H.E.S.S. observations of the flaring gravitationally lensed galaxy PKS 1830-211

TL;DR

This paper reports on H.E.S.S. observations of the gravitationally lensed quasar PKS 1830-211 following a Fermi-LAT detected flare, but finds no significant gamma-ray signal above 67 GeV, setting upper limits on its flux.

Contribution

First H.E.S.S. observations of PKS 1830-211 after a Fermi-LAT flare, providing constraints on very-high-energy gamma-ray emission from a lensed quasar.

Findings

No significant gamma-ray excess detected by H.E.S.S.

Established upper limits on PKS 1830-211 flux above 67 GeV.

Compared H.E.S.S. upper limits with Fermi-LAT flare spectrum extrapolation.

Abstract

PKS 1830-211 is a known macrolensed quasar located at a redshift of z=2.5. Its high-energy gamma-ray emission has been detected with the Fermi-LAT instrument and evidence for lensing was obtained by several authors from its high-energy data. Observations of PKS 1830-211 were taken with the H.E.S.S. array of Imaging Atmospheric Cherenkov Telescopes in August 2014, following a flare alert by the Fermi- LAT collaboration. The H.E.S.S observations were aimed at detecting a gamma-ray flare delayed by 20-27 days from the alert flare, as expected from observations at other wavelengths. More than twelve hours of good quality data were taken with an analysis threshold of $\sim67$ GeV. The significance of a potential signal is computed as a function of the date as well as the average significance over the whole period. Data are compared to simultaneous observations by Fermi-LAT. No photon excess or significant signal is detected. An upper limit on PKS 1830-211 flux above 67 GeV is computed and compared to the extrapolation of the Fermi-LAT flare spectrum.

Figures

Tables

Table 1. Table 1: Analysis results of observations of PKS 1830-211 by H.E.S.S.

Reconstruction	$N_{\mathrm{ON}}$	$N_{\mathrm{background}}$	significance ($\sigma$)
Mono	1641	1649.2	-0.2
Combined	935	954.4	-0.6

Full text

††pubyear: 2019††pagerange: H.E.S.S. observations of the flaring gravitationally lensed galaxy PKS 1830-211–LABEL:lastpage

1Centre for Space Research, North-West University, Potchefstroom 2520, South Africa

2Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, D 22761 Hamburg, Germany

3Max-Planck-Institut für Kernphysik, P.O. Box 103980, D 69029 Heidelberg, Germany

4Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland

5National Academy of Sciences of the Republic of Armenia, Marshall Baghramian Avenue, 24, 0019 Yerevan, Republic of Armenia

6Yerevan Physics Institute, 2 Alikhanian Brothers St., 375036 Yerevan, Armenia

7Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, D 12489 Berlin, Germany

8University of Namibia, Department of Physics, Private Bag 13301, Windhoek, Namibia

9GRAPPA, Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands

10Department of Physics and Electrical Engineering, Linnaeus University, 351 95 Växjö, Sweden

11Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, D 44780 Bochum, Germany

12Institut für Astro- und Teilchenphysik, Leopold-Franzens-Universität Innsbruck, A-6020 Innsbruck, Austria

13School of Physical Sciences, University of Adelaide, Adelaide 5005, Australia

14LUTH, Observatoire de Paris, PSL Research University, CNRS, Université Paris Diderot, 5 Place Jules Janssen, 92190 Meudon, France

15Sorbonne Université, Université Paris Diderot, Sorbonne Paris Cité, CNRS/IN2P3, Laboratoire de Physique Nucléaire et de Hautes Energies, LPNHE, 4 Place Jussieu, F-75252 Paris, France

16Laboratoire Univers et Particules de Montpellier, Université Montpellier, CNRS/IN2P3, CC 72, Place Eugène Bataillon, F-34095 Montpellier Cedex 5, France

17IRFU, CEA, Université Paris-Saclay, F-91191 Gif-sur-Yvette, France

18Astronomical Observatory, The University of Warsaw, Al. Ujazdowskie 4, 00-478 Warsaw, Poland

19Aix Marseille Université, CNRS/IN2P3, CPPM, Marseille, France

20Instytut Fizyki Ja̧drowej PAN, ul. Radzikowskiego 152, 31-342 Kraków, Poland

21Funded by EU FP7 Marie Curie, grant agreement No. PIEF-GA-2012-332350

22School of Physics, University of the Witwatersrand, 1 Jan Smuts Avenue, Braamfontein, Johannesburg, 2050 South Africa

23Laboratoire d’Annecy de Physique des Particules, Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, LAPP, 74000 Annecy, France

