Broadband Spectral Energy Distribution Research Articles

A comparative analysis of the active galactic nucleus and star formation characteristics of broad- and narrow-line Seyfert 1 galaxies

We report here our comparative analysis of the active galactic nucleus (AGN) and star formation (SF) characteristics of a sample of narrow-line Seyfert 1 (NLS1) and broad-line Seyfert 1 (BLS1) galaxies. Our sample consisted of 373 BLS1 and 240 NLS1 galaxies and spanned the redshift 0.02 $<$ $z$ $<$ 0.8. The broad-band spectral energy distribution, constructed using data from the ultra-violet to the far-infrared, was modelled using CIGALE to derive the basic properties of our sample. We searched for differences in stellar mass (M$_ ast $), star formation rate (SFR), and AGN luminosity (L$_ AGN $) in the two populations. We also estimated new radiation-pressure-corrected black hole masses for our sample of BLS1 and NLS1 galaxies. While the virial black hole mass (M$_ BH $) of BLS1 galaxies is similar to their radiation-pressure-corrected M$_ BH $ values, the virial M$_ BH $ values of NLS1 galaxies are underestimated. We found that NLS1 galaxies have a lower M$_ BH $ of log (M$_ BH M$_ odot ) = 7.45 pm 0.27 and a higher Eddington ratio of log ($ Edd $) = $-$0.72 pm 0.22 than BLS1 galaxies, which have log (M$_ BH M$_ odot ) and $ Edd $ values of 8.04 pm 0.26 and $-$1.08 pm 0.24, respectively. The distributions of M$_ ast $, SFR, and specific star formation (sSFR = SFR/M$_ ast $) for the two populations are indistinguishable. This analysis is based on an independent approach and contradicts reports in the literature that NLS1 galaxies have a higher SF than BLS1 galaxies. While we found that L$_ AGN $ increases with M$_ ast $, L$_ SF $ flattens at high M$_ ast $ for both BLS1 and NLS1 galaxies. The reason may be that SF is suppressed by AGN feedback at M$_ ast $ higher than sim 1011 M$_ odot $ or that the AGN fuelling mechanism is decoupled from SF. Separating the sample into radio-detected and radio-undetected subsamples, we found no difference in their SF properties suggesting that the effect of AGN jets on SF is negligible.

Probing the Gamma-Ray Emission Region of Five TeV Flat Spectrum Radio Quasars

The location of γ-ray emission of blazars remains a contested topic, inspiring the development of numerous investigative techniques to address this issue. In this work, we analyzed Fermi γ-ray light curves in the GeV and MeV bands, employing the discrete cross-correlation function method to discern time lags between the two bands. For 4C +21.35, Ton 599, B2 1420+32, and PKS 1510-089, we identified a time lag spanning several days, while for PKS 1441+25, the time lag was not statistically found. The results imply that the soft photons necessary for inverse Compton scattering predominantly originate from the dusty torus in the first four sources, whereas for PKS 1441+25, they seem to be sourced mainly from the broad-line region. Further analysis of the opacity (τ γ γ ) and the GeV spectra study supports the conclusion that the location of the dissipation region must be beyond the BLR to avoid significant absorption. Notably, for PKS 1441+25, the emission region is also posited to lie outside yet proximate to the BLR. The parameters of describing the emission region were obtained by fitting broadband spectral energy distribution with contemporaneous observation data. Our findings suggest that for the five TeV FSRQs, during TeV flaring events, the jet appears to maintain an equilibrium between the energy density of the magnetic field and that of the particles for all investigated sources, with the exceptions of 4C +21.35 and PKS 1441+25. In terms of the overall jet power, particle energy is the dominant contributor, and the observed blazar radiation cannot be solely attributed to the magnetic field, except in the case of 4C +21.35. Consequently, magnetic reconnection is unlikely to be the primary mechanism behind particle acceleration in these systems.

