Upper limits on persistent gravitational waves using folded data and the full covariance matrix from Advanced LIGO’s first two observing runs

The stochastic gravitational-wave background (SGWB) created by astrophysical sources in the nearby Universe is likely to be anisotropic. Upper limits on SGWB anisotropy have been produced for all major data-taking runs ground-based laser interferometric detectors. However, due challenges involved numerically inverting pixel-to-pixel noise covariance matrix, which necessary setting upper limits, searches accounted angular correlations map using spherical harmonic basis, where regularization was relatively easier. This approach better suited though extended sources. Moreover, upper-limit maps two different bases are seemingly different. While may consistent within statistical errors, it important check whether results would remain if full matrix used pixel basis. Here, we use Fisher information create of anisotropy. We first perform an unmodeled search persistent, directional folded data from (O1) and second (O2) observing Advanced LIGO show that with published LIGO-Virgo Collaboration (LVC). then explore various ways account singular-value decomposition Bayesian schemes. do not find evidence any signal LVC errors. Through injection study, they valid 95% is, limit a less than injected strength 5% pixels. Remarkably, that, nuances schemes, simplest method convolved (dirty) normalized variance, analysis, provides as good elaborate schemes matrix. Hence, recommend continuing this simple method, more explored obtain stronger limits.