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SUMMARY:Probing Anisotropies in the Gravitational Wave Background with Pul
 sar Timing Arrays
DTSTART:20260512T103000Z
DTEND:20260512T113000Z
DTSTAMP:20260503T112400Z
UID:indico-event-9339@scitalks.tifr.res.in
DESCRIPTION:Speakers: Deepali Agarwal (University of Texas\, Brownsville\,
  USA)\n\nRecent pulsar timing array (PTA) observations provide evidence fo
 r a common-spectrum process with emerging spatial correlations\, consisten
 t with a gravitational-wave background (GWB). While its origin remains unc
 ertain---ranging from supermassive black hole binaries to cosmological sou
 rces---the signal is currently consistent with isotropy\, with no confirme
 d detection of anisotropies. However\, effects such as large-scale structu
 re\, Poisson fluctuations from a finite number of sources\, and line-of-si
 ght projection can induce anisotropies\, particularly in an astrophysical 
 background\, making them a promising probe of the signal's origin.\nIn thi
 s talk\, I will briefly review methods for mapping the angular distributio
 n of GWB intensity with PTAs and present recent results addressing two key
  questions. First\, assuming a statistically isotropic universe\, I will e
 xamine how anisotropies affect the recovery of the Hellings-Downs correlat
 ion curve when used as a validation tool without explicitly modeling aniso
 tropy. Second\, I will discuss how the non-uniform sky response of PTAs le
 ads to mode suppression and multipole coupling. In practice\, analyses rel
 y on truncating spherical harmonic expansions at a finite angular scale\, 
 often chosen to equal the number of distinct pulsar pairs in an array\, Np
 air\, following the counting argument that cross-correlations provide at m
 ost Npair independent constraints. Commonly used truncations at low multip
 oles fail to capture all the information available in the array\, leading 
 to leakage-induced bias in the reconstructed angular power spectrum. This 
 necessitates truncating analyses at higher multipole\, determined by the p
 ulsar positions on the sky rather than solely by Npair. However\, extendin
 g to higher multipoles renders the inverse problem ill-conditioned\, requi
 ring regularization techniques such as truncated singular value decomposit
 ion. Even with these approaches\, only a limited number of angular modes c
 an be robustly constrained\, highlighting an intrinsic trade-off between b
 ias reduction and statistical uncertainty in anisotropy reconstruction.\n\
 nhttps://scitalks.tifr.res.in/event/9339/
LOCATION:AG-66
URL:https://scitalks.tifr.res.in/event/9339/
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