Non-thermal emission in M31 and M33

Context. Spiral galaxies M31 and M33 are among the γ-ray sources detected by the Fermi Large Area Telescope (LAT). Aims. We aim to model the broadband non-thermal emission of the central region of M31 (a LAT point source) and of the disk of M33 (a LAT extended source), as part of our continued survey of non-thermal properties of local galaxies that includes the Magellanic Clouds. Methods. We analysed the observed emission from the central region of M31 (R < 5.5 kpc) and the disk-sized emission from M33 (R ∼ 9 kpc). For each galaxy, we self-consistently modelled the broadband spectral energy distribution of the diffuse non-thermal emission based on published radio and γ-ray data. All relevant radiative processes involving relativistic and thermal electrons (synchrotron, Compton scattering, bremsstrahlung, and free–free emission and absorption), along with relativistic protons (π0 decay following interaction with thermal protons), were considered, using exact emissivity formulae. We also used the Fermi-LAT-validated γ-ray emissivities for pulsars. Results. Joint spectral analyses of the emission from the central region of M31 and the extended disk of M33 indicate that the radio emission is composed of both primary and secondary electron synchrotron and thermal bremsstrahlung, whereas the γ-ray emission may be explained as a combination of diffuse pionic, pulsar, and nuclear-BH-related emissions in M31 and plain diffuse pionic emission (with an average proton energy density of 0.5 eV cm−3) in M33. Conclusions. The observed γ-ray emission from M33 appears to be mainly hadronic. This situation is similar to other local galaxies, namely, the Magellanic Clouds. In contrast, we have found suggestions of a more complex situation in the central region of M31, whose emission could be an admixture of pulsar emission and hadronic emission, with the latter possibly originating from both the disk and the vicinity of the nuclear black hole. The alternative modelling of the spectra of M31 and M33 is motivated by the different hydrogen distribution in the two galaxies: The hydrogen deficiency in the central region of M31 partially unveils emissions from the nuclear BH and the pulsar population in the bulge and inner disk. If this were to be the case in M33 as well, these emissions would be outshined by diffuse pionic emission originating within the flat central-peak gas distribution in M33.

Gamma-ray variability and multi-wavelength insights into the unprecedented outburst from 4C 31.03

The blazar 4C31.03 recently underwent a major γ-ray outburst at the beginning of 2023 after a prolonged quiescent phase. Fermi-LAT reported a daily average flux of 5×10−6 phs cm−2 s−1, which is about 60 times its average value. We investigated this extraordinary outbreak through temporal and multi-wavelength analysis. From the statistical analysis of the γ-ray lightcurves using Bayesian blocks, we identified 3 epochs of prominent flares. The fastest flux decay during this major outburst was observed within 5.5±0.7 hours. The highest energy of γ-ray photons found from the source during the active phase is ∼82GeV. Using the transparency of γ-rays against pair production and light crossing time argument, we could obtain the minimum jet Doppler factor as 17 corresponding to the flaring state. The broadband spectral energy distribution study performed using synchrotron, SSC and EC emission processes supports the external Compton scattering of IR photons as the likely mechanism for the γ-ray emission from the source. The results of this study support the scenario of the emission region in 4C31.03, being located beyond the Broad line region from the central blackhole. The long-term γ-ray flux distribution depicts a double log-normal variability, indicating that two distinct flux states are active in this energy band. The index distribution also reveals a two distinct variability pattern and hints that the γ-ray spectrum can be more precisely described by two photon indices.

Investigating X-ray emission in the GeV-emitting compact symmetric objects PKS 1718–649 and TXS 1146+596

Aims. Compact symmetric objects (CSOs) are thought to represent the first step in the evolutionary path of radio galaxies. In the present study, we investigated the X-ray emission of two CSOs confirmed to emit at GeV energies: PKS 1718–649 and TXS 1146+596. Unveiling the origin of their observed high-energy emission is crucial to establishing the physical parameters of the radio source and understanding how CSOs interact with the surrounding medium. Methods. We combined archival and new NuSTAR observations of PKS 1718–649 and TXS 1146+596 to have broadband X-ray coverage. For both sources, we modeled the broadband spectral energy distribution (SED) from the radio band up to γ-rays in order to derive their physical parameters. We also discuss the role of the ambient medium in confining the source expansion, which we investigate using X-ray obscuration. Results. For the first time, we report X-ray detections of PKS 1718–649 and 1146+596 with NuSTAR at energies higher than 10 keV. Combining Chandra and NuSTAR observations of TXS 1146+596, we reveal the presence of a multitemperature thermal component dominating the soft X-ray spectrum, and we interpret this finding as indicative of an AGN feedback process in action in this source. In addition, we show that two emitting electron populations are necessary to reproduce the observed broadband SED of TXS 1146+596: in our models, the X-ray emission could either be produced by synchrotron radiation or by a weak X-ray corona, or could be an ADAF-type emission. Interestingly, an additional X-ray component, namely a weak corona, is also required for PKS 1718–649. Moreover, we argue that heavily obscured and possibly frustrated sources tend to show different radio sizes with respect to those that are unobscured and free to expand.

Fermi Large Area Telescope Detection of Gamma Rays from the NGC 6251 Radio Lobe

We report the detection of extended γ-ray emission from lobes in the radio galaxy NGC 6251 using observation data from the Fermi Large Area Telescope. The maximum likelihood analysis results show that a radio morphology template provides a better fit than a pointlike source description for the observational data at a confidence level of 8.1σ, and the contribution of lobes constitutes more than 50% of the total γ-ray flux. Furthermore, the γ-ray energy spectra show a significant disparity in shape between the core and lobe regions, with a curved log-parabola shape observed in the core region and a power-law form observed in the lobes. Neither the core region nor the northwest (NW) lobe displays significant flux variations in the long-term γ-ray light curves. The broadband spectral energy distributions of the core region and the NW lobe can be explained with a single-zone leptonic model. The γ-rays of the core region are due to the synchrotron-self-Compton process, while the γ-rays from the NW lobe are interpreted as inverse Compton emission of the cosmic microwave background.

