Spectral energy distribution for Mrk 501 averaged over all observations taken during the multifrequency campaign performed between 2009 March 15 (MJD 54905) and 2009 August 1 (MJD 55044). See Figure 8 below for further details.

Reference:  A. A. Abdo et al.,  Astrophysical Journal 727: 129, 2011 (Fermi-LAT, VERITAS, GASP-WEBT & multi-wavelength partners)

Full text version here

ArXiv version: ArXiV:1011.5260

Contact person: Daniel Gall

We report on the γ-ray activity of the blazar Mrk 501 during the first 480 days of Fermi operation. We find that the average LAT γ-ray spectrum of Mrk501 can be well described by a single power-law function with a photon index of 1.78±0.03. While we observe relatively mild flux variations with the Fermi-LAT (within less than a factor of 2), we detect remarkable spectral variability where the hardest observed spectral index within the LAT energy range is 1.52 ± 0.14, and the softest one is 2.51 ± 0.20. These unexpected spectral changes do not correlate with the measured flux variations above 0.3 GeV. In this paper, we also present the first results from the 4.5-month-long multifrequency campaign (2009 March 15 – August 1) on Mrk501, which included the VLBA, Swift, RXTE, MAGIC and VERITAS, the F-GAMMA, GASP-WEBT, and other collaborations and instruments which provided excellent time and energy coverage of the source throughout the entire campaign. The extensive radio to TeV data set from this campaign provides us with the most detailed spectral energy distribution yet collected for this source during its relatively low activity. The average spectral energy distribution of Mrk501 is well described by the standard one-zone syn- chrotron self-Compton model. In the framework of this model, we find that the dominant emission region is characterized by a size ~0.1 pc (comparable within a factor of few to the size of the partially-resolved VLBA core at 15-43 GHz), and that the total jet power (≃ 10⁴⁴ erg s⁻¹) constitutes only a small fraction (∼ 10⁻³) of the Eddington luminosity. The energy distribution of the freshly-accelerated radiating electrons required to fit the time-averaged data has a broken power-law form in the energy range 0.3 GeV−10 TeV, with spectral indices 2.2 and 2.7 below and above the break energy of 20 GeV. We argue that such a form is consistent with a scenario in which the bulk of the energy dissipation within the dominant emission zone of Mrk 501 is due to relativistic, proton-mediated shocks. We find that the ultrarelativistic electrons and mildly relativistic protons within the blazar zone, if comparable in number, are in approximate energy equipartition, with their energy dominating the jet magnetic field energy by about two orders of magnitude.

Figures from paper (click to get full size image):

Figure 1: Left: Fermi-LAT γ-ray flux in the energy range 0.3 − 400GeV (top panel) and spectral photon index from a power-law fit (bottom panel) for Mrk501 for 30-day time intervals from 2008 August 5 (MJD 54683) to 2009 November 27 (MJD 55162). Vertical bars denote 1σ uncertainties and the horizontal bars denote the width of the time interval. The red dashed line and the red legend show the results from a constant fit to the time interval MJD 54862–54982, while the black dashed line and black legend show the results from a constant fit to the entire 480-day data set. Right: Scatter plot of the photon index vs flux values.

Figure 2: Multifrequency light curves of Mrk501 with 30-day time bins obtained with 3 all-sky-monitoring instruments: RXTE -ASM (2 − 10 keV, first from the top); Swift -BAT (15 − 50 keV, second) and Fermi -LAT for two different energy ranges (0.2 − 2 GeV, third, and > 2 GeV, fourth). The light curves cover the period from2008 August 5 (MJD 54683) to 2009 November 27 (MJD 55162). Vertical bars denote 1σ uncertainties and horizontal bars show the width of the time interval. The horizontal dashed lines and the legends (for all the plots) show the results from a constant fit to the entire 480-day data set.

Figure 3: Fractional variability parameter for 16 months data (2008 August 5 — 2009 November 27) from 3 all-sky-monitoring instruments: RXTE -ASM (2 − 10 keV); Swift - BAT (15 − 50 keV) and Fermi -LAT (two energy ranges 0.2 − 2 GeV and 2 − 300 GeV). The fractional variability was computed according to Vaughan et al. (2003) using the light curves from Figure 2. Vertical bars denote 1σ uncertainties and horizontal bars indicate the width of each energy bin. The horizontal dashed line and the legend show the results from a constant fit.

