[Paper Review] WEBT and XMM-Newton observations of 3C 454.3 during the post-outburst phase. Detection of the little and big blue bumps

C. M. Raiteri, M. Villata+4 more|arXiv (Cornell University)|Aug 21, 2007

Astrophysics and Cosmic PhenomenaPhysics and Astronomy43 references71 citations

TL;DR

This study presents multiwavelength observations of the blazar 3C 454.3 during its post-outburst faint state, detecting a 'little blue bump' in optical bands due to broad emission lines from the broad-line region and a 'big blue bump' in UV data from XMM-Newton's Optical Monitor, indicating thermal emission from the accretion disc. The X-ray spectra require extra absorption beyond the Galactic value, with possible variability or intrinsic spectral curvature.

ABSTRACT

The blazar 3C 454.3 underwent an unprecedented optical outburst in spring 2005. This was first followed by a mm and then by a cm radio outburst, which peaked in February 2006. We report on follow-up observations by the WEBT to study the multiwavelength emission in the post-outburst phase. XMM-Newton observations on July and December 2006 added information on the X-ray and UV fluxes. The source was in a faint state. The radio flux at the higher frequencies showed a fast decreasing trend, which represents the tail of the big radio outburst. It was followed by a quiescent state, common at all radio frequencies. In contrast, moderate activity characterized the NIR and optical light curves, with a progressive increase of the variability amplitude with increasing wavelength. We ascribe this redder-when-brighter behaviour to the presence of a "little blue bump" due to line emission from the broad line region, which is clearly visible in the source SED during faint states. Moreover, the data from the XMM-Newton OM reveal a rise of the SED in the UV, suggesting the existence of a "big blue bump" due to thermal emission from the accretion disc. The X-ray spectra are well fitted with a power-law model with photoelectric absorption, possibly larger than the Galactic one. However, the comparison with previous X-ray observations would imply that the amount of absorbing matter is variable. Alternatively, the intrinsic X-ray spectrum presents a curvature, which may depend on the X-ray brightness. In this case, two scenarios are possible.

Motivation & Objective

• To study the multiwavelength emission of 3C 454.3 in the post-outburst phase following its 2005 optical and X-ray outburst.
• To investigate the nature of the faint-state spectral features, particularly the 'little blue bump' and 'big blue bump', obscured during the outburst.
• To analyze X-ray spectral properties and determine whether observed absorption is variable or due to intrinsic spectral curvature.
• To understand the connection between radio, optical, and X-ray variability, especially the delayed radio outburst and its relation to earlier optical flares.

Proposed method

• Conducted radio, near-infrared, and optical monitoring using the Whole Earth Blazar Telescope (WEBT) from September 2006 to April 2007.
• Performed two XMM-Newton observations in July and December 2006, including X-ray and UV data from the Optical Monitor.
• Constructed broadband spectral energy distributions (SEDs) at different epochs to analyze spectral features.
• Fitted X-ray spectra with power-law models with and without photoelectric absorption, comparing Galactic and extra absorption components.
• Used statistical tests to evaluate the significance of extra absorption and spectral curvature in X-ray data.
• Analyzed variability amplitude trends across wavelengths to infer contributions from non-jet components like the broad-line region and accretion disc.

Experimental results

Research questions

• RQ1 What causes the increasing variability amplitude with longer wavelength in the optical and near-IR bands during the faint state?
• RQ2 Is the 'little blue bump' in the optical SED due to emission lines from the broad-line region, and is it detectable only in faint states?
• RQ3 Does the UV SED feature (spectral break in the U band) indicate thermal emission from the accretion disc, i.e., a 'big blue bump'?
• RQ4 Is the observed X-ray absorption variable, or does it reflect intrinsic curvature in the soft X-ray spectrum?
• RQ5 Can the XMM-Newton X-ray data be better explained by variable extra absorption or by a softening of the intrinsic spectrum due to a high-frequency tail of the big blue bump?

Key findings

• The source was in a faint state from mid-2006 to April 2007, with moderate variability in the near-IR and optical bands, and a fast decline in high-frequency radio flux following the 2006 February peak.
• The 'little blue bump' was clearly visible in the optical SED, peaking around the V and B bands, and attributed to Fe ii and Mg ii emission lines and Balmer continuum from the broad-line region.
• A second, minor bump in the J band likely corresponds to the Hα line emission.
• XMM-Newton's Optical Monitor revealed a spectral break in the U band, indicating a 'big blue bump' likely due to thermal emission from the accretion disc.
• X-ray spectra required extra absorption beyond the Galactic value—approximately 20% in July and 40% in December—suggesting variable or environmental absorption.
• The data do not support a curved intrinsic X-ray spectrum with Galactic absorption; instead, the softening in faint X-ray states may result from flux contribution by the high-frequency tail of the big blue bump, implying possible constant extra absorption with spectral softening.