VHE FSRQs with Fermi-LAT: VHE and even brighter states in high-z FSRQs due to an HBL-like component?

TL;DR

This study analyzes 14 years of Fermi-LAT data to understand VHE emission in flat-spectrum radio quasars (FSRQs). It shows that VHE activity is tied to brighter, harder MeV-GeV states and that a bluer-when-brighter trend emerges above a flux threshold, with low-state SEDs following a power-law and high-state/VHE states adopting a log-parabola shape. High-redshift FSRQs often exhibit MeV-GeV peak upshifts, which in some cases require an additional HBL-like component; in others, the VHE emission is explained by continuation of the particle spectrum with EC-IR-dominated seed fields. The authors argue that VHE in FSRQs is driven by a combination of particle-spectrum extension and spectral transitions or new HBL-like components, particularly at high redshift, enabling GeV–VHE flux enhancements despite EBL attenuation without invoking extraordinary brightening.

Abstract

Very high-energy (VHE) detected flat-spectrum radio quasars (FSRQs) are relatively few despite being the most persistent bright MeV-GeV sources. Focusing on VHE emission, we investigated the spectral and temporal properties of VHE-detected FSRQs using 14-year Fermi-LAT data. All are highly variable (flux-amplitude$>$100) with VHE detection associated with brighter flux states and relatively harder spectra. Above a flux limit, flux anti-correlates with spectral index, exhibiting a bluer-when-brighter trend. The low-flux state spectral energy distributions (SEDs) for all resembles a power-law, while high-flux and VHE-associated states resemble a log-parabola, accompanied by an almost nil (PKS0736+017, PKS1510-089) to marginal (4C+21.35, 3C279) to significant (B21420+32, TON0599, PKS1441+25, S30218+35, PKS0346-27, OP313) MeV-GeV peak-upshift -- more prominent in high-redshift sources. For no/marginal peak-upshift, the VHE emission is consistent with external Comptonization of infrared photons (EC-IR) driven primarily by a power-law continuation of the particle spectrum to higher energies. For those with a significant MeV-GeV peak-upshift, PKS0346-27 and OP313 shows peak-upshift in the synchrotron spectrum, and thus VHE is EC-IR origin, while for others without synchrotron-peak upshift, we attribute the VHE to an HBL-like component with a Compton-Dominance (CD) like FSRQs, with VHE driven primarily by particle spectrum continuation. In some, even high-state SEDs seem to require an HBL-like component. Thus, VHE activities in FSRQs mainly result from particle spectrum continuation, aided by spectral transition or a new HBL-like component with FSRQ-like CD. Such spectral changes naturally brighten the GeV-VHE flux, overcoming extragalactic background light absorption without requiring extraordinary brightening under the traditional EC-IR scenario than normally exhibited.

Figures (6)

• Figure 1: Monthly binned light curve of the 9 VHE FSRQs (Table \ref{['tab: 4.1']}) for the duration of 5 August 2008 to August 2022. The Fermi data is marked with blue points along with the error bars in black. Orange arrows mark the upper limits (TS$<9.0$). The Bayesian block is shown in a brown-colored curve, and the number of bins is given by N. The dashed purple region marks the duration coincident with the VHE detection for which VHE SEDs have been extracted. The horizontal black line is the mean flux with a 3-sigma red color band around the mean. The grey shaded regions mark the duration used for the low flux state SED extraction (ref. §\ref{['sec:lowHighstate']}), and the cyan region is used for the high flux state SED. References for VHE detection are: PKS 0736+017 HESS2020_0736_VHE, PKS 1510-089 HESS2013_1510_VHE_20092017_1510_VHE_2016Aharonian_2023, 4C+21.35 MAGIC2011_4c_VHE_20102015_4c_VHE_2014, 3C 279 HESS2019_3C279_2015_VHE, B2 1420+32 MAGIC2021_B2_2020_VHE, TON 0599 2017ATel_TON_2017, PKS 1441+25 Ahnen2015_pks1441, S3 0218+35 Ahnen2016_S3_HBLtpe, PKS 0346-27 2021ATel_0346_VHE.
• Figure 2: Top: Log(flux) histogram plot for all 9 VHE FSRQs with increasing order of redshift from left to right. The solid orange line is a normal fit, and the dashed blue line represents a lognormal fit. The dotted vertical red line represents the value of the log(flux) at the time of the VHE episode. Bottom: Spectral index ($\alpha/\Gamma$) histogram plot for all 9 VHE FSRQs with increasing order of redshift from left to right. The dotted red line marks the spectral index at the time of the VHE episode, and the dashed blue line marks the mean spectral index.
• Figure 3: Spectral index ($\alpha/\Gamma$) versus flux trend for sources in increasing order of redshift from left to right. Red points mark the MeV-GeV spectral index and photon flux during the time of the VHE episode.
• Figure 4: MeV-GeV SEDs from LAT for the 9 VHE FSRQs in increasing order of redshift during low (Low) and high (High) flux states, along with the SED for the entire observation period (Full) and VHE detected durations (VHE; see Figure §\ref{['fig:LC']}). The black arrows mark the upper limits, with dotted curves showing the Fermi-LAT sensitivity for the corresponding states (§\ref{['sec:DataReduction']}). The numerical factor in the 'VHE' label is a scaling factor for a clearer representation. The solid curves are the best-fit model (PL/LP; see Table \ref{['tab:Params']}) with a 1-$\sigma$ shaded region. The lower panel of the SEDs gives the ratio of $\nu F_{\nu}$ to max($\nu F_{\nu}$), a good proxy for measuring peak shifts. The dotted horizontal line marks the value of 1.0.
• Figure 5: Modeled SED for FSRQ 3C 279 demonstrates four different scenarios based on the location of the emission region (ref \ref{['subsec:seedPh']}). The first three figures correspond to cases (a) to (c) detailed in §\ref{['subsec:seedPh']}, where the emission region is within the BLR. The fourth figure depicts the case (d) when the emission region lies outside the BLR. Grey points represent the data points from Roy2021, while red data points represent the average spectra for the entire duration. The dark blue, dark green, deep pink, dark red, and dark orange lines indicate synchrotron EC-BLR, EC-IR (EC-DT), disk thermal, and SSC components, respectively. The dashed black line represents the total spectrum.