The Remarkably Featureless High-Resolution X-Ray Spectrum of Markarian 478

Abstract

An observation of Mrk 478 using the Chandra Low Energy Transmission Grating Spectrometer is presented. The source exhibited 30%–40% flux variations on timescales of ∼104 s together with a slow decline in the spectral softness over the full 80 ks observation. The 0.15–3.0 keV spectrum is well fitted by a single power law with photon index Γ = 2.91 ± 0.03. Combined with high-energy data from BeppoSAX, the spectrum from 0.15 to 10 keV is well fitted as the sum of two power laws with Γ1 = 3.03 ± 0.04, which dominates below 2 keV, and Γ2 = 1.4 ± 0.2, which dominates above 2 keV (quoting 90% confidence uncertainties). No significant emission or absorption features are detected in the high-resolution spectrum, supporting our previous findings using the Extreme Ultraviolet Explorer but contradicting the earlier claims of emission lines by Hwang & Bowyer. There is no evidence of a warm absorber as found in the high-resolution spectra of many Seyfert 1 galaxies, including others classified as narrow-line Seyfert 1's such as Mrk 478. We suggest that the X-ray continuum may result from Comptonization of disk thermal emission in a hot corona through a range of optical depths.