VHE $γ$-ray observations of bright BL Lacs with the Large-Sized Telescope prototype (LST-1) of the CTAO

TL;DR

This work assesses the commissioning performance of the CTAO Large-Sized Telescope prototype (LST-1) on bright BL Lacs, delivering time-resolved VHE spectra and joint LST-1/Fermi-LAT SEDs for five canonical blazars. Using Bayesian blocks, long-term and intra-night variability are quantified, and LP models are often preferred over power laws in the spectral fits, with EBL absorption folded into the analyses. The study demonstrates a clear harder-when-brighter trend for Mrk 421, tight constraints on intra-night emission region sizes, and notable inter-instrument consistency in spectral shapes. A Blazar Detectability Simulation shows LST-1 could detect VHE emission from sources beyond the gamma-ray horizon up to redshifts around $z\sim 1.2$, highlighting the instrument’s potential for expanding the observable VHE universe and guiding CTAO-era blazar science.

Abstract

Cherenkov Telescope Array Observatory (CTAO) is the next-generation ground-based gamma-ray observatory operating in the energy range from 20 GeV up to 300 TeV, with two sites in La Palma (Spain) and Paranal (Chile). It will consist of telescopes of three sizes, covering different parts of the large energy range. We report on the performance of Large-Sized Telescope prototype (LST-1) in the detection and characterization of extragalactic gamma-ray sources, with a focus on the reconstructed gamma-ray spectra and variability of classical bright BL Lacertae objects, which were observed during the early commissioning phase of the instrument. LST-1 data from known bright gamma-ray blazars - Markarian 421, Markarian 501, 1ES 1959+650, 1ES 0647+250, and PG 1553+113 - were collected between July 10, 2020, and May 23, 2022, covering a zenith angle range of 4 deg to 57 deg. The reconstructed light curves were analyzed using a Bayesian block algorithm to distinguish the different activity phases of each blazar. Simultaneous Fermi-LAT data were utilized to reconstruct the broadband $γ$-ray spectra for the sources during each activity phase. High-level reconstructed data in a format compatible with gammapy are provided together with measured light curves and spectral energy distributions (SEDs) for several bright blazars and an interpretation of the observed variability in long and short timescales. Simulations of historical flares are generated to evaluate the sensitivity of LST-1. This work represents the first milestone in monitoring bright BL Lacertae objects with a CTAO telescope.

Figures (11)

• Figure 1: LST-1 and Fermi-LAT flux light curves for energies above $100\,$GeV (150 GeV for 1ES 1959+650) and $300\,$MeV, respectively. From top to bottom, Mrk 421, Mrk 501, 1ES 1959+650, 1ES 0647+250, and PG 1553+113 are shown. Vertical gray lines show the bin edges obtained from applying the Bayesian block algorithm and the horizontal red lines and shaded areas show the average flux in each block and its standard deviation, respectively. The Crab Nebula flux is depicted with a horizontal gray dashed line aleksic2015measurement.
• Figure 2: Correlation between PWL spectral index $\alpha$ and the night-wise integral flux, as observed with LST-1 above 100 GeV for Mrk 421. Filled circles and open triangles indicate spectrum fits that result in a $p$-value above and below 0.27%, respectively.
• Figure 3: Intra-night variability of Mrk 421 flare on MJD 59717. The light curve above 100 GeV is constructed with 4 data runs and the total exposure time is $\sim53$ min. A time-binning of $\sim3.3$ min is used. Note that "run-wise" corresponds to one wobble observation time, which is $\sim13$ min in this data. The solid orange curve corresponds to a fit with the function given in Eq. \ref{['eq:risefall']}. Gray shows the Crab Nebula flux above 100 GeV as in Fig. \ref{['fig:Fermi_LST_LC']}.
• Figure 4: Spectral energy distribution of Mrk 421, segmented into six blocks identified by the Bayesian block algorithm, with filled circles representing LST-1 observations and open squares denoting Fermi-LAT observations. The hatched and shaded areas indicate the statistical uncertainties of the spectral fits for Fermi-LAT (PWL model) and LST-1 (LP model) including EBL absorption, respectively, with each dataset fitted independently. The corresponding model parameters are shown in Tables \ref{['tab:Mrk421_LST_Fermi_SED']} and \ref{['tab:fermi_fit']}.
• Figure 5: Joint-fit SED of LST-1 (filled circle) and Fermi-LAT (open square) using a LP model with EBL included. The shaded area shows a statistical error in the spectrum fit.