24Landessternwarte, Universität Heidelberg, Königstuhl, D 69117 Heidelberg, Germany

25Université Bordeaux, CNRS/IN2P3, Centre d’Études Nucléaires de Bordeaux Gradignan, 33175 Gradignan, France

26Oskar Klein Centre, Department of Physics, Stockholm University, Albanova University Center, SE-10691 Stockholm, Sweden

27Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, D 72076 Tübingen, Germany

28Laboratoire Leprince-Ringuet, Ecole Polytechnique, CNRS/IN2P3, F-91128 Palaiseau, France

29APC, AstroParticule et Cosmologie, Université Paris Diderot, CNRS/IN2P3, CEA/Irfu, Observatoire de Paris, Sorbonne Paris Cité, 10, rue Alice Domon et Léonie Duquet, 75205 Paris Cedex 13, France

30Univ. Grenoble Alpes, CNRS, IPAG, F-38000 Grenoble, France

31Department of Physics and Astronomy, The University of Leicester, University Road, Leicester, LE1 7RH, United Kingdom

32Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland

33Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24/25, D 14476 Potsdam, Germany

34Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen Centre for Astroparticle Physics, Erwin-Rommel-Str. 1, D 91058 Erlangen, Germany

35DESY, D-15738 Zeuthen, Germany

36Obserwatorium Astronomiczne, Uniwersytet Jagielloński, ul. Orla 171, 30-244 Kraków, Poland

37Centre for Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87-100 Torun, Poland

38Department of Physics, University of the Free State, PO Box 339, Bloemfontein 9300, South Africa

39Department of Physics, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501, Japan

40Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), The University of Tokyo Institutes for Advanced Study (UTIAS), The University of Tokyo, 5-1-5 Kashiwa-no-Ha, Kashiwa City, Chiba, 277-8583, Japan

41Department of Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

42RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan

43Now at Physik Institut, Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland

44Now at Institut de Ciències del Cosmos (ICC UB), Universitat de Barcelona (IEEC-UB), Martí Franquès 1, E08028 Barcelona, Spain

⋆ Corresponding author

† Deceased

1 The PKS 1830-211 gravitationally lensed quasar

PKS 1830-211 is a high redshift (z=2.5 (Lidman et al., 1999)) Flat Spectrum Radio Quasar (FSRQ) which has been detected in all wavelengths from radio to high-energy gamma-rays. It is a known gravitationally lensed object with two compact images of the quasar nucleus visible in the radio (Jauncey et al., 1991) and optical (Meylan et al., 2005) passbands. The Einstein ring, well visible at radio frequencies, comes from the imaging of the quasar jet (Kochanek & Narayan, 1992). The quasar source is lensed by a foreground galaxy at z=0.89 (Wiklind & Combes, 1996). The angular size of the Einstein ring and separation of compact images is roughly 1 arcsecond so that it cannot be resolved with high-energy instruments such as H.E.S.S. (50 GeV-50 TeV range) or Fermi-LAT (100 MeV-100 GeV range). PKS 1830-211 is seen as a bright, high-energy source by the Fermi-LAT instrument and had several flaring periods during the decade of Fermi-LAT observations. PKS 1830-211 is listed in the 1FHL (Ackermann et al., 2013) and the 3FHL (Ajello et al., 2017) catalogues with a photon index above 10 GeV of $3.55\pm 0.34$ which corresponds to the average "low-state" spectrum. No significant curvature in the spectrum was detected. Photons up to 35 GeV, potentially detectable by H.E.S.S. have been observed by Fermi-LAT (Ajello et al., 2017). Observations of these very high energy photons and the measurement of the very high energy tail of the spectrum would give useful constraints on EBL at redshift z=2.5.

Since the components of the lens cannot be resolved at high or very high energy, the evidence for lensing was searched indirectly on the observed light curve. Due to the different travel paths, the light curves of the two compact components of the lens have a relative time delay, measured in the radio (Lovell et al., 1998) and microwave (Wiklind & Combes, 2001) pass-bands, of $26\pm 5$ days. Barnacka et al. (2011) have studied the first three years of the Fermi-LAT light curve with cepstral and autocorrelation methods. Evidence for a delay of $27.5\pm 1.3$ days was found with a 3 $\sigma$ significance. The time delay between the compact images of PKS 1830-211 was also studied by the Fermi-LAT collaboration (Abdo et al., 2015). They selected several flaring periods and calculated the autocorrelation function of the light curve. No significant peak was found. A possible peak of $\sim 20$ days was found with a 1-day binning of the data, which could be attributed to the $\sim 20$ days separation between two flaring events and perhaps to gravitational lensing. Barnacka et al. (2015) have argued that the time delay measured by high-energy instruments could be very different than the value measured by radio telescopes. The delay measured by Lovell et al. (1998) is obtained from the emission of the compact images. Since the jet of the PKS 1830-211 source is imaged close to the Einstein ring, the time difference between the intial burst and its lensed image can be much smaller if the source of high-energy emission is located inside the jet.

