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)

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

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

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. 2008,2013,2015; 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

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

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

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

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

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

Red “timing” noise Most young pulsars show red spin noise PSR J1024-0719 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 B1937+21 SKA and gravitational waves | Ryan Shannon

Pulse-profile variations in MSPs Arrival times 10cm Profile Residual profiles Evidence for shape variations in PSR J1643-1224 from Parkes (Shannon et al. 2016) Not likely ISM induced Can correct for variations (i.e. Lentati & Shannon 2015) SKA and gravitational waves | Ryan Shannon

Pulse-profile variations in MSPs Profile residual Residual arrival times PSR J0437-4715 (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

Instrumental noise Without correction After correction 03/05/10 PDFB3 and PDFB4 firmware updated at UT0415 (MJD 55319.18) 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

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 J1909-3744 Strong steep red noise in J0437-4715 Inconsistent with J1909-3744 Consistent with Shannon & Cordes (2010) spin noise Weak shallow red noise in J1713+0747 No red noise in J1744-1134 RMS 100 ns over 11 years J1909-3744 J0437-4715 J1713+0747 J1744-1134 SKA and gravitational waves | Ryan Shannon

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 J1909-3744 SKA and gravitational waves | Ryan Shannon

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

Updates on the best pulsar PPTA J1909-3744 10-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

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

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

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