---
abstract: 'The CELESTE atmospheric Cherenkov detector ran until June 2004. It has observed the blazars Mrk 421, 1ES 1426+428 and Mrk 501. We significantly improved our understanding of the atmosphere using a LIDAR, and of the optical throughput of the detector using stellar photometry. The new data analysis provides better background rejection. We present our light curve for Mrk 421 for the 2002-2004 season and a comparison with X-ray data and the 2004 observation of 1ES 1426+428. The new analysis will allow a more sensitive search for a signal from Mrk 501.'
address: 'CENBG, Domaine du Haut-Vigneau, BP 120, 33175 Gradignan Cedex, France'
author:
- 'Brion, E.'
- and the CELESTE collaboration
title: 'Blazar observations above 60 GeV: the Influence of CELESTE’s Energy Scale on the Study of Flares and Spectra'
---

CELESTE,Cherenkov,Mrk 421,1ES 1426+428,Mrk 501 95.85.Pw ,98.54.Cm

Introduction
============

CELESTE was a Cherenkov experiment using 53 heliostats of the former Électricité de France solar plant in the French Pyrenees at the Thémis site. It detected Cherenkov light from electromagnetic showers produced in the atmosphere by the $\gamma$-rays coming from high energy astrophysical sources. The light is reflected to secondary optics and photomultipliers installed at the top of the tower. Finally it is sampled to be analysed (Paré 2002).

To constrain the energy scale of the experiment we improved the optics simulation, now in good agreement with the data. The data analysis has also been improved so that we have better background rejection. We present the light curve for Mrk 421.

Constraining the energy scale {#sec:simulation}
=============================

The simulation has been reexamined to reduce the uncertainties on the energy scale of the experiment (Brion 2003). The LIDAR operating on the site for atmospheric monitoring provided a better determination of the atmospheric extinction (Bussóns Gordo 2004). A stellar photometry study, focussing on the comparison between simulations and data on bright stars’ currents, has been done on the star 51 UMa (M$_\mathrm{B}=6.16$) which is in the field of view (FOV) of Mrk 421. This showed that the old simulation was too optimistic. All mirror reflectivities were decreased subsequent to new measurements. The nominal focussing of the heliostats was degraded after a study of star image sizes. We also verified the photomultiplier gains. The results for the old data set (40 heliostats, new is 53 helisotats, see § \[sec:analysis\]) are presented in figure \[fig:simulation\]: the effect of these changes is smaller for $\gamma$-ray showers (an extended light source) than for stars (point sources).

![ON$-$OFF illumination from the star 51 UMa (M$_\mathrm{B}=6.16$) in the FOV of Mrk 421 as a function of hour angle for pointing at 11 km: the new simulation with our corrections (red stars) fits the data well (black squares) whereas the old simulation (blue circles) was 50 % too high.[]{data-label="fig:simulation"}](brion_fig1.ps){width="7.0cm"}

This study helped us to perform our selection criteria for the data: all data that are too low in currents on the star have also low trigger rates. In order to trigger CELESTE, the heliostats, except the *veto* (see § \[sec:veto\]), are split into 6 groups. For each of them, the analog sum of signals gives the first level trigger. Then, a logical pattern is defined on the majority of the triggering groups. Thus, in the case of the source Mrk 421, we reject all data with trigger rates under 20 Hz for the old data set and under 16 Hz for the new data set.

We also looked at the proton rates as a standard candle for the detector which should be stable for good quality nights. These rates are determined with high offline threshold cuts to avoid trigger bias, and are therefore low (typical trigger rates $\sim 22$ Hz). We’ve shown that they are correlated with the currents on star 51 UMa for selected data (new data set, figure \[fig:ProtonRate\] (a)). The data with low rates are now still rejected but perhaps some of them could be corrected. Indeed, three doubtful zones, A, B and C on the figure, can be distinguished. The zone A can be interpreted as bad nights with thick cloud cover (weak star, little Cherenkov light), the zone B as nights with aerosols and above average extinction of Cherenkov and starlight, and the zone C as nights with high clouds that stop starlight but not Cherenkov light. The data in this last zone may therefore be used. Figure \[fig:ProtonRate\] (b) shows the correlation between the proton rate and the trigger rate for the same data set. Defining a selection criteria for each type of acquisition, and based only on these rates, would be very interesting for sources that don’t have any star nearby for a photometry study.

