Hard X-ray emission of the luminous infrared galaxy NGC 6240 as observed by NuSTAR

¹ ASDC−ASI, via del Politecnico, 00133 Roma, Italy
e-mail: puccetti@asdc.asi.it
² INAF−Osservatorio Astronomico di Roma, via Frascati 33, 00078 Monte Porzio Catone (RM), Italy
³ INAF−Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁴ EMBIGGEN Anillo, Concepción, Chile
⁵ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, 306, Santiago 22, Chile
⁶ Millenium Institute of Astrophysics, Casilla 360 Santiago, Chile
⁷ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado 80301, USA
⁸ Department of Astronomy & Astrophysics, 525 Davey Laboratory, Pennsylvania State University, University Park, PA 16802, USA
⁹ Institute for Gravitation and the Cosmos, Pennsylvania State University, University Park, PA 16802, USA
¹⁰ Department of Physics, 104 Davey Laboratory, Pennsylvania State University, University Park, PA 16802, USA
¹¹ Cahill Center for Astrophysics, California Institute of Technology, 1216 East California Boulevard, Pasadena, CA 91125, USA
¹² Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Mail Stop 169-221, Pasadena, CA 91109, USA
¹³ Yale Center for Astronomy and Astrophysics, Physics Department, Yale University, PO Box 208120, New Haven, CT 06520-8120, USA
¹⁴ Department of Physics, Durham University, Durham DH1 3LE, UK
¹⁵ Instituto de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Gran Bretana N 1111, Playa Ancha, Valparaíso, Chile
¹⁶ Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA
¹⁷ DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 327, 2800 Lyngby, Denmark
¹⁸ Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
¹⁹ School of Physics & Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK
²⁰ Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA
²¹ Institute for Astronomy, Department of Physics, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland
²² Dipartimento di Matematica e Fisica, Universitá degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
²³ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA

Received: 14 August 2015
Accepted: 14 October 2015

Abstract

We present a broadband (~0.3−70 keV) spectral and temporal analysis of NuSTAR observations of the luminous infrared galaxy NGC 6240 combined with archival Chandra, XMM-Newton, and BeppoSAX data. NGC 6240 is a galaxy in a relatively early merger state with two distinct nuclei separated by ~1.̋5. Previous Chandra observations resolved the two nuclei and showed that they are both active and obscured by Compton-thick material. Although they cannot be resolved by NuSTAR, we were able to clearly detect, for the first time, both the primary and the reflection continuum components thanks to the unprecedented quality of the NuSTAR data at energies >10 keV. The NuSTAR hard X-ray spectrum is dominated by the primary continuum piercing through an absorbing column density which is mildly optically thick to Compton scattering (τ ≃ 1.2, N_H ~ 1.5 × 10²⁴ cm^-2). We detect moderately hard X-ray (>10 keV) flux variability up to 20% on short (15−20 ks) timescales. The amplitude of the variability is largest at ~30 keV and is likely to originate from the primary continuum of the southern nucleus. Nevertheless, the mean hard X-ray flux on longer timescales (years) is relatively constant. Moreover, the two nuclei remain Compton-thick, although we find evidence of variability in the material along the line of sight with column densities N_H ≤ 2 × 10²³ cm^-2 over long (~3−15 yr) timescales. The observed X-ray emission in the NuSTAR energy range is fully consistent with the sum of the best-fit models of the spatially resolved Chandra spectra of the two nuclei.