Radio Searches of GW Counterparts Current and future capabilities Dale A. Frail National Radio Astronomy Observatory

Talk outline. What is the expected strength of the radio signal? – Afterglow component. Early and Late. (robust) – Prompt counterpart (speculative). How do we detect the radio signal of a GW trigger? – The quiescent and transient radio sky. A primer. – Current and future radio facilities. – Three search strategies (in order of probability of success) What follow-up would we want to do? What can we be doing today to help the field? 2

3 ν m ≈Γ 4 (i.e radio AG traces trans-relativistic ejecta)

Afterglow Radio Signal – Robust Early radio emission (~days, weeks) – SHB have lower average redshifts, energy and circumburst densities compared to long duration GRBs – Only two SHB detected in radio out of ~25 Swift events. GRB (z=0.257) and GRB (z=0.546) – Best estimate is =100 μ Jy and =0.5 – Predicts 10’s mJy at 1-10 GHz for d=200 Mpc

Afterglow Radio Signal – Robust Van Eerten et al. (2010). Early, on-axis Late, off-axis L-GRB Late-time radio detects AG independent of beaming

Afterglow Radio Signal – Robust Early radio emission (~days, weeks) – SHB have lower average redshifts, energy and circumburst densities compared to long duration GRBs – Only two SHB detected in radio out of ~25 Swift events. GRB (z=0.257) and GRB (z=0.546) – Best estimate is =100 μ Jy and =0.5 – Predicts 10’s mJy at 1-10 GHz for d=200 Mpc Late-time radio emission (~months) – Outflow expands, becomes quasi-isotropic and non-relativistic. A late-time radio turn on independent of original jet direction. – For reasonable SHB parameters t=30 days, F=0.3 mJy at 1.4 GHz at 300 Mpc (Nakar et al. in prep)

Prompt Radio Signal – Speculative Gravitationally excited MHD waves (Postnov & Pshirkov 2009) – Predicts 12.5 Kilo-Jy at 100 MHz for d=200 Mpc Rotational energy of post-merger object ( Moortgat & Kuijpers 2004) – Predicts 50 Mega-Jy at 30 MHz for d=200 Mpc Emission from PSR-like magnetosphere (Hansen & Lyutikov 2001) – Predicts 1 milli-Jy at 400 MHz for d=200 Mpc “Back of the envelope” approach – Radio emission is seen in all high energy processes where there are relativistic particles and magnetic fields – Assume that of energy of a SHB goes into a prompt radio signal – Average fluence for SHB is erg cm -2. Duration 0.1 s – Predicts 1 kilo-Jy at 1 GHz

Quiescent and Transient Radio Sky. Primer. Isotropic source distribution on sky – Above 1 mJy source populations are AGN dominated – Below 1 mJy star-formation galaxies start to emerge The transient radio sky is quiet – GHz flux density range 0.1 mJy to 10 Jy is well studied by several (heterogeneous surveys) – Transients are to of quiescent population e.g. Levinson et al. NVSS/FIRST comparison Ofek et al. survey Important implication is that radio false EM-GW detection rate will be small (<0.1 deg 2 at 1 mJy) … and any background events are likely to be AGN, and hence easily filtered out.

The Quiescent Radio Sky is Isotropic 9 J. Condon

Quiescent and Transient Radio Sky Isotropic source distribution on sky – Above 1 mJy source populations are AGN dominated – Below 1 mJy star-formation galaxies start to emerge The transient radio sky is quiet – GHz flux density range 0.1 mJy to 10 Jy is well studied by several (heterogeneous surveys) – Transients are to of quiescent population e.g. Levinson et al. NVSS/FIRST comparison Ofek et al. survey Important implication is that radio false EM-GW detection rate will be small (<0.1 deg 2 at 1 mJy) … and any background events are likely to be AGN, and hence easily filtered out.

The Transient Radio Sky is Quiet 11 Ofek et al. (2011)

Quiescent and Transient Radio Sky Isotropic source distribution on sky – Above 1 mJy source populations are AGN dominated – Below 1 mJy star-formation galaxies start to emerge The transient radio sky is quiet – GHz flux density range 0.1 mJy to 10 Jy is well studied by several (heterogeneous surveys) – Transients are to of quiescent population e.g. Levinson et al. NVSS/FIRST comparison Ofek et al. survey Important implication is that radio false EM-GW detection rate will be small (<0.1 deg 2 at 1 mJy) … and any background events are likely to be AGN, and hence easily filtered out.

Radio facilities for GW-EM Counterpart Searches: 2011 and Beyond 13 EVLA WSRT/ Apertif LOFAR ASKAP MWA MeerKAT

Radio facilities for GW-EM Counterpart Searches Radio Facility Observing Freq. Field of View 1 hr rms BeamStart Date ASKAP1.4 GHz30 deg 2 30 uJy20”2013 Apertif1.4 GHz8 deg 2 50 uJy15”2013 MeerKA T 1.4 GHz1.5 deg 2 35 uJy15”2013 EVLA1.4 GHz0.25 deg 2 7 uJy1.3-45”2010 EVLA327 MHz5 deg 2 2 mJy5-18”2011 LOFAR MHz50 deg 2 1 mJy5”2011 EVLA74 MHz100 deg 2 50 mJy25-80”2011 MWA MHz1000 deg 2 8 mJy300”2011+ LOFAR15-80 MHz500 deg 2 8 mJy120” (Only Apertif, EVLA, LOFAR has demonstrated noise perfprmance)

Radio facilities for GW-EM Counterpart Searches: ASCAP Australian-lead effort m antennas Operates at 1.4 GHz Focal-plane array technology to give 30 deg 2 FoV 1-hrs, rms~30 uJy (claimed) 75% of the time given to Key Science Projects (25% open) – Continuum sky survey 40X deeper than NVSS – Slow and fast transient searches 2013 delivery (optimistic) 15