![(a) Proton rate as a function of ON$-$OFF current for heliostat E03 that sees 51 UMa when pointing Mrk 421. For a typical photomultiplier tube gain of $5.6\times10^{4}$, $2.5$ A corresponds to 0.28 p.e./ns, as seen in figure \[fig:simulation\]. (b) Proton rate as a function of trigger rate for data on Mrk 421.[]{data-label="fig:ProtonRate"}](brion_fig2.eps "fig:"){width="6.0cm"} ![(a) Proton rate as a function of ON$-$OFF current for heliostat E03 that sees 51 UMa when pointing Mrk 421. For a typical photomultiplier tube gain of $5.6\times10^{4}$, $2.5$ A corresponds to 0.28 p.e./ns, as seen in figure \[fig:simulation\]. (b) Proton rate as a function of trigger rate for data on Mrk 421.[]{data-label="fig:ProtonRate"}](brion_fig3.eps "fig:"){width="6.0cm"}

Analysis improvement {#sec:analysis}
====================

Since the 2000 status of the experiment (de Naurois 2002) three main changes were made for the analysis. First, the selection criteria of the data are stricter for the current and trigger rate stability, and as we’ve shown for the trigger and proton rate value (work in progress). The experiment has been upgraded from 40 to 53 heliostats. We use part of them to broaden our narrow FOV. Finally, we have found a new method to exploit the FADC information to reject the hadronic background (Manseri 2004 a, b). The second and third point are developed hereafter.

The *veto* configuration {#sec:veto}
------------------------

Hadronic showers have a more chaotic and extended development than electromagnetic showers. To measure the extent of the shower, we artificially broaden the FOV: as before, all heliostats aim at 11 km above the ground in the direction of the source, where the maximum of the shower is supposed to occur in our energy range. But 12 heliostats, distributed around the edge of the field, sample a ring of 150 m around that point (figure \[fig:PrincipeVeto\]).

Because of the compactness of electromagnetic showers, the light does not illuminate these 12 heliostats, named [*veto*]{}, contrary to hadronic showers (figure \[fig:NbVetoMC\]). So we require that no *veto* be illuminated.

![Distribution of the number of illuminated *veto* heliostats (simulations).[]{data-label="fig:NbVetoMC"}](brion_fig4.eps){width="\linewidth"}

![Distribution of the number of illuminated *veto* heliostats (simulations).[]{data-label="fig:NbVetoMC"}](brion_fig5.eps){width="\linewidth"}

FADC information
----------------

To detect low energy $\gamma$-rays, we use the sum of the individual digitized signals to increase the signal-to-background ratio. The summation includes a correction for the sphericity of the wavefront, assumed to be centered in the 11 km plane. Assuming a wrong position for this center (impact parameter) broadens the sum: the height-over-width ratio, $(H/W)$, decreases. We compute $(H/W)$ for different assumed positions. The impact parameter is the position for which the $(H/W)$ ratio is maximum, denoted by $(H/W)_{max}$. This is valid for $\gamma$-rays (figure \[fig:Timing\] (a)) but not for protons for which the wavefront is not spherical (figure \[fig:Timing\] (b)). A measurement of the flatness of these 2D-distributions is given by the following estimator: $$\xi = \mathrm{average}\,\left(\frac{\displaystyle(H/W)_{200m}}{\displaystyle(H/W)_{max}}\right)_{\mbox{\small over 24 positions}},$$ where $(H/W)_{200m}$ is the average of $(H/W)$ over 24 positions along a ring 200 m from the maximum position. For $\gamma$-rays there is a clear maximum and this estimator takes low values. For hadrons, it is usually larger (figure \[fig:Timing\] (c)). We have applied $\xi<0.35$.

![Left: the shape of the FADC sum depends on the position of the centre of the wavefront we assume. Right: views of the height-over-width ratio (H/W) computed for discrete positions in the 11 km plane (a) for a 100 GeV $\gamma$ ray and (b) for a 500 GeV proton. (c) Distribution of the $\xi$ parameter for $\gamma$-rays and OFF data.[]{data-label="fig:Timing"}](brion_fig6.eps){width="\textwidth"}

A cut on this estimator is the biggest contribution to an improvement in Crab sensitivity from 2.2 $\sigma/\sqrt{\mathrm{h}}$ to 5.8 $\sigma/\sqrt{\mathrm{h}}$. The Crab detection is also stable (figure \[fig:Crab\]).

