PKS1830-211: a flaring gravitationally lensed galaxy

June 2019

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The Extragalactic Background Light (EBL) is the diffuse electromagnetic radiation which permeates the Universe. The largest part of the energy density of EBL is contributed by the Cosmic Microwave Background (CMB). The second highest contribution is the infrared light which is due in part to starlight. The level of infrared EBL at a given redshift is thus a measure of stellar light emission in the past, and depends on galactic evolution. Light from distant very high energy sources is absorbed by the EBL at very high energy and affects the observed spectra of these sources. The amount of EBL can therefore be measured by studying absorption features in the spectra of distant sources (see our source of the month and reference [1]).

Gravitational lensing is a process where light from distant sources is concentrated by gravitational deflection towards the observer, in complete analogy to optical mirages (Fig. 1). As in mirages and other atmospheric phenomena [2], light can travel by different paths and produce multiple apparent images of the same object. Because of the light concentration, lensed objects are expected to be seen further away than other sources.

PKS 1830-211 is a high redshift (z=2.5) Flat Spectrum Radio Quasar which is known to be lensed by a galaxy at redshift z=0.89 [3]. It has been extensively studied in all bands, from radio to high-energy gamma rays. The radio (insert from Fig.1) and X-ray observations show two compact images of the quasar core with an angular separation of the order of an arcsecond and an elliptical Einstein ring. Due to different travel paths, the time arrival of photons from the two compact images have a relative delay. This delay has been measured in the radio passband to be 26 ± 5 days [4].

Fig. 1: Illustration of gravitational lensing. The light from the distant PKS 1830-211 blazar is deflected by an intervening galaxy at z=0.89. The insert on the bottom left hand side shows a MERLIN image at 5 GHz. Two compact images of the quasar core and an Einstein ring are seen.

PKS 1830-211 has been detected in high-energy gamma rays by the Fermi-LAT instrument onboard the Fermi satellite. Fermi-LAT has detected photons up to 35 GeV [5], close to the energy range of H.E.S.S. II. Since high-energy instruments do not have a sufficient angular resolution to separate the two core images, the measurement of time delay between images is challenging at these energies. Time delays ranging between 20 days to more than 30 days have been claimed. The best opportunity to measure the time delay at high energies is a follow-up on a flare and a search for the delayed echo. H.E.S.S observations of PKS 1830-211 were triggered by a flare alert posted by the Fermi-LAT team on August 2, 2014 [6]. The flare actually started on July 27 and lasted for four days.

Fig. 2: Significance of the excess number of photons in the direction of PKS 1830-211 versus date in MJD (Modified Julian Date), obtained with the large CT5 telescope of H.E.S.S.. No excess is seen. The red arrow shows the date of the delayed flare for a lensing time delay of 27 days.

PKS 1830-211 was observed by the five telescopes of the H.E.S.S. array between August 12, 2014 and August 26, 2014, to allow for the detection of delayed flares with time delays ranging from 16 to 30 days. More than 12 hours of high quality data were analysed. Two telescope configurations were used for reconstruction. The first reconstruction ("Mono"), aimed at obtaining a very low energy threshold using only the large CT5 telescope. The other reconstruction ("Stereo") provides a larger energy coverage, but a higher energy threshold and uses all five telescopes of the H.E.S.S. array.

As illustrated on Figure 2, the delayed flare of PKS 1830-211 was not detected during the observation period by either H.E.S.S or Fermi-LAT. Either the flare did not repeat, or was too faint to be detected. Since the Fermi-LAT instrument detected photons in the energy range of tens of GeV from PKS 1830-211, a detection by the H.E.S.S. array could be possible in the future thanks to its low energy threshold (Fig. 3). Such detection would put interesting constraints on the EBL models at high redshifts.

Fig. 3: Spectrum of PKS 1830-211 from Fermi-LAT observations between July 27 and July 30, 2014, and in August 2014. The black lines show the influence of EBL on the spectra predicted by the models of Gilmore et al. [7] (black dash-dotted line), Finke et al. [8] (dotted line) and Franceschini et al. [9] (black solid line).


[1] H. E. S. S. Collaboration (H. Abdalla et al.), "Measurement of the EBL spectral energy distribution using the VHE γ-ray spectra of H.E.S.S. blazars", Astronomy & Astrophysics, 606 (2017) 59
[2] J.F. Nye, "Natural focusing and fine structure of light", Institute of Physics, Bristol and Philadelphia (1999)
[3] T. Wiklind and F. Combes, "The redshift of the gravitational lens of PKS1830-211 determined from molecular absorption lines", Nature 379 (1996) 139
[4] J.E.J. Lovell et al., "The Time Delay in the Gravitational Lens PKS 1830-211", ApJ 508 (1998) L51
[5] Fermi-LAT collaboration (M. Ajello et al.), "3FHL: The Third Catalog of Hard Fermi-LAT Sources", ApJS 232 (2017) 18
[6] F. Krauss et al., The Astronomer's Telegram 6361 (2014)
[7] R.C. Gilmore et al., "Semi-analytic modelling of the extragalactic background light and consequences for extragalactic gamma-ray spectra", MNRAS 422 (2012) 3189
[8] J.D. Finke, S. Razzaque, C.D. Dermer, "Modeling the Extragalactic Background Light from Stars and Dust", ApJ 712 (2010) 238
[9] A. Franceschini, G. Rodighiero, M. Vaccari, "Extragalactic optical-infrared background radiation, its time evolution and the cosmic photon-photon opacity" A&A 487 (2008) 837