1. Detecting gravitational waves with the SKA: Recent results and open questions
Present how we have limits on GW levels to constrain the massive black hole population
Ryan Shannon (Research Fellow, ICRAR-Curtin/CSIRO)

2. Outline Goal: Highlight recent results in pulsar timing arrays
Improved data analysis techniques
What we need to figure out before SKA PTA commences
Gravitational waves and pulsar timing
Suppressing noise in PTA experiments
Recent limits on the gravitational wave signal and implications for galaxy evolution
Emergence of noise in the best pulsar
SKA and gravitational waves | Ryan Shannon

3. Gravitational waves and pulsar timing
GWs propagating across pulsar-Earth line of sight alter arrival time of the observed pulses
Signal has quadropolar correlation between pulsars (pulsar timing array, PTA)
Three groups working together
Europe (EPTA)
North America (NANOGrav)
Australia (Parkes PTA)
International Pulsar Timing Array (IPTA)
Differences to LIGO:
Lower GW frequencies by ~ 1010
Different path to detection
Ephemeral (LIGO) vs gradual (PTA)
Image: David Champion
SKA and gravitational waves | Ryan Shannon

4. Sources of GWs in Pulsar Band
Sensitive to GWs with frequencies between 3 and 300 nHz (get TOAs every week for 10 years)
Massive Black Hole Binaries (Jaffe & Backer 2003, Sesana et al ,2013,2015; Ravi et al. 2012,2014,2015)
Cosmic Strings and Super Strings (Damour & Vilenkin 2005, Sanidas et al. 2012, Lentati et al. 2015)
Cosmological QCD Phase Transition (Witten 2007, Caprini et al. 2010).
Stochastic backgrounds
Gravitational wave memory (Wang et al. 2015, Madison et al. 2015, Lasky et al. 2016)
Single sources
SKA and gravitational waves | Ryan Shannon

5. Why do we care about supermassive black holes?
How do galaxies form? Why do today’s galaxies look the way they do? What did the first galaxies look like?
What is the cause/effect of supermassive black holes and galaxy evolution?
When galaxies merge, what happens to their SMBHs?
Too close together to resolve using optical telescopes.
Reside in dense environments.
Gravitational waves enable us to study the centres of galaxies invisible otherwise!
SKA and gravitational waves | Ryan Shannon

6. The Stochastic Background
Time (arb)
ΔT (arb)
Earth
Pulsar
E+P
Resid.
Coherent combination
The stochastic background is the superposition of GWs from many sources
Background induces red power spectrum in residuals (more power at lowest frequency of GWs).
The background is characterized by a characteristic strain amplitude spectrum hc(f) with amplitude Ac (10-15)and spectral index α (-2/3).
180
Relative Angle
(deg) 90
hc(f) = Ac f-α
RMS contributions to residuals ~20 ns over 5 years: need to combine signal from a group of pulsars (pulsar timing array, PTA).
SKA and gravitational waves | Ryan Shannon

7. How do we detect the background?
Problem: Other sources of noise can look like gravitational waves (e.g., pulsar spin noise)
Solution: Search for correlation in arrival times between multiple pulsars:
Pulsar Timing Array
Credit: David Champion
Observed (absence of) correlation (Arzoumanian et al. 2016)
Expected correlation
SKA and gravitational waves | Ryan Shannon

8. Correcting for the ISM Red: 50 cm Black: 20 cm Blue: 10 cm
Largest red-noise signal in data set:
Variations in line of sight eelctron content dispersion measure (DM).
Proportional to λ2.
Need to remove red signal associated with DM variations without removing red signal associated with GWB
Include model of λ-independent in DM correction algorithm (Shannon 2011, Keith et al. 2013, Lee et al. 2014, Lentati et al. 2014)
Manchester et al. (2013)
SKA and gravitational waves | Ryan Shannon

9. Super dispersive ISM Multipath propagation/scattering:
Scintillation and pulse broadening (Cordes & Shannon 2010)
Frequency-dependent dispersion measures (Cordes, Shannon & Stinebring 2016)
Effects predicted to be worse at low frequency
Hints of excess noise in low frequency some pulsars (Lentati et al. 2016)
Need multi-freq observation to identify this effect
Is avoidance (i.e. higher frequency) the best strategy (Shannon & Cordes 2016)
Questions for the SKA:
Can DM correction be done in band?
Need to determine role of low-frequency observations (LOFAR/MWA/SKA-low)
Simulated propagation effects at 0.7 and 1.4 GHz
Shannon & Cordes (2016)
SKA and gravitational waves | Ryan Shannon