Figure 4: Spectral energy distribution for Mrk501 from Fermi-LAT during the period from 2008 August 5 (MJD 54683) to 2009 November 27 (MJD 55162). The black line depicts the result of the unbinned likelihood power-law fit, the red contour is the 68% uncertainty of the fit, and the black data points show the energy fluxes in differential energy ranges. The legend reports the results from the unbinned likelihood power-law fit in the energy range 0.3 − 400 GeV.

Figure 5: Spectral energy distribution for Mrk501 from Fermi-LAT for six 30-day time intervals: MJD 54832–54862 (top left), MJD 54862–54892 (top right), MJD 54892–54922 (middle left), MJD 54922–54952 (middle right), MJD 54952–54982 (bottom left) and MJD 54982–55012 (bottom right). In all the panels, the black line depicts the result of the unbinned likelihood power-law fit, the red contour denotes the 68% uncertainty of the fit, and the black data points show the energy fluxes computed for differential energy ranges. The blue arrows denote 95% upper limits, which were computed for the differential energy ranges with a signal of T S n region size decreased by an order of magnitude (tvar ≃ 0.35 day). See paper for further discussion.

Figure 6: Time and energy coverage during the multifrequency campaign. For the sake of clarity, the minimum observing time displayed in the plot was set to half a day.

Figure 7: Spectral energy distribution for Mrk 501 from Fermi -LAT for several time intervals of interest. The panel (a) shows the SED for the time period before the multifrequency campaign (MJD 54683–54901), the panel (b) for the time interval corresponding to the multifrequency campaign (MJD 54905–55044) and the panel (c) for the period after the campaign (MJD 55044–55162). In all panels, the black line depicts the result of the unbinned likelihood power-law fit, the red contours denote the 68% uncertainty of the power-law fit and blue arrows denote upper limits at 95% confidence level, which were computed for the differential energy ranges with a signal of TS

Figure 8: Spectral energy distribution for Mrk 501 averaged over all observations taken during the multifrequency campaign performed between 2009 March 15 (MJD 54905) and 2009 August 1 (MJD 55044). The legend reports the correspondence between the instruments and the measured fluxes. Further details about the instruments are given in §5.1. The optical and X-ray data have been corrected for Galactic extinction, but the host galaxy (which is clearly visible at the IR/optical frequencies) has not been subtracted. The TeV data from MAGIC and VERITAS have been corrected for the absorption in the extragalactic background light using the model reported in Franceschini et al. (2008). The VERITAS data from the time interval MJD 54952.9–54955.9 were removed from the data set used to compute the average spectrum, and are depicted separately in the SED plot (in green diamonds). See paper for further details.

Figure 9: Top panel: Enlargement of the γ-ray energy range from Figure8. Bottom panel: Same SED as in the top panel, but with the Fermi -LAT data from the multifrequency campaign split in two data sets: MJD 54952–54982 (open blue squares) and the rest (filled blue circles).

Figure 10: The SSC model fits to the broadband emission spectrum of Mrk501, averaged over all the observations made during the multifrequency campaign performed between 2009 March 15 (MJD 54905) and 2009 August 1 (MJD 55044). The red bow-tie in the figure corresponds to the 68% containment of the power-law fit to the average Fermi-LAT spectrum (photon index 1.74 ± 0.05). The dotted black curve denotes the fit to the starlight emission of the host galaxy assuming a template of a luminous elliptical as given in Silva et al. (1998). The details of the model are given in §6. The black curves correspond to the main set of the model parameters considered (variability timescale tvar ≃ 4 days), while the red dot-dashed curves to the alternative set of the model parameters with the emission region size decreased by an order of magnitude (tvar ≃ 0.35 day). See paper for further discussion.

Figure 11: The jet comoving energy density of ultrarelativistic electrons per logarithmic energy bin, γUe′(γ), as a function of the electron Lorentz factor γ (solid black curve). For comparison, the comoving energy density of the magnetic field (solid red line) and syn- chrotron photons (dotted blue line) are shown. The dashed blue curve denotes the comoving energy density of synchrotron photons which are inverse-Compton upscattered in the Thom- son regime for a given electron Lorentz factor γ.

Figure 12: The decomposition of the SSC continuum for Mrk 501. The data points are the same as in the bottom panel of Figure 9. The SSC fit to the average spectrum is denoted by the solid black curve. Top: Contributions of the different segments of the electron spectrum Comptonizing the whole synchrotron continuum. Bottom: Contributions of the different segments of the electron spectrum (as in the top panel) Comptonizing different segments of the synchrotron continuum.