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Periodic Report Summary 1 - GW ASAP (Gravitational Wave Astrophysics and Analysis with Pulsars)

1. Publishable Summary

There are three main objectives to GW ASAP. The first is the Host Identification Project, where I identify local galaxies which are massive enough to host supermassive black hole binary (SMBHB), using galaxy catalogues such as 2MASS. Some of these binaries will be emitting gravitational waves (GWs) in the pulsar timing array (PTA) band, and may contribute to local anisotropy in the nanoHertz GW background. By seeding these galaxies with SMBHBs, I create a realistic GW background which can be used to assess, and map, the local degree of GW anisotropy. The second goal is to integrate anisotropic searches into various PTA data analysis libraries, thereby providing us with a new and more general tool to search for GWs with PTAs. The map of the GW anisotropy will enable us to restrict the priors that we use in the likelihood function evaluation— an important step for direct GW detection. The third objective is to use the anisotropy method to search for resolvable GW sources, e.g. SMBHBs. The anisotropic search routine can be extended to a multipole moment determined by the angular resolution of the PTA, and can serve as a coarse all-sky filter to search for GW “hotspots”. We can then run the search for single sources in these restricted areas of higher probability. In summary, during my outgoing phase I largely completed Objectives 1 and 2 from GW ASAP, led developments in astrophysics and cosmology in all three PTAs (EPTA, NANOGrav and PPTA), and explored the multi-messenger potential of fast radio bursts.

- Work performed since the beginning of the project
Since beginning this project, I have used galaxy catalogues to search for nearby SMBHB host galaxies which could contribute to anisotropy in the nanohertz GW background (Objective 1), and have integrated theoretical techniques used to search for anisotropy into major PTA libraries (Objective 2). Starting with a rigorous analysis of the assumptions made in the PTA GW searches (Mingarelli & Sidery 2014), I co-led the first search for anisotropy using EPTA data (Taylor, Mingarelli et al. 2015; Figure 1 attached). I am now leading the NANOGrav effort to constrain stochastic GW background anisotropy (Mingarelli for NANOGrav, in prep; Figure 1 attached), and contributed to the theoretical development of PTA science which can be done with the Square Kilometre Array (Janssen et al. 2014). Using EPTA & NANOGrav data, I analyzed extensive datasets (Desvignes et al. 2016; Arzoumanian et al. 2015), placing limits on the isotropic stochastic background and developing new skills to set limits on the primordial GWs from inflation (Lentati et al. 2015; Arzoumanian et al. 2016). The primordial limits garnered much interest, and led to several spontaneous collaborations, described below. In Arzoumanian et al., 2016, I led the implementation of new techniques to search for a turnover in the GW strain spectrum, due to the individual SMBHBs coupling with their environment. Such couplings are predicted from final parsec physics — without external environmental factors such as interactions with gas and stars, the binary may stall at a separation of 1pc, not merging in a Hubble time. This work also led to my writing of a rebuttal letter to Shannon et al. (2015), who claimed that the non-detection of the GW background was mysterious. I clarify that while this is disappointing, it is unsurprising due to our ignorance of the final parsec astrophysics (Mingarelli for NANOGrav 2016). Work is now almost complete for Objective 1 (Figure 1 (right) attached), where I use the 2MASS galaxy catalogue to search for SMBHB hosts.

Spontaneous Collaborations: Opportunities for spontaneous collaborations have arisen during my outgoing phase, which have produced high-impact publications in the fields of cosmology and multi-messenger astronomy. After reading about the new cosmological constraints I produced for Lentati et al. (2015), I was approached by Paul Lasky to work on Parkes PTA data.
Together we developed new ways of constraining cosmological parameters, using data from GW experiments across 29 decades in frequency (Lasky, Mingarelli et al., PRX, 2016; Figure 2, attached). I also helped to develop a new formalism where one can map the nanohertz GW background using Cosmic Microwave Background methods, in order to retain polarization information of the background (Gair et al., PRD, 2014). We then applied this method to ground-based interferometers (Romano et al., PRD, 2015), which is the first time PTA methods have informed ground-based interferometer GW searches. I also had the opportunity to collaborate with Prof. Janna Levin of Columbia University, who was visiting my Outgoing Host Institution (Caltech). Together with Joe Lazio, we developed a model for fast radio burst progenitors, called “black hole batteries”, where a neutron star’s magnetic field interacts with a black hole before merger. These fast radio bursts have a distinctive double peak, with the possibility of a third if the signal is strong enough, and could also produce GWs in the LIGO band (Mingarelli, Levin, Lazio, ApJL, 2015).

