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 α = ∆ d z d l = 1 � • Isaac Newton (1704) d l ∇ ⊥ Φ c 2 • Henry Cavendish (1784) • Johann Soldner (1801) α = 2 G M • Newtonian (Soldner): c 2 r ↓ α α = 4 G M • relativistic (Einstein 1915): c 2 r 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 • spacetime • sources ⋆ cosmology • lenses ⋆ relativity • propagation effects ⋆ new physics? titlepage introduction summary bonus back forward − 1 +1 fullscreen 5/20

Measuring distances with time-delays source • distance ratios known Dds • angles measurable s n e l • geometry can be determined Ds • need one length for scale Dd use time-delay ! � observer Refsdal (1964), MNRAS 128, 307 : ∆t ∝ D d D s ∝ 1 can determine Hubble constant! � D ds H 0 titlepage introduction summary bonus back forward − 1 +1 fullscreen 6/20

Current results H 0 ∈ [0 , 150] Ω m ∈ [0 . 05 , 0 . 5] [ Wong et al. (2020), MNRAS, arXiv:1907.04869 ] All H 0 : 71 . 0 +2 . 9 B1608 (Suyu+2010, Jee+2019) − 3 . 3 RXJ1131 (Suyu+2014, Chen+2019) H 0 : 78 . 2 +3 . 4 − 3 . 4 HE0435 (Wong+2017, Chen+2019) probability density H 0 : 71 . 7 +4 . 8 J1206 (Birrer+2019) − 4 . 5 WFI2033 (Rusu+2019) H 0 : 68 . 9 +5 . 4 PG1115 (Chen+2019) − 5 . 1 H 0 : 71 . 6 +3 . 8 − 4 . 9 H 0 : 81 . 1 +8 . 0 − 7 . 1 H 0 : 73 . 3 +1 . 7 − 1 . 8 50 60 70 80 90 H 0 [kms − 1 Mpc − 1 ] 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 µ sec! ⋆ 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 < µ sec • repeating FRBs ⋆ time delay for each burst ⋆ Universe expands, delays increase by ∼ 10 − 10 per year � hundreds of µ sec 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 1.0 1.0 1.0 0.8 0.8 0.8 [ Wucknitz et al. (2020), arXiv:2004.11643 ] 0.6 0.6 0.6 0.4 0.4 0.4 0.2 0.2 0.2 0.0 0.0 0.0 65 70 75 0.28 0.30 0.32 1.10 1.05 1.00 0.95 0.90 H 0 [km s 1 Mpc 1 ] w M 0.33 0.90 0.90 0.32 0.95 0.95 0.31 1.00 1.00 0.30 M w w 0.29 1.05 1.05 0.28 1.10 1.10 0.27 0.28 0.30 0.32 65 70 75 65 70 75 H 0 [km s 1 Mpc 1 ] H 0 [km s 1 Mpc 1 ] M 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 deg 2 (CHIME), 30 deg 2 (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 deg 2 • 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 � E j ( t )e 2 π i θ x j /λ � 2 � � � ∑ I ( θ , t ) = � j • computing scales with N 2 per sample • for regular x j and θ : use FFT • scales as N log N • 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 m 2 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: � E j ( t )e 2 π i θ x j /λ � 2 � � � ∑ I ( θ , t ) = beamforming � j | E j ( t ) | 2 + ∑ = ∑ E j ( t ) ¯ e 2 π i θ ( x j − x k ) /λ E k ( t ) imaging � �� � � �� � j FFT j � = k visibility • N Tel telescopes, N θ beams, dense array: N Tel ∼ N θ sampling rate data: R , beams: r • scaling of computations R N Tel N θ = R N 2 ⋆ beamforming: R N 2 ⋆ imaging: Tel + r N θ log N θ titlepage introduction summary bonus back forward − 1 +1 fullscreen 21/20