Light Echoes of Transients and Variables in the Local Universe

Abstract

Astronomical light echoes, the time-dependent light scattered by dust in the vicinity of varying objects, have been recognized for over a century. Initially, their utility was thought to be confined to mapping out the three-dimensional distribution of interstellar dust. Recently, the discovery of spectroscopically useful light echoes around centuries-old supernovae in the Milky Way and the Large Magellanic Cloud has opened up new scientific opportunities to exploit light echoes.In this review, we describe the history of light echoes in the local Universe and cover the many new developments in both the observation of light echoes and the interpretation of the light scattered from them. Among other benefits, we highlight our new ability to classify outbursting objects spectroscopically, view them from multiple perspectives, obtain a spectroscopic time series of the outburst, and establish accurate distances to the source event. We also describe the broader range of variable objects with properties that may be better understood from light-echo observations. Finally, we discuss the prospects of new light-echo techniques not yet realized in practice.

Observational Clues to the Progenitors of Type Ia Supernovae

“…A light echo found by Rest et al (2005) indicates an explosion that is 600 ± 200 years old, and Rest, Sinnott & Welch (2012) indicate that the echo's spectrum confirms a SN Ia classification (but the spectrum is still unpublished). In this case, Edwards, Pagnotta & Schaefer (2012) again using HST imaging, are able to rule out the presence of surviving post-main-sequence donor stars, but 27 main-sequence stars, that are close enough to the explosion center, cannot be excluded as surviving companions.…”

LATE-TIME PHOTOMETRY OF TYPE IA SUPERNOVA SN 2012cg REVEALS THE RADIOACTIVE DECAY OF 57Co

“…Light-echo flux is dominated by the light of the SN around peak, so to first order the color of the light echo is similar to the color of the SN at peak. In addition, the scattering by the dust favors blue light, which can shift the color of the light curve bluewards by a few tenths of a magnitude, depending on the scattering angle (e.g., Rest et al 2012).…”

Asymmetry in the Outburst of Sn 1987a Detected Using Light Echo Spectroscopy

“…We calculate the integrated scattering function for each LE (i.e., scattering angle) using the method described in Rest et al (2012b), using the "LMC avg" carbonaceous-silicate grain model of Weingartner & Draine (2001). We then fit the model spectrum to the observed LE spectrum (see, e.g., lower panel of Figure 3) varying three parameters: the reddening (E(B − V ) assuming R v = 3.2), a normalization constant, and a small flux offset (to account for small errors in sky subtraction).…”

“…Rest et al (2011a) used LE spectroscopy to detect asymmetries in the outburst of the Cas A SN, while Rest et al (2012a) recently obtained LE spectroscopy from the "Great Eruption" of η Carinae. We refer the reader to Rest et al (2012b) for a review of LE spectroscopy emphasizing these more recent results. Given the acceleration of the field of LE spectroscopy in recent years, a detailed study of spectra of the well-known LE system of SN 1987A can provide a foundation for future studies, as well as provide new insight into the explosion of SN 1987A.…”