10. Red “timing” noise Most young pulsars show red spin noise
PSR J
Most young pulsars show red spin noise
Rotation instabilities?
Magnetospheric torque changes?
Open question: is this a generic property of MSPs too?
Distant companions/asteroid belts?
Can have similar spectral properties to GWB
Solution: need to (at least) model the presence of red noise in datasets
Triage bad pulsars
PSR B
SKA and gravitational waves | Ryan Shannon

11. Pulse-profile variations in MSPs
Arrival times
10cm Profile
Residual profiles
Evidence for shape variations in PSR J from Parkes (Shannon et al )
Not likely ISM induced
Can correct for variations (i.e. Lentati & Shannon 2015)
SKA and gravitational waves | Ryan Shannon

12. Pulse-profile variations in MSPs
Profile residual
Residual arrival times
PSR J (brightest MSP)
7-yr of modern data at 10-cm
Hints that timing noise is connected to pulse shape variations
Calibration/intrinsic to the pulsar?
Lessons for SKA: Need to in incorporate algorithms to search/correct shape variations (much easier with high S/N observations)
SKA and gravitational waves | Ryan Shannon

13. Instrumental noise Without correction After correction
03/05/10 PDFB3 and PDFB4 firmware updated at UT0415 (MJD ) to remove delays, i.e. to make time-stamp refer to time of signal arrival at digitiser input. In case of PDFB4, the PTU was also changed to remove the two-bin delay. (V6,V3)
After correction
Lesson for SKA: Keep track of instrument changes + propagation time through signal path
SKA and gravitational waves | Ryan Shannon

14. A new Parkes Pulsar Timing Array Limit
11-year dataset
Only consider 10cm data
Low frequency data contains excess (non dispersive) red noise and not as sensitive
Select pulsar based on timing precision
No evidence for red noise in PSR J
Strong steep red noise in J
Inconsistent with J
Consistent with Shannon & Cordes (2010) spin noise
Weak shallow red noise in J
No red noise in J
RMS 100 ns over 11 years J J J J
SKA and gravitational waves | Ryan Shannon

15. How do the models stack up?
Pconsistent
0.7% 9%
New algorithms
Bayesian methodology to marginalize over pulsar timing model and search for stochastic contributions to TOAs
Ac < 1.0x10-15 (95% CL)
0.3-4%
6-20%
Sensitivity of 5 year SKA campaign on PSR J
SKA and gravitational waves | Ryan Shannon

16. Scenarios that can explain low limit
Slow galaxy mergers (green)
Disfavored because of known galaxy merger mechanisms
Recoil of SMBH after merger/disruption of binary (purple)
Not favored observationally
Stalled mergers (red)
Final-parsec problem not solved
Environmentally driven mergers (grey/red)
Over-solve final-parsec problem
Low occupation fraction of seed SMBH in early universe (not displayed)
Supermassive black hole merger timeline
SKA and gravitational waves | Ryan Shannon

17. Updates on the best pulsar
PPTA J cm observations
(12 yr – through March 2016)
Emergence of red noise in our data sets
Need to refer TOAs to inertial frame
Solar system barycentre
Ephemerides published by NASA/JPL (spacecraft navigation and pulsar timing)
Blue: DE421 Red: DE418
Dashed: projection
SKA and gravitational waves | Ryan Shannon

18. Choose your own ephemeral adventure
Simulated arrival times over > 40 years
Simulated in DE418 and measured in DE421
(Ephemeris error shows dipolar correlation in arrival times)
Before fitting
After
fitting
After fitting for Msaturnt, Right Ascension, Declination
ΔMsat = 3x10-10 Msun
(same as for real data set)
SKA and gravitational waves | Ryan Shannon

19. Improving Sensitivity: short-term
Improved instrumentation
Wide (radiofrequency) bandwidth receiving systems (6:1 observing bandwidth)
Increases sensitivity (×2), observing throughput, better correction for refractive interstellar medium
Additional telescope time
Increase sensitivity
Better understand systematics / improve calibration of the signal
meerKAT/SKA-low
Observe more pulsars
Large-area sky surveys for new pulsars
Optimize scheduling
SKA and gravitational waves | Ryan Shannon

20. Summary
Our understanding of astrophysical noise is improving and guiding PTA observations
New methods and re-analysis are increasing sensitivity
Limits on GWB are being used to constrain the massive black hole population. We are now exclude GW signals from vanilla models of galaxy evolution.
IPTA dataset should immediately improve limit.
New facilities / longer datasets /more pulsars will enable us to detect GWs with pulsars.
Amazing physics and astrophysics along the way to GW detection
SKA and gravitational waves | Ryan Shannon