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1. The NANOGrav 12.5 yr Data Set: Bayesian Limits on Gravitational Waves from Individual Supermassive Black Hole Binaries

   https://doi.org/10.3847/2041-8213/acdbc7  

   Arzoumanian, Zaven ; Baker, Paul T. ; Blecha, Laura ; Blumer, Harsha ; Brazier, Adam ; Brook, Paul R. ; Burke-Spolaor, Sarah ; Bécsy, Bence ; Casey-Clyde, J. Andrew ; Charisi, Maria ; et al ( July 2023 , The Astrophysical Journal Letters)

   Pulsar timing array collaborations, such as the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), are seeking to detect nanohertz gravitational waves emitted by supermassive black hole binaries formed in the aftermath of galaxy mergers. We have searched for continuous waves from individual circular supermassive black hole binaries using NANOGrav’s recent 12.5 yr data set. We created new methods to accurately model the uncertainties on pulsar distances in our analysis, and we implemented new techniques to account for a common red-noise process in pulsar timing array data sets while searching for deterministic gravitational wave signals, including continuous waves. As we found no evidence for continuous waves in our data, we placed 95% upper limits on the strain amplitude of continuous waves emitted by these sources. At our most sensitive frequency of 7.65 nHz, we placed a sky-averaged limit ofh₀< (6.82 ± 0.35) × 10⁻¹⁵, andh₀< (2.66 ± 0.15) × 10⁻¹⁵in our most sensitive sky location. Finally, we placed a multimessenger limit of$\mathit{}<(1.41\pm 0.02)\times {10}^9{M}_{\odot }$on the chirp mass of the supermassive black hole binary candidate 3C 66B.

    

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2. Multimessenger Gravitational-wave Searches with Pulsar Timing Arrays: Application to 3C 66B Using the NANOGrav 11-year Data Set

   https://doi.org/10.3847/1538-4357/ababa1  

   Arzoumanian, Zaven ; Baker, Paul T. ; Brazier, Adam ; Brook, Paul R. ; Burke-Spolaor, Sarah ; Bécsy, Bence ; Charisi, Maria ; Chatterjee, Shami ; Cordes, James M. ; Cornish, Neil J. ; et al ( September 2020 , The Astrophysical Journal)

   Abstract When galaxies merge, the supermassive black holes in their centers may form binaries and emit low-frequency gravitational radiation in the process. In this paper, we consider the galaxy 3C 66B, which was used as the target of the first multimessenger search for gravitational waves. Due to the observed periodicities present in the photometric and astrometric data of the source, it has been theorized to contain a supermassive black hole binary. Its apparent 1.05-year orbital period would place the gravitational-wave emission directly in the pulsar timing band. Since the first pulsar timing array study of 3C 66B, revised models of the source have been published, and timing array sensitivities and techniques have improved dramatically. With these advances, we further constrain the chirp mass of the potential supermassive black hole binary in 3C 66B to less than (1.65 ± 0.02) × 10 9   M ⊙ using data from the NANOGrav 11-year data set. This upper limit provides a factor of 1.6 improvement over previous limits and a factor of 4.3 over the first search done. Nevertheless, the most recent orbital model for the source is still consistent with our limit from pulsar timing array data. In addition, we are able to quantify the improvement made by the inclusion of source properties gleaned from electromagnetic data over “blind” pulsar timing array searches. With these methods, it is apparent that it is not necessary to obtain exact a priori knowledge of the period of a binary to gain meaningful astrophysical inferences. 

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3. Precision Timing of PSR J0437–4715 with the IAR Observatory and Implications for Low-frequency Gravitational Wave Source Sensitivity

   https://doi.org/10.3847/1538-4357/abeb64  

   Lam, M. T. ; Hazboun, J. S. ( April 2021 , The Astrophysical Journal)

   Abstract While observations of many high-precision radio pulsars of order ≲1 μ s across the sky are needed for the detection and characterization of a stochastic background of low-frequency gravitational waves (GWs), sensitivity to single sources of GWs requires even higher timing precision. The Argentine Institute of Radio Astronomy (IAR; Instituto Argentino de Radioastronomía) has begun observations of the brightest known millisecond pulsar, J0437−4715. Even though the two antennas are smaller than other single-dish telescopes previously used for pulsar timing array (PTA) science, the IAR’s capability to monitor this pulsar daily, coupled with the pulsar’s brightness, allows for high-precision measurements of pulse-arrival time. While upgrades of the facility are currently underway, we show that modest improvements beyond current plans will provide IAR with unparalleled sensitivity to this pulsar. The most stringent upper limits on single GW sources come from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). Observations of PSR J0437−4715 will provide a significant sensitivity increase in NANOGrav’s “blind spot” in the sky where fewer pulsars are currently being observed. With state-of-the-art instrumentation installed, we estimate the array’s sensitivity will improve by a factor of ≈2–4 over 10 yr for 20% of the sky with the inclusion of this pulsar, as compared to a static version of the PTA used in NANOGrav’s most recent limits. More modest instrumentation results in factors of ≈1.4–3. We identify four other candidate pulsars as suitable for inclusion in PTA efforts. International PTA efforts will also benefit from inclusion of these data, given the potential achievable sensitivity. 

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4. The NANOGrav Search for Signals from New Physics: MCMC chains

   https://doi.org/10.5281/zenodo.8083620  

   NANOGrav, The Collaboration ( January 2023 , Zenodo)

   MCMC chains for the GWB analyses performed in the paper "The NANOGrav 15 yr Data Set: Search for Signals from New Physics". 

