Using Casper DSP Technology to Map the Black Hole in the Galactic Center

Mark Wagner1,3, Mel Wright 1, Henry Chen6, Matt Dexter1, Shep Doeleman5, Lincoln Greenhill6, Vinayak Nagpal1, Rurik Primiani4, Jonathan Weintroub4, Dan Werthimer1,3

1:Berkeley Wireless Research Center 2:Department of Physics and Astronomy, 3:Space Sciences Lab 4:Harvard University 5:Massachusetts Institute of Technology 6:University of California Los Angeles

Black Holes, How Do They Work?Black Holes are one of the most extreme physical objects known to occur in our universe. Pre-dicted by Einstein’s General Theory of Relativity, they occur when an objects own force of gravityexceeds all other forces of internal pressure, resulting in a singularity. Black holes are believedto reside at the center of most galaxies, including our own Sagitarius A* (SgrA*). Observingand imaging a black hole would allow us to study it’s spin, the physics of the accretion disc, andGeneral Relativity in the strong field regime. Using CASPER technology with the Very LongBaseline Interferometer (VLBI) Event Horizon Telescope (EHT) we have been able to probe thegalactic center, and within a few years be able to image SgrA* with a resolution close to the eventhorizon.

SgrA* Evidence for a Black HoleThere is a great deal of evidence suggesting SgrA* is a 4x106 solar mass black hole at the galacticcenter. This includes mass estimates from orbitting stars (Gillessen et.al 2008, Ghez. et el. 2008),proper motion limits on SgrA* itself (Backer & Sramek 1999, Reid & Brunthaler 2004), and sizelimits on SgrA* from VLBI (Bower et al. 2004, Shen et al. 2005, Doeleman et al. 2008). The

results from these observations imply a mass den-sity for SgrA* greater than 9x1022 solar massesper cubic parsec. Close observational correlationbetween the mass of our galaxy and it’s veloc-ity dispersion suggest strong connection betweenformation of the black hole and the galaxy itself.The best evidence thus far is from studying theproper motion of stars near SgrA* (seen left). Us-ing VLBI for the EHT will increase our under-standing and add to the mounting evidence for aSuper Massive Black Hole at the galactic center.

Event Horizon TelescopeThe Event Horizon Telescope (EHT) is a network of (sub)mm wavelength antennas that are linkedtogether to function as a Global VLBI array. At observing wavelengths of 1.3mm and 0.8mm,the angular resolution of the EHT approaches 20 micro arcseconds. For SgrA*, the 4 millionsolar mass black hole at the Galactic Center, the EHT array has already resolved structures onthe scale of the Event Horizon. Similar scale structures have also been detected at the base of therelativistic jet in M87, a giantelliptical galaxy harboring a blackhole that is 2000 times more mas-sive than that powering SgrA*. Cur-rently IBOB and BEE2 boards andCASPER firmware are used to phaseup and process the signals from theantennas, then pass the data to theMark VB data recorder. Closure phase(O = φ1+φ2−φ3) is used for calli-bration as phase errors are subtractedout. In 2007 SgrA* was observedwith ARO/SMT, CARMA, and theJCMT, which allowed an estimateto be made on the Schwarzschild ra-dius 0.37AU .

CASPER Libraries and Hardware

CASPER FPGA based hardware is now used in phasing up the 8 SMA antennas with 1GHzBandwidth and coherently storing the data. The CASPER technology allows for the phased ar-ray technology that will improve sensitivty and allow for sublmm observations where there isminimal dispersion by the Inter-Stellar Medium which goes as λ2. The next generation architec-ture of the proposed broadband phased array processor upgrade, based on the ROACH Virtex-5,is targeted for deployment later this year. This will allow double the bandwidth and increasedsensitivity.

What Would We See?VLBI simulations of SgrA* have given us a good idea of what to expect from imaging. The left-most images (below) are models that have been scatter broadened by the Inter Stellar Medium.The middle panels show images reconstructed using a 7-station λ = 0.8mm wavelength arraythat could reasonably scheduled within the next 3-5 years. The right panel shows images

reconstructed using 13-station ar-ray that could be assembled withinthis decade. Below on the Left,a symmetric emitting surface sur-rounding a black hole is gravita-tionally lensed to appear larger thanits true diameter. The solid blackline shows the apparent diameterwith lensing by a black hole, andthe dashed line with no lensing. Theintrinsic size of SgrA* observed with1.3mm VLBI (horizontal red line)is smaller than the minimum ap-parent size of the black hole event

horizon suggesting that the submmemission of SgrA* must be offsetfrom the black hole position. Thiscan be understood in the contextof General relativistic MHD accre-tion disk simulations (right), whichexhibit compact regions of emis-sions due to Doppler enhancementof the approaching side of an ac-cretion disk.

Improving Sensitivity

By enhancing EHT sensitivity through work on receivers, phased arrays, wideband VLBI back-ends, and high-speed VLBI recorders, these time-variable changes can be resolved and monitoredon 10-second time scales. Using simulations, which model flares in SgrA* as orbiting hot-spots inthe accretion flow, EHT can potentially time these orbits and provide new estimates of the blackhole spin. A planned upgrade to SMA and CARMA this year will allow us to form beams atthese locations, improving the image and sensitivity from the 2007 observations. There has alsobeen significant progress made at the Atacama Large Millimeter/submillimeter Array (ALMA)in Chile, which will be phased up and contributing to the EHT within a few years.

CARMAThe Combined Array for Research in Millimeter-wave Astronomy (CARMA) was created bymoving the six 10-meter telescopes from Caltech’s Owens Valley Radio Observatory and nine6-meter telescopes from the Berkeley-Illinois-Maryland Association array to a new location atCedar Flat in the Inyo Mountains near Bishop, CA.

SMAThe Sub-Millimeter Array (SMA) consists of eight 6-meter antennas capable of operating atwavelengths as short as 0.3 mm on Mauna Kea, Hawaii.