Observing Cosmic Dawn with the LWA-1

PIs: Judd Bowman (ASU), Greg Taylor (UNM)

Jake Hartman (JPL) Jayce Dowell, Joe Craig (UNM)Steve Ellingson (Virginia Tech)

Jackie Monkiewicz Arizona State University

The “Dark Ages” and Cosmic Dawn

2/20

Dark Ages: z = 1,100 to z~ 40 • matter-dominated• H & He are neutral•1st structures collapsing

Cosmic Dawn: z = 40-20•1st stars & galaxies•1st QSOs?•Early heating, reionization of small bubbles

Cosmic Dawn project purpose:

3/20

Detect/constrain signal of 1st generation of stars in 21-cm absorption of hydrogen at z ~ 30

REQUIRES:

1.Low frequency experiment, 10-100 MHz LWA-1

2.Long Integration time

3.Very accurate bandpass calibration

4. Novel beamforming techniques

Cosmic Dawn in 21 cm:

(Furlanetto 2006, Pritchard & Loeb 2010) 4/20

Tb = Ts – Tcmb

Seen against CMB:

Thermal History of IGM

Cosmic Dawn in 21 cm:

• 1st stars create absorption trough

• Additional heating sources mitigate trough

(Furlanetto 2006, Pritchard & Loeb 2010) 5

Tb = Ts - Tcmb

Observing strategy:

REQUIREMENTS:Very good bandpass calibration!

• Looking for broad, shallow absorption trough• need > 104 S/N in any spectral channel

But only need ~10% accuracy in absolute power…

6/20

A

E

D

B

C

20 40 60 80 100 120 140 160 180

ν (MHz)

ΔT21

(m

K)

+50

0

−50

−100 (Prit

char

d &

Loe

b 20

10)

Observing strategy:

STRATEGY:Simultaneously observe bright calibrators & dark (low Tsys) science field

• 2 x 19.6 MHz beams on bright calibrator• 2 x 19.6 MHz beams on science field• 520 hours on-sky

6/20

Observing Strategy -- COMPLICATION:

• Frequency variation of beam shape couples of foreground structure to sidelobes

mistake sources drifting through sidelobes for 21-cm spectral features?

8/20

1.0

0.8

0.6

0.4

0.2

0.0−15 −10 −5 0 5 10 15

Offset (degrees)

Rel

ativ

e ga

in

74 MHz

38 MHz

Novel Beamforming Strategies:Mitigate potential foreground-frequency coupling of sidelobes:

9/20

1. Defocusing (e.g. gaussian smoothing)

2. Sidelobe steering

3. Nulling

4. Sidelobe shimmering

5. “Optimized” beam-forming(account for mutual coupling of antennas)

Work to date:

10/20

• Learning the LWA Software Library! (and PYTHON in general)

• Phase-and-Sum Beamforming with TBN

• Raster Mapping of TBN Beam (pseudo-beams)

Phase-and-Sum Beamforming:

11/20

Phase-and-Sum Beamforming:

12/20

Find bursting Sun produces much better coefficientsthan Cyg A --- not surprising?

Raster Mapping:

13/20

Use bright source in TBN data to map structure of sidelobes --- “Pseudo-beam”

Raster Mapping – Variation with elevation

14/20

Cyg A:

Transit

EL = 83 deg

-1 hour

EL = 76 deg

-2 hours

EL = 65 deg

15/20

Cas A NCP

@ Cyg A transit

EL = 46 deg

-2 hours before Cyg A transit

EL = 34 deg

@ Cyg A transit

EL = 34 deg

… What is going on in the North/Northeast during the Cyg A transit on Sept 21, 2011??

16

PASI started recording Sept 23, 2011:

http://www.phys.unm.edu/~lwa/lwatv/55827.mov

Pseudo-beam Maps over full frequency range:

17/20

Acquired TBN observations of 4 frequency groups:

87 MHz80 MHz73 MHz

71 MHz64 MHz57 MHz

…corresponding to 4 DRX tunings for main Cosmic Dawn observations

55 MHz48 MHz41 MHz

39 MHz32 MHz25 MHz

Beam 1Tuning 1

Beam 1Tuning 2

Beam 2Tuning 1

Beam 2Tuning 2

Test our Beamforming Strategies:

18/17

1. Defocusing (e.g. gaussian smoothing)

2. Sidelobe steering

3. Nulling

4. Sidelobe shimmering

5. “Optimized” beam-forming(account for mutual coupling of antennas)

Which is the “quickest and dirtiest”?

Test our Beamforming Strategies:

19/17

1. Defocusing (e.g. gaussian smoothing)

2. Sidelobe steering

3. Nulling

4. Sidelobe shimmering

5. “Optimized” beam-forming(account for mutual coupling of antennas)

Which is the “quickest and dirtiest”?

Pseudo-beam Maps over full frequency range:

20/20

Apply some of our novel beam-forming strategies

Defocusing is simplest, fastest apply Gaussian to antenna gains

\

Acquire raster maps of customized DRX beams,compare with TBN predictions, confirm shape

LWA-OCD Project Outputs:

21/20

Beamforming: Detailed measurements of beamStrategies for custom beamforming

Deep integrations:Very high S/N spectra of bright calibratorsVery high S/N spectra of diffuse Galaxy

(including high-level H recombination lines)• Serendipitous radio transients

Lots of opportunity for RFI mitigation!

Detection/contraints on First Light absorption trough.

END

RFI environment at LWA:

22

The “Dark Ages” and Cosmic Dawn

23/17

z = redshift (decreases with increasing time)

z = ∞ z = 0

The “Dark Ages” and Cosmic Dawn

24/17

Wouthuysen-Field Effect:

25

Phase-and-sum Beamforming:

26/17

•Use bright source to back out coefficients•Only works for narrow (< 10 KHz bandwidth)

insensitive to 2 offsets

Delay-and-sum Beamforming:

•Do phase-and-sum over full LWA frequency range•Solve for true delays for each antenna

System noise for LWA:

27From Pihlstrom, 2012, internal memo

Acronyms:

28