First Results from NuSTAR Observations of Mkn 421

¹ Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA
² Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA
³ Kavli Institute for Particle Astrophysics and Cosmology, SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA
⁴ Max-Planck-Institut für Physik, D-80805, München, Germany
⁵ DTU Space – National Space Institute, Technical University of Denmark, Elektrovej 327, 2800 Lyngby, Denmark
⁶ INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, I-23807 Merate, Italy
⁷ Laboratoire Leprince Ringuet, Ecole Polytechnique, F-91128 Palaiseau, France
⁸ ASI – Science Data Center, via Galileo Galilei, I-00044 Frascati, Italy
⁹ Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA
¹⁰ Physics Department, Washington University, St. Louis, MO 63130-4899, USA
¹¹ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
¹² School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA
¹³ Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125, USA
¹⁴ INAF – Osservatorio Astronomico di Roma, via Frascati 33, I-00040 Monteporzio Catone, Italy
¹⁵ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
¹⁶ Dipartimento di Scienza e Alta Tecnologia, Universitá dell’Insubria, Via Valleggio 11, I-22100 Como, Italy
¹⁷ Department of Physics, Yale University, New Haven, CT 06520-8121, USA
¹⁸ Space Science Institute, Boulder, CO 80301, USA
¹⁹ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA

^a e-mail: mislavb@astro.caltech.edu

Published online: 9 December 2013

Abstract

Mkn 421 is a nearby active galactic nucleus dominated at all wavelengths by a very broad nonthermal continuum thought to arise from a relativistic jet seen at a small angle to the line of sight. Its spectral energy distribution peaks in the X-ray and TeV γ-ray bands, where the energy output is dominated by cooling of high-energy electrons in the jet. In order to study the electron distribution and its evolution, we carried out a dedicated multi-wavelength campaign, including extensive observations by the recently launched highly sensitive hard X-ray telescope NuSTAR, between December 2012 and May 2013. Here we present some initial results based on NuSTAR data from January through March 2013, as well as calibration observations conducted in 2012. Although the observations cover some of the faintest hard X-ray flux states ever observed forMkn 421, the sensitivity is high enough to resolve intra-day spectral variability. We find that in this low state the dominant flux variations are smooth on timescales of hours, with typical intra-hour variations of ≲ 5%. We do not find evidence for either a cutoff in the hard X-ray spectrum, or a rise towards a high-energy component, but rather that at low flux the spectrum assumes a power law shape with a photon index of approximately 3. The spectrum is found to harden with increasing brightness.