EPTA – Exploiting 25 years of data

Siyuan ChenOn behalf of the EPTA

IPTA Catch-up Meeting23 Sep 2020

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EPTA DR1

● Desvignes et al. 2016; Caballero et al. 2016

● 42 millisecond pulsars (MSPs), “Historical” backend data, T=(6.9-24.1) yr

● Major upgrade from 2011 release (van Haasteren et al. 2011)

● 2011: EPTA Gravitational-wave background (GWB) strain upper limit: A=6x10-15 (95% CL) @ α=-2/3

● 2015/16 limit (Lentati et al. 2015): A=3x10-

15 (95% CL) @ α=-2/3,

● first time to see a suspicious possible common red signal (CRS) in EPTA pulsar data

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EPTA DR1 GWB search/detection analysis

6 MSP search Confirmation by multiple

codes and checks Some variation in GWB

parameters depending on sampler used, but consistent within uncertainties

Passed all basic data-systematics checks

CRS spectral index Vs strain amplitude(multiple Bayesian codes; fixed DE421 ephemeris)

EPTA 2014-15 GWB search analysis

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EPTA DR1 GWB search/detection analysis

Hellings-Downs (HD) curve?:Cross-correlation measurement precision still low, however:

1) Bayesian factor disfavoured a clock signal (monopolar correlation)2) GW-parameter posterior distributions did not depend on Solar-system ephemeris used3) Pulsar-position randomization was flattening the strain amplitude posterior distribution

=> Suspicion of possible GWB signal to be further monitored

CRS cross-correlation curve (multiple methods/codes; fixed DE421 ephemeris)

EPTA 2014-15 GWB search analysis

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EPTA DR1 GWB search/detection analysis

Frequentist statistic: Fixed spectral index α=-2/3(GW-driven supermassive black holes); fixed DE421 ephemeris

Max. likelihood (Λ) estimator (top= Λ, bottom = Λ distribution)

Optimal statistic (OS) (Anholm et al 2009).Bottom=Null hypothesis Amplitude distributionGreen line = Max. Likelihood value

All analyses pointed to amplitude ~2x10-15 for spectral index α=-2/3, in case this was a GWB, but HD measurement missing

EPTA 2014-15 GWB search analysis

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EPTA DR2

Major extension of the DR1; adds about ~10 years of new/improved data (some overlap with old data)

Initial focus on 6 pulsars for prelim DR2 v0 with more pulsars to be added to DR2 v1 sequentially

Includes several new observing systems with coherent dedispersion

Incorporated for the first time TOAs from the Large European Array for Pulsars (LEAP) project

=> Major challenge: data from 5+1 telescopes

Presentation of preliminary GWB analysis on the prelim DR2 v0; all results are preliminary

Work ongoing to finalize dataset and analyze it in more detail

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EPTA DR2

Includes several new observing systems with coherent dedispersion:

Effelsberg: L-band receiver with 200(400) MHz; S-band receivers with 200(500) MHz bandwidths

Lovell: ROACH (300 MHz) system replacing DFB at L-band

Nancay: NUPPI (512 MHz) system replacing BON at L and S-band

Sardinia: major upgrade ongoing, L-band and higher freqs planned

Westerbork: Puma2 system replacing Puma1 at low freqs, L and S-band

LEAP: Coherently added observations at L-band with 128 MHz bandwidth from up to 5 telescopes forming a up to 194-m dish

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EPTA DR1 vs DR2 v0

Improving our dataset to improve our GW search results: From EPTA DR1 to DR2Below: TOAs of 6 MSPs used for 2015 analysis. Pulsar noise properties appear consistent so far.

