PULSAR SURVEYS (AO & GBT) • Why? • How deep can we go? (D max , V max ) • Example surveys • Hardware • Funding
Feb 05, 2016
PULSAR SURVEYS(AO & GBT)
• Why?
• How deep can we go? (Dmax, Vmax)
• Example surveys
• Hardware
• Funding
Why more pulsars?
• Extreme Pulsars:
• P < 1 ms P > 5 sec
• Porb < hours B > 1013 G
• V > 1000 km s-1
• Population & Stellar Evolution Issues
• Physics payoff (GR, LIGO, GRBs…)
• Serendipity (strange stars, transient sources)
• New instruments (AO, GBT, SKA) can dramatically increase the volume searched (galactic & extragalactic)
Simulated DM vs l histogram (50k pulsars)
How Low Can We Go?Dmax = D (S / Smin1 )1/2 Nh
1/4
Smin1 = single harmonic threshold = m Ssys /( T)1/2
m = no. of sigma Nh = no. of harmonics that maximize harmonic sum
Nh 0 for heavily broadened pulses
Regimes:
Luminosity limited Dmax Smin1 -1/2
DM/SM limited Dmax Smin1 -x , x<1/2
Finding Dmax
• beam luminosity
• beam widths (core,cone)
• orientation angles (• pulse shape at nominal distance (1 kpc)
Dmax = Dnom [H(Nh)/Smin1]1/2
H(Nh) =
max Nh-1/2 Wi WORB WDM WSM WTC WHPF
[ Dmax = Dmax (DM, SM) iterate ]
SEARCH VOLUME:
VS = S D3max
DETECTION VOLUME:
Vd = S 0Dmax dD D2np/np(sun)
Regimes for Dmax
Luminosity limited: (r -2 law)
Dispersion limited: t ch DM / 3
Scattering limited: t SM5/6 / 4.4
Time constant limited: tTC
tTC(min) = (ch)-1
Dmax example
Dmax vs Lp
Dmax vs. P (0.43 GHz)
Dmax vs. P (larger Lp)
Dmax vs. P (1.4 GHz)
Dmax for B1933+16 (L band)
Implications
• After maximizing T (RFI,TAC constraints), the control parameters for Dmax are l,b,,Nch
optimal directions to search (modulo RF and where pulsars are)
• Coherent dedispersion for searches?(not worth it if scattering limited… better to put processing power into binary searches)
AO, GBT, Parkes
Compare AO,GBT & Parkes(Lband)
Ssys Nch T Smin1 d/dT
(Jy) (MHz) (s) (Jy) (hr/deg2)
AO 3.6 400 1024 300 73 42/Nb
GBT 16 400 1024 900 190 4.5/Nb
Parkes 36 288 96 2100 360 1 (Nb=13)
Compare AO,GBT & Parkes(Lband)
DMc Dmax for Lp=10 mJy kpc2 ,l=30,b=5
ms 33 ms 89 ms
AO 27 3 kpc 8 kpc 8 kpc
GBT 54 2.8 5 5
Parkes 28 1.3 4 4
Strawman AO Surveys
L band7 beams400 MHz/512 channels/beam (multi WAPP)
300 s/beam 6 hr/deg2
3000 hr 500 deg2
Search volume 3 to 20 x Parkes MB (l,b,P dependent)
S band? Advantage for very fast,weak pulsars& flat spectrum pulsars at low b
AO at S,L,P bands
OPTIMAL DIRECTIONS
• AO advantage: collecting area smaller channel bandwidths
• choose directions where Parkes MB is luminosity or DM limited.(SM limited less advantage per decrease in Smin1)
• e.g. along spiral arm tangents Cygnus region | b | > few degrees (period dependent)
Shopping List
• Multibeam system (Feeds/Rx) e.g. 7 @ L
• Digital backends (multi WAPP)
• Data storage
• Processing
• Followup
• $$$ for all of the above
Ideas
• Multibeam systems: e.g. Rick Fisher’s focal plane sampling + beamforming system
• Digital backends: AO: WAPP x 4 x NbeamsGBT: GBT correlator + fast dump
• Storage/processing: Moore’s law
• Followup: dedicated timing telescopes (85ft, 1HT, 100ft @ AO?)
• $$$: NSF MRI consortium proposal, private funding?