Julian H. Krolik

Publications (2)6.02 Total impact

• Source

  Available from: arxiv.org

  Article: Intrinsic Curvature in the X-Ray Spectra of BL Lacertae Objects

  Eric S. Perlman, Greg Madejski, Markos Georganopoulos, Karl Andersson, Timothy Daugherty, Julian H. Krolik, Travis Rector, John T. Stocke, Anuradha Koratkar, Stefan Wagner, Margo Aller, Hugh Aller, and Mark G. Allen
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  ABSTRACT: We report results from XMM-Newton observations of 13 X-ray bright BL Lacertae objects, selected from the Einstein Slew Survey sample (SSS). The survey was designed to look for evidence of departures of the X-ray spectra from a simple power-law shape (i.e., curvature and/or line features) and to find objects worthy of deeper study. Our data are generally well fit by power-law models, with three cases having hard (Γ < 2; dN/dE E-Γ) spectra that indicate synchrotron peaks at E 5 keV. Previous data had suggested a presence of absorption features in the X-ray spectra of some BL Lac objects. In contrast, none of these spectra show convincing examples of line features in either absorption or emission, suggesting that such features are rare among BL Lac objects, or, more likely, are artifacts caused by instrumental effects. We find significant evidence for intrinsic curvature [steepening by dΓ/d(log E) = 0.4 ± 0.15] in 14 of the 17 X-ray spectra. This cannot be explained satisfactorily via excess absorption, since the curvature is essentially constant from 0.5-6 keV, an observation that is inconsistent with the modest amounts of absorption that would be required. We use the XMM-Newton Optical Monitor data with concurrent radio monitoring to derive broadband spectral energy distributions and peak frequency estimates. From these, we examine models of synchrotron emission and model the spectral curvature we see as the result of episodic particle acceleration.
  The Astrophysical Journal 12/2008; 625(2):727. · 6.02 Impact Factor
• Source

  Available from: arxiv.org

  Article: BL Lac X-ray Spectra: simpler than we thought

  Eric S. Perlman, Timothy Daugherty, Anuradha Koratkar, Grzegorz Madejski, Karl Andersson, Julian H. Krolik, Hugh Aller, Margo Aller, John T. Stocke, Travis A. Rector, Paolo Padovani, Mark Allen, Stefan Wagner
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  ABSTRACT: We report results from {\it XMM-Newton} observations of thirteen X-ray bright BL Lacertae objects, selected from the {\it Einstein} Slew Survey sample. The spectra are generally well fit by power-law models, with four objects having hard ($\alpha<1; F_\nu \propto \nu^{-\alpha}$) spectra that indicates synchrotron peaks at $>5$ keV. None of our spectra show line features, indicating that soft X-ray absorption ``notches'' must be rare amongst BL Lacs, rather than common or ubiquitous as had previously been asserted. We find significant curvature in most of the spectra. This curvature is almost certainly intrinsic, as it appears nearly constant from 0.5 to 6 keV, an observation which is inconsistent with the small columns seen in these sources.
  04/2004;