J0613-0200

J1012+5307

J1600-3053

J1713+0747

J1744-1134

J1909-3744

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Single pulsar – Upper limits (DR1 vs DR2 v0)

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Detection analysis (DR1 vs DR2 v0)

Varied spectral indexFixed spectral index

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Fixed Solar system ephemeris vs BayesEphem (DR1)

Upper limit analysis Detection analysis

Results seem independent between fixing or modelling the SSE; DR2 v0 analyses appear also consistent

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BayesEphem vs EphemGP (DR2 v0)

Results seem independent between fixing or modelling the SSE; DR1 analyses appear also consistent

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EPTA DR2 v0 GWB search/detection analysis

CRS still present – now with evidence for possible HD correlationBelow a DR1 vs DR2 comparison; DE436 ephemeris + BayesEphem

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CRS Free spectrum vs Power Law comparison

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Example model selection: Bayes Factor (BF) (DR1 vs DR2 v0)

Pulsar intrinsic red noise only

Common-uncorrelated

HD Correlation

Monopole Correlation

Dipole Correlation

HD + Monopole

HD + common uncorr.

HD + Dipole

0.7 0.06

– 0.34

– 0.3

– 0.03

0.1

0.213.15 0.17

– 2.24

– 1.79

0.09

0.03

0.45

BF=Log10(Bayes Factor) values for model comparison/selection: Each BF favoring(+ve) or disfavoring(-ve) the addition of the parameter at the right of the arrow to to the model at the start of the arrow

Values for fixed α=-2/3 Ephemeris DE436+ BayesEphem Blue=DR1; Orange=DR2

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Alternative (frequentist) analysis: Optimal statistic spatial correlation

DE436 ephemeris

S/N = sqrt(2*ln(BF))

BF = exp(S/N^2 / 2)

DR2 v0DR1

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EPTA GWB search – A Summary of recent Enterprise runs

Pulsar – Upper limit Lentati et al. 2015 DR1 (enterprise) DR2 (enterprise)

J0613-0200 9.7e-15 8.2e-15 6.3e-15

J1012+5307 8.3e-15 8.3e-15 6.5e-15

J1600-3053 2.1e-14 2.2e-14 1.1e-14

J1713+0747 4.4e-15 4.2e-15 4.6e-15

J1744-1134 7.0e-15 7.4e-15 5.5e-15

J1909-3744 5.2e-15 5.0e-15 4.8e-15

GWB fixed gamma 3.0e-15 2.7e-15 3.4e-15

GWB varied gamma 1.3e-14 1.6e-14 1.1e-14

Model comparison: Spatial correlation

Monopole vs none:DR1 → DR2

Dipole vs none:DR1 → DR2

HD vs none:DR1 → DR2

Log10 Bayes factor – 0.34 → -2.24 – 0.3 → -1.79 0.06 → 0.17

Frequentist OS S/N 0.14 → 1.06 – 0.04 → 0.74 0.34 → 0.81

More DR2 pulsar data in prep. for further work on search/detectionGoal is to better sample the cross-correlation curve

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EPTA – Current Status

• EPTA data based on regular observations with 5 telescopes (Effelsberg, Lovell, Nancay, Sardinia, Westerbork) with combined weekly cadence

• For nearly 10 years, monthly LEAP runs coherently adding EPTA dishes to up to 194-m dish• Additionally, low-frequency MSP monitoring with LOFAR core, GLOW stations, now

NenuFAR• DR1 analysis in 2014-2015 suggested a common red-noise process but of unclear origin,

opted to do more work before publicizing• Since then trying to understand origin (GW, instrumental, ephemerides, clocks, samplers

etc.) in order to avoid false-positive detections• Decision to improve data and data acquisition systems while working on paper leading to

DR2 rather than publishing new limits● DR2 contains excellent new data incl. LEAP showing very promising and interesting results.

Work in progress

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EPTA – Towards 10k days of data

● Comparing Single pulsar noise analysis between enterprise/temponest/fortytwo (incl. Timing parameters)

● Testing different Solar System ephemerides and models (BayesEphem/EphemGP/LINIMOSS) bridging the uncertainties

● Testing different pipelines/samplers for consistency● Checking the effects of various DM/ISM correction schemes● Scintillation and Scattering effects● Using LEAP to cross check different telescopes● Include more pulsars, curate dataset● More collaboration in IPTA