Fast Transients: A Tutorial. Dale A. Frail NRAO

Transcription

1 Fast Transients: A Tutorial Dale A. Frail NRAO

2 Tutorial Outline 1. Fast transients What are they and why are they interesting now? 2. Optimal Search Strategies The problem and some recent progress 3. Fast transient experiments Ongoing and future

3 Why now? V. L. Ginzburg

4 Time Domain as a Discovery Axis. Why?

5 Time Domain as a Discovery Axis. Why? Sensitivity ALMA EVLA VLBA MerKATT, Apertif, ASKAP, LOFAR Spectral Range Field-of-View

6 LOFAR WSRT-Apertif ASKAP LWA MeerKAT

7 Fast Transients: A pragmatic definition Intense, short duration (ct<1 s) activity from a compact object Pulse profile affected by plasma propagation effects, including dispersive smearing and interstellar scattering Fast transients can be either cataclysmic or repeatable Probe extremes of gravity, magnetism and/or states of matter Requires a non-thermal, i.e. coherent emission mechanism There is no theory that can predict a priori what the specific intensity will be for a pulsed, coherent source

8 Radio Phase Space

9 Plasma Propagation I: Dispersion Lorimer and Kramer (2005) Cold plasma dispersive delay, i.e. radio signals at lower frequencies arriving later than higher ones Without correction, pulse is temporally smeared DM (Galactic pole)=30 pc cm -3 or electron column N e =10 20 cm -2 Negative: Computational overhead. Must search for unknown DM in data reduction Positive: Excellent probe of ionized gas (i.e. IGM) 9

10 Plasma Propagation II: Scattering Multi-path scattering results in a broadening of pulsed signal Fluence is conserved A frequency wall below which you cannot see pulsations under some given period P > 5τ scat Single pulses have an advantage over period pulses Deal with ISS through appropriate observing strategy. Lorimer and Kramer (2005) 10

11 Classes of fast transients: neutron stars Repeating, galactic sources with coherent emission processes Pulsars, nulling pulsars, magnetars, rotating radio transients (RRATs) and Crab giant pulses all originate from magnetized, rotating neutron stars Pulsar Nulling Pulsar RRAT Burke-Spolar astro-ph/ RRAT

12 Classes of fast transients: RRATs Identified as single, dispersed pulses with durations 2-30 msec Repeat stochastically every few minutes to a ~hours at same DM Periods from 0.4 s to 7 s Concentrated in the galactic plane Some evidence that RRATs bridge an evolutionary gap between normal pulsars and magnetars in the P-Pdot diagram McLaughlin et al. (2006)

13 Classes of fast transients: RRATs Identified as single, dispersed pulses with durations 2-30 msec Repeat stochastically every few minutes to a ~hours at same DM Periods from 0.4 s to 7 s Concentrated in the galactic plane Some evidence that RRATs bridge an evolutionary gap between normal pulsars and magnetars in the P-Pdot diagram Karako-Argaman et al. astro-ph/

14 Classes of fast transients: Flare Stars The Sun, magnetically active stars and brown dwarfs all undergo dramatic flaring behavior Burst durations last typically 10s to 10 s min Relatively rare or understudied Typically nearby, d<30 pc Strongly circularly polarized Osten and Bastian (2006) 60 s AD Leonis Spangler & Moffett (1976) 14

15 Known, Unknowns See notes from Siemens text Also mention ultrafast transients

16 Radio Transient Phase Space: Simplified Galactic Extragalactic Crab giant pulses Normal and msec pulsars Nulling PSRs RRATs Rotating magnetars (extrasolar planets) Tidal disruption events (TDEs) Long duration GRBs Relativistic supernovae (SNe) Short duration GRBs Type II SNe Type I b/c SNe Novae and CVs Soft gamma-ray repeaters HMXBs and LMXBs GC transients Flare stars Brown Dwarfs Short duration Long duration

17 Radio Transient Phase Space: Simplified Repeated Catastrophic Crab giant pulses Normal and msec pulsars Nulling PSRs RRATs Rotating magnetars (extrasolar planets) Tidal disruption events (TDEs) Long duration GRBs Relativistic supernovae (SNe) Short duration GRBs Type II SNe Type I b/c SNe Novae and CVs Soft gamma-ray repeaters HMXBs and LMXBs GC transients Flare stars Brown Dwarfs Coherent Incoherent

18 The Lorimer Transient An intense (30 Jy), short (5 msec), dispersed signal Seen in 3 of 13 Parkes beams Dispersion measure (375 pc cm -3 ) in excess of galactic + SMC value Pulse appears scatter broadened Implied extragalactic (d~1 Gpc) Second, less convincing, event. DM=745 pc cm -3 (Keane et al 2011) Four new candidates from Parkes HTRU survey (see J3-2) Lorimer et al. (2007)