Radio facilities for GW-EM Counterpart Searches: Apertif Dutch effort Upgrade of WSRT using FPAs m antennas Demonstrated peformance Operates at 1.4 GHz 8 deg 2 FoV 1-hrs, rms~50 uJy 75% of the time will be given to Key Science Projects (25% open) – Proposals in April operation 16

Radio facilities for GW-EM Counterpart Searches: MeerKAT South African-lead effort m antennas Operates GHz. Expansion plans GHz and GHz Focal-plane array technology to give 1.5 deg 2 FoV 1-hrs, rms~35 uJy (claimed) 75% of the time given to Key Science Projects (25% open) – Continuum sky survey – Slow and fast transient searches 2013 delivery of 1.4 GHz 17

Radio facilities for GW-EM Counterpart Searches: EVLA The 500-lb gorilla of radio astronomy m antennas Upgrade project almost finished. Will deliver order of magnitude increase in continuum sensitivity 1-50 GHz + 74 and 327 MHz 1-hrs, rms~7 uJy at 1.4 GHz Responds to external triggers Sub-arrays can be used to image a large error box 18

Radio facilities for GW-EM Counterpart Searches: EVLA The 500-lb gorilla of radio astronomy m antennas Upgrade project almost finished. Will deliver order of magnitude increase in continuum sensitivity 1-50 GHz + 74 and 327 MHz 1-hrs, rms~7 uJy at 1.4 GHz Responds to external triggers Sub-arrays can be used to image a large (irregular) error box 19

Radio facilities for GW-EM Counterpart Searches: LOFAR Dutch-lead European project 36 Dutch stations, 8 Euro stations MHz & MHz Key Science Projects – Continuum sky survey – Slow and fast transient searches Real-time pipeline + alert system and external triggers all planned RSM will monitor 25% of sky Million source survey in Radio sky monitor (RSM)

How might we best detect radio signals? Three strategies in order of chance of success – Afterglow search at late times for off-axis emission 0.1 to 1 mJy Timescales of a month EVLA, ASKAP, MerrKAT, Apertif – Afterglow search for on-axis event Bright but rare (i.e. beamed) 1-10 mJy Timescales of days EVLA, ASKAP, MerrKAT, Apertif – Search for prompt signal 1 mJy to 1 MJy (i.e. highly uncertain) Low frequency arrays. LOFAR, MWA, electronically steered in response to GW trigger Signal will be dispersively delayed

How might we best detect prompt signal? Prompt signal will suffer dispersive delay and scattering Sources of dispersive delay – Our Galaxy, IGM, host galaxy and circumburst medium Expect DM=1000 pc cm -3, or delays of 13 min at 75 MHz Dispersive delay scales as ν -2 Scattering effects (due to turbulence) are more difficult of estimate. – 0.1 to 4 s at 75 MHz – Scattering scales as ν -4.4 DM (pc cm -3 ) Lorimer and Kramer (2005)

How might we best detect prompt signal? Prompt signal will suffer dispersive delay and scattering Sources of dispersive delay – Our Galaxy, IGM, host galaxy and circumburst medium Expect DM=1000 pc cm -3, or delays of 13 min at 75 MHz Dispersive delay scales as ν -2 Scattering effects (due to turbulence) are more difficult of estimate. – 0.1 to 4 s at 75 MHz – Scattering scales as ν -4.4 DM (pc cm -3 )

What follow-up would we want to do? Panchromatic modeling to derive real estimates of energy and circumburst density. Direct VLBI imaging of the relativistic shock from the afterglow and any radio-emitting detritus from the merger Sub-milliarcsecond resolution An simple VLBA imaging project. Easier than GRB (z=0.17) Rule of thumb: If LIGO can detect a merger, the VLBA can image it. GRB z=0.17 (800 Mpc) Pihlstrom et al. (2007) 1 mas at 100 Mpc is 0.5 pc

What follow-up would we want to do? Panchromatic modeling to derive real estimates of energy and circumburst density. Direct VLBI imaging of the relativistic shock from the afterglow and any radio-emitting detritus from the merger Sub-milliarcsecond resolution An simple VLBA imaging project. Easier than GRB (z=0.17) Rule of thumb: If LIGO can detect a merger, the VLBA can image it. GRB z=0.17 Pihlstrom et al. (2007) 1 mas at 100 Mpc is 0.5 pc Bietenholz et al. (2003) SNe 1993J at d=4 Mpc

What follow-up would we want to do? Panchromatic modeling to derive real estimates of energy and circumburst density. Direct VLBI imaging of the relativistic shock from the afterglow and any radio-emitting detritus from the merger Sub-milliarcsecond resolution An simple VLBA imaging project. Easier than GRB (z=0.17) Rule of thumb: If LIGO can detect a merger, the VLBA can image it. GRB z=0.17 Pihlstrom et al. (2007) 1 mas at 100 Mpc is 0.5 pc Bietenholz et al. (2003)

What can we be doing today to help field? Continue to study GW populations – AM CVn stars – Core collapse (relativistic) SNe – Short-hard bursts Characterize the quiescent and transient radio sky to flux densities of 10 uJy Develop robust systems to respond to external triggers – Capability to carry out real-time response of radio telescopes to transients is rare – Nasu radio transients are an interesting test case. Bright, short lived with poor localization.

Conclusions Radio counterpart searches are a powerful tool – Predict a bright signal 1-10 mJy – Independent of beaming – Short latency is not needed. (Mañana!) – False positives are relatively unimportant A “bonanza” of new radio facilities is coming on line at just the right times for the next generation GW detectors 28

The future looks bright Come and join the GW-EM adventure 29