Efficient wide-area sky monitoring Olaf Wucknitz - - PowerPoint PPT Presentation

Efficient wide-area sky monitoring

Olaf Wucknitz

wucknitz@mpifr-bonn.mpg.de

Future Trends in Radio Astronomy Instrumentation Bonn/online, 21–22 September 2020

Efficient wide-area sky monitoring

• Motivation: Lensed FRBs
• Need for wide-area monitoring
• Existing instruments
• Beamforming
• FFT arrays

titlepage introduction summary bonus back forward −1 +1 fullscreen 2/20

Gravitational lensing: the idea

• Isaac Newton (1704)

α = ∆dz dl = 1 c2

• dl ∇

⊥Φ

• Henry Cavendish (1784)
• Johann Soldner (1801)
• Newtonian (Soldner):

α = 2 G c2 M r ↓

• relativistic (Einstein 1915):

α = 4 G c2 M r

α

titlepage introduction summary bonus back forward −1 +1 fullscreen 3/20

Rings and multiple images

titlepage introduction summary bonus back forward −1 +1 fullscreen 4/20

Fields of study in lensing

• sources
• lenses
• propagation effects
• spacetime

⋆ cosmology ⋆ relativity ⋆ new physics?

titlepage introduction summary bonus back forward −1 +1 fullscreen 5/20

Measuring distances with time-delays

Ds Dds Dd

• bserver

source l e n s

• distance ratios known
• angles measurable
• geometry can be determined
• need one length for scale
• use time-delay !

Refsdal (1964), MNRAS 128, 307 : ∆t ∝ DdDs Dds ∝ 1 H0

• can determine Hubble constant!

titlepage introduction summary bonus back forward −1 +1 fullscreen 6/20

Current results

50 60 70 80 90

H0 [kms−1 Mpc−1]

probability density

H0 : 71.0+2.9

−3.3

H0 : 78.2+3.4

−3.4

H0 : 71.7+4.8

−4.5

H0 : 68.9+5.4

−5.1

H0 : 71.6+3.8

−4.9

H0 : 81.1+8.0

−7.1

H0 : 73.3+1.7

−1.8

H0 ∈ [0, 150] Ωm ∈ [0.05, 0.5]

All B1608 (Suyu+2010, Jee+2019) RXJ1131 (Suyu+2014, Chen+2019) HE0435 (Wong+2017, Chen+2019) J1206 (Birrer+2019) WFI2033 (Rusu+2019) PG1115 (Chen+2019)

[ Wong et al. (2020), MNRAS, arXiv:1907.04869 ]

titlepage introduction summary bonus back forward −1 +1 fullscreen 7/20

Problem solved?

• No!
• mass-model degeneracies

⋆ degeneracy between lens and source ⋆ e.g. mass-sheet degeneracy ⋆ hard to break without additional info!

• ‘tension’ with CMB and BAO measurements
• There is something we don’t understand!

titlepage introduction summary bonus back forward −1 +1 fullscreen 8/20

Fast radio bursts (FRBs)

• short (msec) bright radio bursts
• unknown source nature
• some repeating
• small coherent sources
• some localised: extragalactic
• gravitationally lensed FRBs?

⋆ measure time delays to msec or even µsec! ⋆ galactic interferometry (few km resolution!)

titlepage introduction summary bonus back forward −1 +1 fullscreen 9/20

Lorimer burst

[ Lorimer (2007), Science 318, 777 ]

titlepage introduction summary bonus back forward −1 +1 fullscreen 10/20

Cosmology with lensed FRBs

• ‘images’ show as coherent delayed copies
• can correlate signals
• coherent time delays precise to < µsec
• repeating FRBs

⋆ time delay for each burst ⋆ Universe expands, delays increase by ∼ 10−10 per year hundreds of µsec for a few years

• can see the Universe expanding!
• eliminate mass model by combining time delay and its evolution