- Main Results
A publication list is below. Furthermore, I gave invited talks at McGill University (Montreal, Canada), NASA Headquarters (Washington, DC), NASA’s Goddard Spaceflight Center (Greenbelt, MD), Canadian Institute for Advanced Research meeting (Whistler, BC), Caltech (Pasadena, USA), and Columbia University (New York, USA). I presented my work at the American Physical Society Meeting (Apr 2015 & 2016) NANOGrav and IPTA meetings, and the American Astronomical Society Meeting (Jan 2015). Outreach included 6 podcasts, an invited article for Scientific American and appearance on Amy Poehler’s “Smart Girls”— an online show with millions of viewers aimed at inspiring young women (see Sec 5).

(1) Z. Arzoumanian et al. (including C. M. F. Mingarelli), ApJ 821, Issue 1, id. 13, (2016)
(2) S. Babak et al. (including C. M. F. Mingarelli), MNRAS, Vol 455, (2016)
(3) N. Caballero et al. (including C. M. F. Mingarelli), MNRAS, Vol 457, (2016)
(4) G. Desvignes et al. (including C. M. F. Mingarelli), MNRAS, Vol 458, (2016)
(5) J. R. Gair, J. D. Romano, S. R. Taylor, C. M. F. Mingarelli, PRD 90, 082001, (2014)
(6) G. Janssen et al. (including C. M. F. Mingarelli), Proceedings of Science, PoS(AASKA14) 037, (2014)
(7) P. Lasky, C. M. F. Mingarelli et al., Phys. Rev. X, Vol 6, Issue 1, id. 011035 (2016)
(8) L. Lentati et al. (including C. M. F. Mingarelli), MNRAS, Vol 458, (2016)
(9) L. Lentati, S. R. Taylor, C. M. F. Mingarelli et al., MNRAS, Vol 453, (2015)
(10) C. M. F. Mingarelli for NANOGrav, arXiv:1602.06301, (2016)
(11) C. M. F. Mingarelli, J. Levin, T. J. W. Lazio, ApJ Letters 814, L20, (2015)
(12) C. M. F. Mingarelli, T. Sidery, PRD 90, 062011, (2014)
(13) J. D. Romano, S. R. Taylor, N. J. Cornish, J. Gair, C. M. F. Mingarelli, R. van Haasteren, PRD 92, 042003, (2015)
(14) S. R. Taylor, M. Vallisneri, J. A. Ellis, C. M. F. Mingarelli, T. J. W. Lazio, R. van Haasteren, ApJ Letters, 819, L6, (2016)
(15) S. R. Taylor, C. M. F. Mingarelli et al., Phys. Rev. Lett. 115, 041101, (2015)
(16) J. P. W. Verbiest et al. (including C. M. F. Mingarelli), MNRAS, Vol 457, (2016)

- Expected final results and their potential impact and use
By largely achieving the anisotropic analysis goal before completing the Host Identification Project, I took advantage of the momentum developed by working with the EPTA & NANOGrav data analysis libraries. Techniques from Taylor, Mingarelli et al. (2015) are used in Mingarelli for NANOGrav (2016; in prep), together with new and novel ways of searching for anisotropy (Objective 2). The second data release from the IPTA will likely be available by September 2016, at which point I will commence the analysis of the data, and the investigation into using anisotropic searches to search for continuous (or resolvable) GW sources — Objective 3, which is about to commence.
By giving invited talks on GW ASAP, I’ve been able to expand my network of experts who support my Host Identification Project (first objective; using galaxy catalogues to build a GW background) to include Prof. Chung Pei Ma and Dr. Steve Croft of UC Berkeley, and Dr. Jenny Greene of Princeton University. This project is reaching maturity and is expected to be submitted to ApJ Letters by September 2016. The impact of GW ASAP is already being felt: this new line of work has attracted high-profile astrophysicists to work with me on pulsar timing arrays, expanding the reach of the field and the scope of future projects.
By the end of GW ASAP, I expect to add at least 3 more 1st author publications to GW ASAP (the NANOGrav anisotropic paper, the Host Identification Project and the IPTA anisotropic analysis), to bring the total number of publications to 19, 18 of which are peer-reviewed and 5 of which are 1st author.

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