1ES1218+304 - A flaring blazar with an unusually hard spectrum PDF Print
Night-by-night VHE light curve for 1ES 1218+304 is shown as measured from 2008 December 29 (MJD 54829) to 2009 April 23 (MJD 54944). See Figure 3 below for more details.


Discovery of variability in the very high energy gamma-ray emission of 1ES1218+304 with VERITAS




Reference:  V. A. Acciari et al. (The VERITAS Collaboration), Astrophysical Journal 709: L163-L167, 2010

Full text version

ArXiv version: ArXiV:1001.2590

Contact person: Asif Imran


First detected by the Ariel-5 X-ray satellite, 1ES 1218+304 was one of the earliest BL Lac objects to be discovered based on its X-ray emission. The source was later classified as a high-frequency peaked BL Lac (HBL) object based on the position of its synchrotron peak at a high frequency. Located at a redshift of z=0.182, 1ES 1218+304 belongs to a group of blazars that exhibit unusually hard VHE spectra considering their redshifts. As part of the Key Science Project in 2008-2009, VERITAS carried out an intensive monitoring campaign of the source to characterize emission properties of one of these distant hard spectra blazars in more detail.

Our observations reported an excess of 1155 gamma ray events, corresponding to a 21.8 sigma detection in 27.2 hours of live time. The resulting time-averaged differential energy spectrum is well described by a power law and the average integral photon flux is Φ(E>200~GeV) = (18.4 ± 0.9stat)× 10-12 cm-2s-1 (~ 7% of Crab Nebula flux). The light curve from 1ES 1218+304 shows (see figure) strong nightly flux variations between January 25, 2009 and February 5, 2009. The highest gamma-ray flux recorded on January 30, 2009 reached ~20% of the Crab Nebula flux.

Variability is a prominent feature in the emission properties of blazars. However, to date, all of the hard spectra blazars (1ES 1101-232, 1ES 0347-121, 1ES 0229+200, 1ES1218+304, RGB J0710+591) appear to emit at a baseline level. The increased flux from 1ES 1218+304 is the first clear example of significant variation from a baseline flux. The extended jet emission model suggested a separate but very hard, slowly varying emission component to explain the hardness of the VHE spectra of high redshift blazars. According to this model, electrons in the extended jet inverse-Compton scatter cosmic microwave background photons to TeV energies over a large emission region (R ~ pc scale). However, the requirement for a large emission region is excluded by the R < 0.01 pc constraint imposed by the observed day-scale variability. Therefore, the flaring behavior from 1ES 1218+304 implies that the above model of emission in the  extended jet is unlikely to be the sole explanation for the extreme hardness in the intrinsic spectrum of 1ES 1218+304.


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


Figure 2: Observed differential energy spectrum of 1ES 1218+304 from the 2008-2009 data set. The solid line shows the power law fit to the data (filled circles) with a spectral index value of -3.07 ± 0.09 (stat). This agrees, within statistical uncertainties, with the previous VERITAS measurement of -3.08 ± 0.34 (stat) (dashed line).
Figure 3 (shown above): Night-by-night VHE light curve for 1ES 1218+304 is shown as measured from 2008 December 29 (MJD 54829) to 2009 April 23 (MJD 54944). The open circles represent the integral flux above 200 GeV, Φ(E > 200 GeV) from 1ES 1218+304 assuming a spectral shape, dN/dE ~ E^−Γ with Γ=3.07. The dashed-dotted line corresponds to the average integral flux of Φ (E>200 GeV) = 18.4 ± 0.9 (stat) x 10^-12 cm^-2 s^-1. The inset shows the flux variations for the flaring nights in more detail. An exponential function, e^(λ t) (dashed line) is used to describe the rise and fall time for flux variations giving λ_rise = 0.86 (Χ^2/dof = 3.7/2) and λ_fall = -0.47 (Χ^2/dof = 7.2/6), respectively. The characteristic flux doubling time is estimated to be on the order of one day.

Last Updated on Thursday, 27 May 2010 02:25

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