic problem, where the
peak performance cannot be reached, GRay is two orders of magnitude faster than
existing CPU-based ray tracing codes. This performance enhancement allows more
effective searches of large parameter spaces when comparing theoretical
predictions of images, spectra, and lightcurves from the vicinities of compact
objects to observations. GRay can also perform on-the-fly ray tracing within
general relativistic magnetohydrodynamic algorithms that simulate accretion
flows around compact objects. Making use of this algorithm, we calculate the
properties of the shadows of Kerr black holes and the photon rings that
surround them. We also provide accurate fitting formulae of their dependencies
on black hole spin and observer inclination, which can be used to interpret
upcoming observations of the black holes at the center of the Milky Way, as
well as M87, with the Event Horizon Telescope.

37
The categories of this paper are: supr-con
18
The categories of this paper are: econ
5
The categories of this paper are: gr-qc
11
The categories of this paper are: hep-lat
3
The categories of this paper are: physics
26
The categories of this paper are: bayes-an
32
The categories of this paper are: atom-ph
9
The categories of this paper are: nlin
12
The categories of this paper are: nucl-th
23
The categories of this paper are: solv-int
1
The categories of this paper are: cs
7
The categories of this paper are: hep-th
17
The categories of this paper are: q-fin
22
The categories of this paper are: chao-dyn
0
The categories of this paper are: hep-ph
29
The categories of this paper are: funct-an