Browsing by Author "Norman, Colin"

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    A Large Population of Obscured AGN in Disguise as Low-luminosity AGN in Chandra Deep Field South
    (2020) Lambrides, Erini L.; Chiaberge, Marco; Heckman, Timothy; Gilli, Roberto; Vito, Fabio; Norman, Colin
    Population synthesis models of actively accreting supermassive black holes (or active galactic nuclei (AGN)) predict a large fraction that must grow behind dense, obscuring screens of gas and dust. Deep X-ray surveys are thought to have provided the most complete and unbiased samples of AGN, but there is strong observational evidence that a portion of the population of obscured AGN is being missed. In this paper, we use a sample of AGN derived from the deepest X-ray survey to date, the Chandra 7 Ms GOODS-South Survey, to investigate the nature of low-flux X-ray sources. We make full use of the extensive multiwavelength coverage of the GOODS-South field and cross-match our objects with wavelengths from the radio to the IR. We find that the low X-ray flux AGN in our sample have X-ray luminosities that indicate low-luminosity AGN classification, while their radio, IR, and optical counterparts indicate moderate to powerful AGN classification. We find that the predicted column densities are, on average, an order of magnitude higher than the calculated column densities via X-ray detections for X-ray-faint sources. We interpret our results as evidence of obscured AGN disguised as low-luminosity AGN via their X-ray luminosities. When we compare the estimation of the obscured AGN space density with and without these objects, we find a difference of 40% in the lowest X-ray luminosity regime probed by our sample.
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    SN 2019tsf: Evidence for Extended Hydrogen-poor CSM in the Three-peaked Light Curve of Stripped Envelope of a Type Ib Supernova
    (2025) Zenati, Yossef; Wang, Qinan; Bobrick, Alexey; DeMarchi, Lindsay; Glanz, Hila; Rozner, Mor; Jencson, Jacob E.; Rest, Armin; Metzger, Brian D.; Margutti, Raffaella; Gómez, Sebastián; Smith, Nathan; Toonen, Silvia; Bright, Joe S.; Norman, Colin; Foley, Ryan J.; Gagliano, Alexander; Krolik, Julian H.; Smartt, Stephen J.; Villar, Ashley V.; Narayan, Gautham; Fox, Ori; Auchettl, Katie; Brethauer, Daniel; Clocchiatti, Alejandro; Coelln, Sophie V.; Coppejans, Deanne L.; Dimitriadis, Georgios; Dorozsmai, Andris; Drout, Maria; Jacobson-Galan, Wynn; Gao, Bore; Ridden-Harper, Ryan; Kilpatrick, Charles Donald; Laskar, Tanmoy; Matthews, David; Smith, Ken W.; Stauffer, Candice McKenzie; Rest, Sofia; Stroh, Michael C.; Strolger, Louis-Gregory; Terreran, Giacomo; Pierel, Justin D. R.; Piro, Anthony L.
    We present multiband ATLAS and ZTF photometry for SN 2019tsf, a Type Ib stripped-envelope supernova (SESN). The slow spectral evolution could be associated with an uncommon explosion mechanism specific to this SN. Possible explanations include fallback accretion onto a compact remnant or a long-lived central engine, both of which could provide extended energy injection responsible for the late-time rebrightening and unusual spectral features. The rebrightening observations represent the latest photometric measurements of a multipeaked Type Ib SN. As late-time photometry and spectroscopy suggest no hydrogen, the potential circumstellar material (CSM) must be H-poor. The absence of a nebular phase and the lack of narrow emission lines in the late-time spectra (>142 days) of the SNe suggest that any CSM interaction is likely asymmetric and enveloped by the SN ejecta. However, an extended CSM structure is evident through a follow-up radio campaign with the Karl G. Jansky Very Large Array (VLA), indicating a source of bright optically thick radio emission at late times, which is highly unusual among H-poor SESNe. We attribute this phenomenology to an interaction of the supernova ejecta with asymmetric CSM, potentially disk-like, and we present several models that may explain the origin of this rare Type Ib supernova. We propose a warped disk model in which a tertiary companion—commonly present around massive stars—perturbs the progenitor's CSM, producing density enhancements that may explain the observed multipeaked SN 2019tsf light curve. This SN 2019tsf is a unique SN Type Ib among the recently discovered class of SNe that undergo mass transfer at the moment of explosion....