[Paper Review] WEBT multiwavelength monitoring and XMM-Newton observations of BL Lacertae in 2007-2008. Unveiling different emission components

C. M. Raiteri, M. Villata+9 more|arXiv (Cornell University)|Sep 9, 2009

Astrophysics and Cosmic PhenomenaPhysics and Astronomy59 references2 citations

TL;DR

This study presents multiwavelength observations of BL Lacertae from 2007–2008, combining 37 optical-to-radio telescopes with three XMM-Newton X-ray pointings. It proposes a two-component synchrotron plus inverse-Compton emission model, plus thermal disc radiation, to explain the UV excess and variable X-ray spectrum, finding a disc temperature >20,000 K and luminosity >6×10⁴⁴ erg s⁻¹, indicating significant accretion activity.

ABSTRACT

In 2007-2008 we carried out a new multiwavelength campaign of the Whole Earth Blazar Telescope (WEBT) on BL Lacertae, involving three pointings by the XMM-Newton satellite, to study its emission properties. The source was monitored in the optical-to-radio bands by 37 telescopes. The brightness level was relatively low. Some episodes of very fast variability were detected in the optical bands. The X-ray spectra are well fitted by a power law with photon index of about 2 and photoelectric absorption exceeding the Galactic value. However, when taking into account the presence of a molecular cloud on the line of sight, the data are best fitted by a double power law, implying a concave X-ray spectrum. The spectral energy distributions (SEDs) built with simultaneous radio-to-X-ray data at the epochs of the XMM-Newton observations suggest that the peak of the synchrotron emission lies in the near-IR band, and show a prominent UV excess, besides a slight soft-X-ray excess. A comparison with the SEDs corresponding to previous observations with X-ray satellites shows that the X-ray spectrum is extremely variable. We ascribe the UV excess to thermal emission from the accretion disc, and the other broad-band spectral features to the presence of two synchrotron components, with their related SSC emission. We fit the thermal emission with a black body law and the non-thermal components by means of a helical jet model. The fit indicates a disc temperature greater than 20000 K and a luminosity greater than 6 x 10^44 erg/s.

Motivation & Objective

• To disentangle the complex broad-band emission components in BL Lacertae beyond the standard synchrotron-self-Compton (SSC) model.
• To investigate the origin of the UV excess and soft X-ray excess observed in XMM-Newton data.
• To determine whether multiple non-thermal components or additional emission mechanisms are required to explain the spectral energy distributions (SEDs).
• To constrain the accretion disc properties and black hole mass using simultaneous radio-to-X-ray data.
• To test the validity of the helical jet model in explaining the observed variability and spectral features.

Proposed method

• Multiwavelength monitoring using 37 telescopes across radio to optical bands, with simultaneous XMM-Newton X-ray observations.
• Spectral energy distribution (SED) construction from contemporaneous radio-to-X-ray data at XMM-Newton observation epochs.
• Fitting of X-ray spectra with single and double power laws to assess curvature and soft excess presence.
• Modeling of thermal emission using a black body law for the accretion disc.
• Fitting of non-thermal components using the helical jet model of Villata & Raiteri (1999), assuming two distinct emitting regions in a helical jet.
• Comparison of X-ray spectral variability with previous observations from BeppoSAX and other satellites to assess spectral state evolution.

Experimental results

Research questions

• RQ1 What causes the prominent UV excess observed in the SEDs of BL Lacertae during the 2007–2008 campaign?
• RQ2 Why does the X-ray spectrum show a soft excess that varies dramatically between observations?
• RQ3 Can the observed SEDs be explained by a single synchrotron-self-Compton (SSC) model, or are multiple non-thermal components required?
• RQ4 What is the origin of the X-ray spectral curvature, and how does it vary over time?
• RQ5 Is the observed emission consistent with a helical jet structure hosting two distinct emitting regions?

Key findings

• The X-ray spectrum is best fitted by a double power law, indicating a concave shape, especially when accounting for a molecular cloud along the line of sight.
• The UV excess is attributed to thermal emission from an accretion disc with a temperature exceeding 20,000 K and luminosity greater than 6×10⁴⁴ erg s⁻¹.
• The broad-band SEDs require two synchrotron components with their associated SSC emission, indicating two distinct emitting regions in the jet.
• The accretion disc luminosity implies a lower limit on the accretion rate of 0.2 M⊙ yr⁻¹, assuming a radiative efficiency of η ≈ 0.06.
• The black hole mass is constrained to be greater than 6×10⁶ M⊙ if the luminosity approaches the Eddington limit.
• The model reproduces the GeV spectrum with a photon index of approximately 2 and is consistent with the TeV spectrum observed by MAGIC in 2005.