![Stability of the Crab detection after hour angle efficiency corrections. Line is $6.5\pm0.5\ \gamma/$min.[]{data-label="fig:Crab"}](brion_fig7.ps){width="7.0cm"}

Blazar observations {#sec:blazars}
===================

This stable analysis and good sensitivity provide a good detection of the Mrk 421 flares. A 19 $\sigma$ detection during 10 h since October 2002 gives a mean of 5.6 $\gamma/$min. The light curve is presented in figure \[fig:Mrk421\]. With the new acceptances we will deduce a spectral measurement for Mrk 421 with smaller uncertainties.

![Light curve of Mrk 421. Left: since 2002 seen by CELESTE (red points in $\gamma$/min corrected for hour angle efficiency, 1 point is 20 min data, Crab transit rate shown for reference) and RXTE/ASM (blue squares in mCrab, 6 day bins). Right: light curve of Mrk 421 zoomed on the $17^{\mathrm{th}}$ of March 2004 flare seen by CELESTE.[]{data-label="fig:Mrk421"}](brion_fig8.ps "fig:"){width="6.0cm"} ![Light curve of Mrk 421. Left: since 2002 seen by CELESTE (red points in $\gamma$/min corrected for hour angle efficiency, 1 point is 20 min data, Crab transit rate shown for reference) and RXTE/ASM (blue squares in mCrab, 6 day bins). Right: light curve of Mrk 421 zoomed on the $17^{\mathrm{th}}$ of March 2004 flare seen by CELESTE.[]{data-label="fig:Mrk421"}](brion_fig9.ps "fig:"){width="6.0cm"}

The source 1ES 1426+428 is still not detected with our new analysis during the March 2004 observation for 4.4 h data. Since the data have been taken in the same conditions as for the Crab, we can use the sensitivity of 5.8 $\sigma/\sqrt{\mathrm{h}}$ to remark that for a 3 $\sigma$ observation for these 4.4 h data, the flux $f_\mathrm{1ES\,1426}$ would have to be $f_\mathrm{1ES\,1426} = \frac{3\sigma}{5.8\sigma\sqrt{4.4}}f_\mathrm{Crab} \approx \frac{f_\mathrm{Crab}}{4}$. A better upper limit determination is in preparation.

Finally, the blazar Mrk 501 has been observed during 14.5 h with the previous experiment at 40 heliostats. The old analysis gave a 2.5 $\sigma$ significance (with 850 photons which would give 1 $\gamma/$min). The new analysis will allow an improved investigation of Mrk 501’s behaviour in this unexplored energy range.

Brion, E., Smith, D. and The CELESTE Collaboration, Testing the Energy Scale of the CELESTE experiment, SF2A-2003: Semaine de l’Astrophysique Francaise, pp. 463-+, 2003 (astro-ph/0310148).

Bussóns Gordo, J., Debiais, G., Espigat, P. , LIDAR atmospheric monitoring for the CELESTE gamma-ray experiment, submitted to Astroparticle Physics, 2004.

Manseri, H., Astronomie gamma au-dessus de 30 GeV. Une nouvelles méthode d’identification des rayons $\gamma$ cosmiques à partir du sol avec le détecteur CELESTE, Ph.D. thesis of the École Polytechnique, Paris, France *(in french)*, March 2004.

Manseri, H. and The CELESTE Collaboration, Gamma-ray astronomy above 30 GeV with the CELESTE experiment (1996-2004), International Symposium on High Energy Gamma-Ray Astronomy, 2004.

de Naurois, M., Holder, J., Bazer-Bachi, R. , Measurement of the Crab Flux above 60 GeV with the CELESTE Cerenkov Telescope, ApJ, 566:343-357, February 2002.

Paré, E., Balauge, B., Bazer-Bachi, R. , CELESTE: an atmospheric Cherenkov telescope for high energy gamma astrophysics, NIM, 490:71-89, September 2002.