19 Shadows of Doubt: Perytons Some doubt cast on veracity of Lorimer transient. <V/Vmax>. Where are the fainter events in the Parkes data? 20+ similar events detected Follow cold plasma dispersion law but with important deviations Seen typically in all 13 beams Some hints that they may be some type of atmospheric phenomena (but see Bachi et al. 2012) Burke-Spolaor et al. (2012)

20 Optimal Search Strategies Optimize search for single, dispersed pulses in time domain Search strategy is not the same as that for PSRs Volume of sky surveyed is more important than area One part of the sky is as good as any other Need large instantaneous sensitivity and big field of view Time on the sky is important è commensal Single parabolic dishes detect sources only one way. Interferometers can be deployed to detect sources in several different ways

21 Optimal Search Strategies Dish signal (1..N 0 dishes) Fly's eye Signal to be searched De-dispersion processing AA-low elemental antenna signal Incoherent combination Event detection Storage Off-line processing Station beamforming (1..N 0 stations) Coherent combination Positive result Buffer dump Station beam signal Digitised voltages Store in rolling buffer Colegate and Clarke (2011)

22 Optimal Search Strategies: Survey Speed 2 "A % FoM = Δν $$ eff '' Ω # Tsys & Fender and Bell (2012)

23 Optimal Search Strategies Macquart (2011) Survey speed (SS) is not equivalent to detection rate Survey speed can be sensitive to how the elements of an interferometer are deployed e.g. Fly s Eye and incoherent sum have same SS but later has higher detection rate (by N 1/4 ) Colegate and Clarke (2011) Volumetric detection rate ignores process compute costs Suggest event rate per beam searched as a cost proxy Incoherent strategy beats Fly s Eye, coherent and subarrays in most cases Still forming one or more beams. Not using full power of imaging interferometer. Too expensive.

24 Optimal Search Strategies: Bispectrum New method for fast transient searching (see J3-7) Uses bispectrum (closure visibilities) and closure phases Computationally efficient ( x faster than other methods) Insensitive to position in FoV and calibration Allows real-time searches over the full FoV and BW. Buffered visibility data can be processed to make arcsec image Law and Bower (2012) 24

25 Proof of concept: VLA as a RRAT Trap Law et al. (2012) 16 min of VLA data dumped at 10 msec rates (90 MB s -1 ) RRAT J detected at 13-sigma with correct DM Localization much improved over discovery. Consistent with 3-yr timing solution

26 Radio Transient Phase Space: Simplified Galactic Extragalactic Crab giant pulses Normal and msec pulsars Nulling PSRs RRATs Rotating magnetars Tidal disruption events (TDEs) Long duration GRBs Relativistic supernovae (SNe) Short duration GRBs Type II SNe Type I b/c SNe Novae and CVs Soft gamma-ray repeaters HMXBs and LMXBs GC transients Flare stars Brown Dwarfs Short duration Long duration

27 Current and future fast transient experiments Telescope Experiment Frequency Method Talk Parkes MPS (e.g. HTRU) 1.4 GHz Dish + mulf- feed J3-2 Arecibo PALFA, Astro- pulse 1.4 GHz Dish + mulf- feed ATA- 42 Fly s Eye 1.4 GHz Dish array, fly s eye VLBA V- FASTR 1-90 GHz Dish array, incoherent J3-5 VLA RRAT Trap+, LOBO 1.4 GHz Dish array, bispectrum J3-7,J1-1 WSRT 8gr8 + PuMA 1.4 GHz Dish array, Fed array LWA1 LWA- TV, LEDA- CATS MHz Aperture array, imaging J3-11 LOFAR RSM, AARTFAAC- ASM, ARTEMIS, DRAGNET MHz Aperture array, various MWA RTS MHz Aperture array, imaging ASKAP CRAFT 1.4 GHz Dish array + PAF, various J1-4 AperFf ALERT, ART 1.4 GHz Dish array + PAF, various MeerKAT TRAPUM 1.4 GHz LNSD dish array

28 Current and future fast transient experiments Telescope Experiment Frequency Method Parkes MPS (e.g. HTRU) 1.4 GHz Dish + mulf- feed J3-2 Arecibo PALFA, Astro- pulse 1.4 GHz Dish + mulf- feed ATA- 42 Fly s Eye 1.4 GHz Dish array, fly s eye VLBA V- FASTR 1-90 GHz Dish array, incoherent J3-5 VLA RRAT Trap+, LOBO 1.4 GHz Dish array, bispectrum J3-7,J1-1 WSRT 8gr8 + PuMA 1.4 GHz Dish array, Fed array LWA1 LWA- TV, LEDA- CATS MHz Aperture array, imaging J3-11 LOFAR RSM, AARTFAAC- ASM, ARTEMIS, DRAGNET MHz Aperture array, various MWA RTS MHz Aperture array, imaging ASKAP CRAFT 1.4 GHz Dish array + PAF, various J1-4 AperFf ALERT, ART 1.4 GHz Dish array + PAF, various MeerKAT TRAPUM 1.4 GHz LNSD dish array