PKS 1830-211 is monitored by Fermi-LAT and its light curve is posted on the internet 111https://fermi.gsfc.nasa.gov/ssc/data/access/lat/msl_lc/source/PKS_1830-211 on a daily basis. H.E.S.S. observations of PKS 1830-211 were triggered by an alert posted by the Fermi-LAT team on August 2, 2014 (Krauss et al., 2014). The flare seen by the Fermi-LAT instrument started on July 27 and lasted $\sim$4 days. The H.E.S.S. observations are described in Section 2 and data analysis in Section 3. The H.E.S.S. limits are compared to the Fermi-LAT signal in Section 4 and discussed in Section 5.

2 H.E.S.S. observations

The very-high-energy (50 GeV-50 TeV range) gamma-ray observatory of the H.E.S.S. collaboration consists of five Imaging Atmospheric Cherenkov Telescopes (IACTs) located in the Khomas Highland of Namibia ($23^{\circ}$ 16’ 18” S, $16^{\circ}$ 30’ 1” E), 1800 m above sea level. From January 2004 to October 2012, the array was a four-telescope instrument, with telescopes labeled CT1-4. Each of the telescopes, located at the corners of a square with a side length of 120 m has an effective mirror surface area of 107 m2, and is able to detect cosmic gamma-rays in the energy range 0.1 – 50 TeV . In October 2012, a fifth telescope CT5, with an effective mirror surface area of 600 m2 and an improved camera (Bolmont et al., 2014) was installed at the center of the original square, giving access to energies below 100 GeV (H.E.S.S. Collaboration et al., 2017).

PKS 1830-211 was observed by the five telescopes of the H.E.S.S. IACT array between August 12 2014 and August 26 2014, to allow for the detection of delayed flares with time delays ranging from 20 to 27 days. The observations were taken at an average zenith angle of 12 degrees.

3 Data analyses

This paper is based on a sample of 12.4 hours of high quality data. Data selection cuts have been described in (H.E.S.S. Collaboration et al., 2017). Data were next analyzed with the Model analysis (de Naurois & Rolland, 2009) and cross-checked with the ImPACT analysis (Parsons & Hinton, 2014), the two methods giving compatible results. The two analyses use different calibration chains. With both reconstruction chains, data of CT5 were analyzed either alone (Mono reconstruction) or combined with the CT1-4 data (Combined reconstruction). The Mono reconstruction has an energy threshold of 67 GeV. The Combined reconstruction has a higher threshold of 144 GeV, but a larger effective area.

A point source is searched at the location of PKS 1830-211. Fig. 1 shows the distribution of the squared angular distance $\theta^{2}$ of candidate photons from the target position. This distribution, obtained in the Mono analysis, is compared to the background from hadrons mis-identified as photons. The background is calculated with the ring background method (Berge et al., 2007), other methods giving similar results.

Table 1 summarizes the number of candidate photons in the signal region, the expected background and the significance of the excess, calculated with Li and Ma formula 17 (Li & Ma, 1983).

No significant excess of photons over background is seen by H.E.S.S. at the position of PKS 1830-211. A similar search using the Combined analysis also gives a negative result.

Because of the very soft spectrum measured by Fermi LAT in the low state, PKS 1830-211 has a chance of being detectable by H.E.S.S. only during flares. The delayed flare lasts only less than about 4 days, however, due to the uncertainties on the date of the flare, it could have happened at any time between August 17 = MJD 56886 (time delay of 20 days) and August 24 = MJD 56893 (radio time delay of 27 days) as explained in Section 1. Fig. 2 shows the evolution over time of significance, binned by 28-minute runs. No significant daily photon excess was detected during the H.E.S.S. observation period.

4 Flux upper limits and comparison to the Fermi-LAT spectra

The non-detection by H.E.S.S. translates into 99% confidence level (C.L.) upper limits on the average very-high-energy flux of PKS 1830-211 during H.E.S.S. observations. These upper limits are shown in Fig. 3. Red (resp. blue) arrows show the limits obtained from the Mono (resp. Combined) analysis and the corresponding solid lines show the effect of deabsorption using the Extragalactic Background Light (EBL) model of Gilmore et al. (2012).