   The data is provided in pickle format. Each file contains a NumPy array with the MCMC chain (with burn-in already removed), and a dictionary with the model parameters' names as keys and their priors as values. You can load them as

   with open ('path/to/file.pkl', 'rb') as pick: temp = pickle.load(pick) params = temp[0] chain = temp[1]

   The naming convention for the files is the following:

   • igw: inflationary Gravitational Waves (GWs)
   • sigw: scalar-induced GWs
     • sigw_box: assumes a box-like feature in the primordial power spectrum.
     • sigw_delta: assumes a delta-like feature in the primordial power spectrum.
     • sigw_gauss: assumes a Gaussian peak feature in the primordial power spectrum.
   • pt: cosmological phase transitions
     • pt_bubble: assumes that the dominant contribution to the GW productions comes from bubble collisions.
     • pt_sound: assumes that the dominant contribution to the GW productions comes from sound waves.
   • stable: stable cosmic strings
     • stable-c: stable strings emitting GWs only in the form of GW bursts from cusps on closed loops.
     • stable-k: stable strings emitting GWs only in the form of GW bursts from kinks on closed loops.
     • stable-m: stable strings emitting monochromatic GW at the fundamental frequency.
     • stable-n: stable strings described by numerical simulations including GWs from cusps and kinks.
   • meta: metastable cosmic strings
     • meta-l: metastable strings with GW emission from loops only.
     • meta-ls metastable strings with GW emission from loops and segments.
   • super: cosmic superstrings.
   • dw: domain walls
     • dw-sm: domain walls decaying into Standard Model particles.
     • dw-dr: domain walls decaying into dark radiation.

   For each model, we provide four files. One for the run where the new-physics signal is assumed to be the only GWB source. One for the run where the new-physics signal is superimposed to the signal from Supermassive Black Hole Binaries (SMBHB), for these files "_bhb" will be appended to the model name. Then, for both these scenarios, in the "compare" folder we provide the files for the hypermodel runs that were used to derive the Bayes' factors.

   In addition to chains for the stochastic models, we also provide data for the two deterministic models considered in the paper (ULDM and DM substructures). For the ULDM model, the naming convention of the files is the following (all the ULDM signals are superimposed to the SMBHB signal, see the discussion in the paper for more details)

   • uldm_e: ULDM Earth signal.
   • uldm_p: ULDM pulsar signal
     • uldm_p_cor: correlated limit
     • uldm_p_unc: uncorrelated limit
   • uldm_c: ULDM combined Earth + pulsar signal direct coupling 
     • uldm_c_cor: correlated limit
     • uldm_c_unc: uncorrelated limit
   • uldm_vecB: vector ULDM coupled to the baryon number
     • uldm_vecB_cor: correlated limit
     • uldm_vecB_unc: uncorrelated limit 
   • uldm_vecBL: vector ULDM coupled to B-L
     • uldm_vecBL_cor: correlated limit
     • uldm_vecBL_unc: uncorrelated limit
   • uldm_c_grav: ULDM combined Earth + pulsar signal for gravitational-only coupling
     • uldm_c_grav_cor: correlated limit
       • uldm_c_cor_grav_low: low mass region  
       • uldm_c_cor_grav_mon: monopole region
       • uldm_c_cor_grav_low: high mass region
     • uldm_c_unc: uncorrelated limit
       • uldm_c_unc_grav_low: low mass region  
       • uldm_c_unc_grav_mon: monopole region
       • uldm_c_unc_grav_low: high mass region

   For the substructure (static) model, we provide the chain for the marginalized distribution (as for the ULDM signal, the substructure signal is always superimposed to the SMBHB signal)

    

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5. The NANOGrav 12.5 yr Data Set: Search for Gravitational Wave Memory

   https://doi.org/10.3847/1538-4357/ad0726  

   Agazie, Gabriella ; Arzoumanian, Zaven ; Baker, Paul T. ; Bécsy, Bence ; Blecha, Laura ; Blumer, Harsha ; Brazier, Adam ; Brook, Paul R. ; Burke-Spolaor, Sarah ; Burnette, Rand ; et al ( February 2024 , The Astrophysical Journal)

   We present the results of a Bayesian search for gravitational wave (GW) memory in the NANOGrav 12.5 yr data set. We find no convincing evidence for any gravitational wave memory signals in this data set. We find a Bayes factor of 2.8 in favor of a model that includes a memory signal and common spatially uncorrelated red noise (CURN) compared to a model including only a CURN. However, further investigation shows that a disproportionate amount of support for the memory signal comes from three dubious pulsars. Using a more flexible red-noise model in these pulsars reduces the Bayes factor to 1.3. Having found no compelling evidence, we go on to place upper limits on the strain amplitude of GW memory events as a function of sky location and event epoch. These upper limits are computed using a signal model that assumes the existence of a common, spatially uncorrelated red noise in addition to a GW memory signal. The median strain upper limit as a function of sky position is approximately 3.3 × 10⁻¹⁴. We also find that there are some differences in the upper limits as a function of sky position centered around PSR J0613−0200. This suggests that this pulsar has some excess noise that can be confounded with GW memory. Finally, the upper limits as a function of burst epoch continue to improve at later epochs. This improvement is attributable to the continued growth of the pulsar timing array.

    

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