[ Wucknitz et al. (2020), A&A submitted, arXiv:2004.11643 ]

titlepage introduction summary bonus back forward −1 +1 fullscreen 11/20

Combination of lensed FRBs

0.28 0.30 0.32

M

0.0 0.2 0.4 0.6 0.8 1.0 65 70 75 H0 [km s

1 Mpc 1]

0.27 0.28 0.29 0.30 0.31 0.32 0.33

M

65 70 75 H0 [km s

1 Mpc 1]

0.0 0.2 0.4 0.6 0.8 1.0 1.10 1.05 1.00 0.95 0.90 w 0.0 0.2 0.4 0.6 0.8 1.0 0.28 0.30 0.32

M

1.10 1.05 1.00 0.95 0.90 w 65 70 75 H0 [km s

1 Mpc 1]

1.10 1.05 1.00 0.95 0.90 w

[ Wucknitz et al. (2020), arXiv:2004.11643 ]

titlepage introduction summary bonus back forward −1 +1 fullscreen 12/20

How to find them

• FRB searches over limited areas (CHIME, ASKAP)
• CHIME finds several per day
• from AGN statistics: one in ∼ 1000 is lensed

⋆ one lensed FRB per year? ⋆ less repeaters

• field of view ca. 250 deg2 (CHIME), 30 deg2 (per

ASKAP dish)

• about 1 % of the visible sky
• need all lensed ‘echoes’ for identification

⋆ will generally be missed

titlepage introduction summary bonus back forward −1 +1 fullscreen 13/20

CHIME

[ https://chime-experiment.ca ]

titlepage introduction summary bonus back forward −1 +1 fullscreen 14/20

FRB sky monitor

• observe large region continuously

⋆ circumpolar region ⋆ several 1000 deg2

• sufficient sensitivity
• sufficient resolution
• will find more FRBs than CHIME
• will not miss lensed ones
• many beams with high time resolution

titlepage introduction summary bonus back forward −1 +1 fullscreen 15/20

FFT beamforming

I(θ,t) =

• ∑

j

Ej(t)e2πiθxj/λ

• 2
• computing scales with N2 per sample
• for regular xj and θ: use FFT
• scales as N logN
• not a new idea

⋆ Otobe et al. (1994), PASJ 46, 503 ⋆ Tegmark & Zaldarriaga (2009), PRD 79, 3530

• used by CHIME in 1-d
• 2-d instrument perfect to find lensed FRBs

titlepage introduction summary bonus back forward −1 +1 fullscreen 16/20

EMBRACE array

[ Torchinsky et al. (2015), JInst 10, C07002 ]

titlepage introduction summary bonus back forward −1 +1 fullscreen 17/20

EMBRACE as FFT array

• SKADS project, Nancay version still exists
• 8×8 m2

4608 elements

• 900–1500 MHz
• analogue beamformer per 2×2 elements
• ∼ 1000 signals, similar to CHIME (2048)
• could observe circumpolar region (e.g. Onsala)
• no missed lensed echoes
• estimated FRB detections: few per day
• hardware cost < 2 million Euros
• ERC funding proposal not successful

titlepage introduction summary bonus back forward −1 +1 fullscreen 18/20

CHORD: extension for CHIME

[ Vanderlinde et al. (2019),arXiv:1911.01777 ]

titlepage introduction summary bonus back forward −1 +1 fullscreen 19/20

Summary

• lensed FRBs are great tool

⋆ cosmology ⋆ galactic interferometers ⋆ other aspects and caveats!

• need continuous monitoring

⋆ of wide area ⋆ with high time resolution

• FFT telescope can do it!
• re-use existing hardware (EMBRACE)?
• use PAF?

[ Wucknitz et al. (2020), A&A submitted, arXiv:2004.11643 ]

titlepage introduction summary bonus back forward −1 +1 fullscreen 20/20

Beamforming vs. correlation

• per frequency channel:

I(θ,t) =

• ∑

j

Ej(t)e2πiθxj/λ

• 2

beamforming = ∑

j

|Ej(t)|2 + ∑

j=k

Ej(t)¯ Ek(t)

• visibility

e2πiθ(xj−xk)/λ

• FFT

imaging

• NTel telescopes, Nθ beams, dense array: NTel ∼ Nθ

sampling rate data: R, beams: r

• scaling of computations

⋆ beamforming: R NTelNθ = R N2 ⋆ imaging: R N2

Tel +r Nθ logNθ

titlepage introduction summary bonus back forward −1 +1 fullscreen 21/20