H.E.S.S. upper limits are compared to Fermi-LAT GeV spectra on Fig 4. The Fermi-LAT observations have been analyzed with Fermi Science Tools v10-r0p5 and Pass8 data, in the Enrico framework (Sanchez & Deil, 2013). The spectral data from 26-30 July 2014 (flare) are well described by a power-law spectrum with an index of $n_{flare}=-2.36\pm 0.17$ for photon energies $>1$ GeV. The relatively high low-energy cut was used to avoid contamination from the Galactic plane. The flare spectrum is much harder than the spectrum measured in the low state of PKS 1830-211, but $n_{flare}$ is compatible with the photon indices of previous flare spectra, as measured by Fermi-LAT (Abdo et al., 2015). The spectrum of PKS 1830-211 obtained from the Fermi-LAT observations within the H.E.S.S. observation window is well described by a power-law with an index of $n_{low}=-2.97\pm 0.44$ above 1 GeV. The value of $n_{low}$ is compatible with the value published in the 3FHL catalogue.

A proper comparison between H.E.S.S. upper limits and the Fermi signal has to take into account the effect of the absorption of the flux of PKS 1830-211 by the EBL and the difference between the flare duration and H.E.S.S. exposure. Since no significant curvature of the spectrum was measured bt the Fermi-LAT collaboration, the unabsorbed spectrum was modeled by a powerlaw. The effect of light absorption by EBL from PKS 1830-211 has been estimated with the models of Gilmore et al. (2012) (black dash-dotted line), Finke et al. (2010) (dotted line) and Franceschini et al. (2008) (black solid line). Fig. 4 shows that there is a substantial difference between the predictions of these models for a source at redshift 2.5 such as PKS 1830-211. Note that EBL absorption could also be affected by the lens environment. Light from lensed AGN is expected to be more absorbed than average, due to the presence of galaxies along the line of sight. Indeed, absorption from the intervening galaxy has been detected by de Rosa et al. (2005) in the X-ray spectrum of PKS 1830-211. However Barnacka et al. (2014) and Böttcher & Thiersen (2016) have argued that gravitational lensing could help gamma-rays from a distant source avoiding excess absorption. The Fermi-LAT flare spectrum from Fig 4 is a 4-night average while the H.E.S.S. exposure amounts to 10 nights of data taking. The steady source upper limits from Fig 3 are thus a factor $\sim\sqrt{10/4}$ too constraining, which is corrected for in Fig 4.

5 Conclusion

No significant delayed flare from PKS 1830-211 was detected by either H.E.S.S. or Fermi-LAT. The flare did not repeat or was too faint to be detected. Fig 4 shows however that the detection of a strong flare would have been possible close to the Mono analysis energy threshold if the level of EBL absorption was at or below the absorption predicted by the model of Franceschini et al. (2008). Due to its lensed nature, observation of flaring event of PKS 1830-211 in the TeV passband could be useful to constrain EBL models at redshift as large as 2.5. The detection of the lensing time delay in future very high energy observations would help pinpoint the spatial origin of the high-energy emission (Barnacka et al., 2015). It would also permit more exotic applications such as constraining photon mass (Glicenstein, 2017) or testing Lorentz Invariance Violation (Biesiada & Piórkowska, 2009).

Acknowledgments

The support of the Namibian authorities and of the University of Namibia in facilitating the construction and operation of H.E.S.S. is gratefully acknowledged, as is the support by the German Ministry for Education and Research (BMBF), the Max Planck Society, the German Research Foundation (DFG), the Helmholtz Association, the Alexander von Humboldt Foundation, the French Ministry of Higher Education, Research and Innovation, the Centre National de la Recherche Scientifique (CNRS/IN2P3 and CNRS/INSU), the Commissariat à l’ Energie Atomique et aux Energies Alternatives (CEA), the U.K. Science and Technology Facilities Council (STFC), the Knut and Alice Wallenberg Foundation, the National Science Centre, Poland grant no. 2016/22/M/ST9/00382, the South African Department of Science and Technology and National Research Foundation, the University of Namibia, the National Commission on Research, Science & Technology of Namibia (NCRST), the Austrian Federal Ministry of Education, Science and Research and the Austrian Science Fund (FWF), the Australian Research Council (ARC), the Japan Society for the Promotion of Science and by the University of Amsterdam. We appreciate the excellent work of the technical support staff in Berlin, Zeuthen, Heidelberg, Palaiseau, Paris, Saclay, Tüebingen and in Namibia in the construction and operation of the equipment. This work benefited from services provided by the H.E.S.S. Virtual Organisation, supported by the national resource providers of the EGI Federation