Pulsar Science with GUPPI

Paul Demorest, NRAO-CV

GUPPI Team: Patrick Brandt, PD, Ron Duplain, John Ford, Randy McCullough,

Scott Ransom, Jason Ray

Pulsar Features

● Neutron star● Only ~10 km across● Mass ~1.4x solar● B-field ~108-12 gauss (~billion x Earth's)● Spin periods >1.5 ms● Broadband radio beam sweeps by Earth

“lighthouse-style”.

Pulsars act as extremely precise astronomical clocks:

Spin period of PSR B1937+21 atMidnight, December 5, 1998:

P = 1.5578064688197945 ms +/- 0.0000000000000004 ms !

~1.5 ms

Pulsars make unique “laboratories” for probing extreme physics.

Timing double-NS binaries allows unique tests of gravity (general relativity).

PSR B1913+16: 1993 Nobel Prize in Physics for Hulse and Taylor! Proves gravitiational waves exist..

Double Pulsar J0737-3039: GBT/GASP timing confirms GR to 0.05% (Kramer et al 2006).

PSR timing can provide precise NS mass measurements.

This constrains physical theories of matter at super-nuclear densities.

(Lattimer & Prakash, 2007)

A pulsar timing array may be able to detect nHz-freq grav. waves within 5 years. (NANOGrav project; http://www.nanograv.org)

This requires timing precision of 10-100 ns over 5-10 year scale!

Pulsar Timing Array GW complementarity:

● Pulsars have unique and demanding observational requirements:● Broad-band signal (high BW = more S/N)● High time resolution (~1 us)● High dynamic range (many ADC bits)● Highly polarized signal (full Stokes)● Interstellar medium disperses the pulses.

Interstellar Dispersion

Interstellar Dispersion

Pulsar Backends

● “Search” backends:● Fast-dump spectrometers● Time resolution 50 to 100 μs● Frequency resolution 25 kHz to 1 MHz● Often traded data quality for more BW

● “Timing” backends:● Pulse period folding● Coherent dedispersion● High time resolution, ~1 μs● Data quality (# bits, polns, etc) more important

Coherent Dedispersion● Removes ISM dispersion (for

known DM) within a freq channel.

● FIR filter applied pre-detection via FFT convolution; filter length ~k to ~M-point.

● Most previous systems are software based and handle ~100 MHz total BW.

GASP system @ GB

Coherent vs Incoherent

Low-frequency observing

GUPPI

● GUPPI = Green Bank Ultimate Pulsar Processing Instrument

● CASPER (FPGA) HW plus 9-node GPU cluster.● Incorporates best features of 5 previous

backends at GB.● Both search and timing/coherent modes.● 100, 200, or 800 MHz total BW● 8-bit ADCs, full-Stokes, flexible parameters (#

channels, integration time, etc).

GUPPI Timeline

● “Pulsar dream machine” first suggested at Fall 2006 GB instrumentation workshop.● At same time, CASPER at UCB is ramping up.

● Initial work in U. Cinn. student project.● Serious development at NRAO starts Fall 2007.● GUPPI1 in operation late 2008.● GPU cluster purchase/development Fall 2009.● GUPPI2 first light December 2009.

GUPPI Status

● GUPPI-1 (Search mode)● In regular operation for >1 year.● Fully integrated with GBT M&C systems!● Already many new PSR discoveries.● Produces < 200 MB/s output data rate.

● GUPPI-2 (Coherent dedispersion)● Heavy development effort Fall 2009.● Reuses many parts of GUPPI-1 design.● Achieved first light December 2009!● Typically 10's of GB per few-hour obs session.

GUPPI architecture:~1 MHz PFB in FPGAsCoherent dedisp in GPUs

XAUI

10 Geswitch;24 Gb/s

IBOB

BEE2

“beef”

GPUs

GUPPI GPU Processing

● NVIDIA GTX 285, programmed w/ CUDA● Each card can dedisperse (FFT-X-IFFT) and

fold >100 MHz BW in real time.● Performance vs DM:

● Max DM ~ 3000 at L-band!

Chirp length (samples)

GUPPI-2 First Light!

December 28th, 100 MHz mode :

GUPPI-2 First Light!

December 30th, 800 MHz mode:

GUPPI-2 First Light!

Cleaned-up 800 MHz mode:

Improved RFI resistance:

(3-level, ACF) (8-bit, PFB)

>10x improvement in BW vs older coherent systems!

Fully utilizes all GBT low-freq receivers (“ultimate”).

Improved S/N ratio, also reduces scintillation variability.

PSR J1713+0747 plot: S. Ransom

GUPPI Science: 350 MHz GBNCC Pulsar Survey

(plot: Anthony Ford)

GUPPI science results: Newly discovered MSPs!

GUPPI science results: High-precision timing / GWs

Expected GW improvement vs time:

ASP

GUPPI

GUPPI science results: Mass of PSR J1614-2230(PD, T. Pennucci, S. Ransom)

Pulsar in 9-day binary system.

Shapiro delay: GR-induced (“warped space-time”) delay as pulses pass by companion star.

Directly measures companion mass.

GUPPI science results: Mass of PSR J1614-2230

~5 years of timing with Spigot, BCPM, GUPPI-1...

... 1 week of timing with GUPPI-2! Excellent Shapiro delay detection...

Orbital inclination = 89.17(2) deg!

Companion mass = 0.500(6) solar!

Pulsar mass = 1.97(4) solar!

(Demorest et al., Nature, 2010)