Rubin-LSST

Abstract:

<jats:title>Abstract</jats:title> <jats:p>We present a formalism to extract the angular power spectrum of fields sampled at a finite number of points with arbitrary positions — a common situation for several catalog-based astrophysical probes — through a simple extension of the standard pseudo-<jats:italic>C<jats:sub>ℓ</jats:sub> </jats:italic> algorithm. A key complication in this case is the need to handle the shot noise component of the associated discrete angular mask which, for sparse catalogs, can lead to strong coupling between very different angular scales. We show that this problem can be solved easily by estimating this contribution analytically and subtracting it. The resulting estimator is immune to small-scale pixelization effects and aliasing, and, most notably, unbiased against the contribution from measurement noise uncorrelated between different sources. We demonstrate the validity of the method in the context of cosmic shear datasets, and showcase its usage in the case of other spin-0 and spin-1 astrophysical fields of interest. We incorporate the method in the public <jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="uri" xlink:href="https://github.com/LSSTDESC/NaMaster" xlink:type="simple">&lt;monospace&gt;NaMaster&lt;/monospace&gt;</jats:ext-link> code.</jats:p>

Abstract:

Fast X-ray transients (FXTs) are extragalactic bursts of X-rays first identified in archival X-ray data and are now routinely discovered in real time by the Einstein Probe, which is continuously surveying the night sky in the soft (0.5–4 keV) X-ray regime. In this Letter, we report the discovery of the second optical counterpart (AT 2024gsa) to an FXT (EP 240414a). EP 240414a is located at a projected radial separation of 27 kpc from its likely host galaxy at z = 0.4018 ± 0.0010. The optical light curve of AT 2024gsa displays three distinct components. The initial decay from our first observation is followed by a rebrightening episode, displaying a rapid rise in luminosity to an absolute magnitude Mr ∼ −21 after two rest-frame days. While the early optical luminosity and decline rate are similar to those of luminous fast blue optical transients, the color temperature of AT 2024gsa is distinctly red and we show that the peak flux is inconsistent with a thermal origin. The third component peaks at Mi ∼ −19 at ≳16 rest-frame days post-FXT, and is compatible with an emerging supernova. We fit the riz-band data with a series of power laws and find that the decaying components are in agreement with gamma-ray burst afterglow models, and that the rebrightening may originate from refreshed shocks. By considering EP 240414a in context with all previously reported known-redshift FXT events, we propose that Einstein Probe FXT discoveries may predominantly result from (high-redshift) gamma-ray bursts, and thus appear to be distinct from the previously discovered lower-redshift, lower-luminosity population of FXTs.