29 Single parabolic dish with multi-feed 13-beams at 1.4 GHz Original backend had analog filterbanks with 96-3 MHz channels, 250 us sample time New digital backend, improves sensitivity to msec pulses Science bonanza Nearly 1000 PSRs found Discovery of RRATs Discovery of Lorimer events Improved algorithms. Site and instrument are well-specified Parkes 64-m

30 Array of dishes: Fly s Eye Experiment 42 6-m dishes used at 1.4 GHz Each dish points in a separate part of the sky. FoV= 150 deg 2 Special purpose digital spectrometer built 128 channels, 200 MHz BW 450 hrs of data, 17 Tb Single pulse search carried out for each pointing Siemion et al. (2012) Allen Telescope Array

31 No Lorimer-like events or perytons BurstA Single Detection 10 6 Bright Extragalactic Radio Pulse #2 Upper Limit 10 5 Deneva et al 2010 (primary beam) 10 4 Burke-Spolaor et al 2010 Lorimer et al 2007 Event Rate (sky day ) Core Collapse Supernovae (per Gpc 3 ) Gamma Ray Bursts (per Gpc 3 ) Keane et al 2011 Siemion et al 2011 (This Work) 10 0 Binary Neutron Star Inspirals (per Gpc 3 ) 10 <2 events Sky -1 day -1 (>44 Jy) Siemion et al 2011 (off axis) Deneva et al 2010 (off axis) Pulse Energy Density (kjy s) Siemion et al (2012)

32 Array of dishes: Incoherent sum 10 25m dishes being used commensally at 0.3 to 90 GHz. 25% of the time at 1.4 GHz. Small FoV but 1000 s hrs of on-sky time ACFs stream through the DiFX correlator, time aligned, summed and search for dispersed pulses VLBI resolution images can be made of any detections VLBA

33 No Lorimer-like events or perytons 318 hrs at 1.4 GHz <2 events Sky -1 day -1 (>44 Jy) Wayth et al (2012) Thompson et al. (2011)

34 Aperture array: Tied array beam Stand-alone LOFAR backend to be used for single stations or tied arrays 48 MHz of beam-formed data at 5 usec rates. Can trade beams for BW CPU+ GPU combination to do DM searches on time series. Prototype for DRAGNET (PI: Hessels) will allow up to 100 tied-array beams ARTEMIS (PI: Karastergiou)

35 Low Band Sky Monitor (aka LOBO) A low frequency observing system that runs 24/7 in parallel with the normal VLA observing program Provides sensitive, widefield data from 60 MHz to 480 MHz (see J1-1) Preliminary design and costing being done with NRAO and NRL Backends Imaging pipeline PSR and Transient detection system EVLA 1024MHz IF0 IF1 ISO ISO ISO RFI Enclosure (modified DTS chassis) IF Amps. Digitize 500 MHz BW at each antenna Uses ROACH2 board Uses new 8 bit samplers from MerrKAT Send down 10 Gb fiber Software correlator ROACH2 katadc ZDOK 1Gsps LVDS Buffer PowerPC + MIB Xilinx Virtex6 SX475T PPC network converter (fiber-tocopper) 10gbE ROACH SFP+ 48V PC Power Supply Steve Durand (2012) Fiber #1 Fiber #2 8Gbps IF0 8Gbps IF1 Compute Node 10GbE NIC #1 10GbE NIC #2 EMI EVLA LVDS 20Hz EVLA M/C (MM fiber) EVLA -48V 35

36 Current and future fast transient experiments Telescope Experiment Frequency Method Talk Parkes MPS (e.g. HTRU) 1.4 GHz Dish + mulf- feed J3-2 Arecibo PALFA, Astro- pulse 1.4 GHz Dish + mulf- feed ATA- 42 Fly s Eye 1.4 GHz Dish array, fly s eye VLBA V- FASTR 1-90 GHz Dish array, incoherent J3-5 VLA RRAT Trap+, LOBO 1.4 GHz Dish array, bispectrum J3-7,J1-1 WSRT 8gr8 + PuMA 1.4 GHz Dish array, Fed array LWA1 LWA- TV, LEDA- CATS MHz Aperture array, imaging J3-11 LOFAR RSM, AARTFAAC- ASM, ARTEMIS, DRAGNET MHz Aperture array, various MWA RTS MHz Aperture array, imaging ASKAP CRAFT 1.4 GHz Dish array + PAF, various J1-4 AperFf ALERT, ART 1.4 GHz Dish array + PAF, various MeerKAT TRAPUM 1.4 GHz LNSD dish array