Modeling X-Ray and Gamma-Ray Emission from Redback Pulsar Binaries

We investigated the multiband emission from the pulsar binaries XSS J12270−4859, PSR J2039−5617, and PSR J2339−0533, which exhibit orbital modulation in the X-ray and gamma-ray bands. We constructed the sources’ broadband spectral energy distributions and multiband orbital light curves by supplementing our X-ray measurements with published gamma-ray results, and we modeled the data using intrabinary shock (IBS) scenarios. While the X-ray data were well explained by synchrotron emission from electrons/positrons in the IBS, the gamma-ray data were difficult to explain with the IBS components alone. Therefore, we explored other scenarios that had been suggested for gamma-ray emission from pulsar binaries: (1) inverse-Compton emission in the upstream unshocked wind zone and (2) synchrotron radiation from electrons/positrons interacting with the kilogauss magnetic field of the companion. Scenario (1) requires that the bulk motion of the wind substantially decelerates to ∼1000 km s−1 before reaching the IBS for increased residence time, in which case the formation of a strong shock is untenable, inconsistent with the X-ray phenomenology. Scenario (2) can explain the data if we assume the presence of electrons/positrons with a Lorentz factor ∼ 108 (∼0.1 PeV) that pass through the IBS and tap a substantial portion of the pulsar voltage drop. These findings raise the possibility that the orbitally modulating gamma-ray signals from pulsar binaries can provide insights into the flow structure and energy conversion within pulsar winds and particle acceleration nearing PeV energies in pulsars. These signals may also yield greater understanding of kilogauss magnetic fields potentially hosted by the low-mass stars in these systems.

Estimating the jet power from Broadband SED modelling of Mkn 501 for different particle distributions

ABSTRACT We consider the broad-band spectral energy distribution of the high-energy-peaked blazar Mkn 501 using Swift-XRT/UVOT, NuSTAR and Fermi-LAT observations taken between 2013 and 2022. The spectra were fitted with a one-zone leptonic model using synchrotron and synchrotron self-Compton emission from different particle energy distributions such as a broken power law, log-parabola, as well as distributions expected when the diffusion or the acceleration time-scale are energy-dependent. The jet power estimated for a broken power-law distribution was ∼1047(1044) erg s−1 for a minimum electron energy γmin ∼ 10(103). However, for electron energy distributions with intrinsic curvature (such as the log-parabola form), the jet power is significantly lower at a few times 1042 erg s−1 which is a few per cent of the Eddington luminosity of a 107 M⊙ black hole, suggesting that the jet may be powered by accretion processes. We discuss the implications of these results.

TeV flaring activity of the AGN PKS 0625–354 in November 2018

Most γ-ray detected active galactic nuclei are blazars with one of their relativistic jets pointing towards the Earth. Only a few objects belong to the class of radio galaxies or misaligned blazars. Here, we investigate the nature of the object PKS 0625−354, its γ-ray flux and spectral variability and its broad-band spectral emission with observations from H.E.S.S., Fermi-LAT, Swift-XRT, and UVOT taken in November 2018. The H.E.S.S. light curve above 200 GeV shows an outburst in the first night of observations followed by a declining flux with a halving time scale of 5.9 h. The γγ-opacity constrains the upper limit of the angle between the jet and the line of sight to ∼10°. The broad-band spectral energy distribution shows two humps and can be well fitted with a single-zone synchrotron self Compton emission model. We conclude that PKS 0625−354, as an object showing clear features of both blazars and radio galaxies, can be classified as an intermediate active galactic nuclei. Multi-wavelength studies of such intermediate objects exhibiting features of both blazars and radio galaxies are sparse but crucial for the understanding of the broad-band emission of γ-ray detected active galactic nuclei in general.

Modeling Blazar Broadband Emission with a Convolutional Neural Network. I. Synchrotron Self-Compton Model

Modeling the multiwavelength spectral energy distributions (SEDs) of blazars provides key insights into the underlying physical processes responsible for the emission. While SED modeling with self-consistent models is computationally demanding, it is essential for a comprehensive understanding of these astrophysical objects. We introduce a novel, efficient method for modeling the SEDs of blazars by the mean of a convolutional neural network (CNN). In this paper, we trained the CNN on a leptonic model that incorporates synchrotron and inverse Compton emissions, as well as self-consistent electron cooling and pair creation–annihilation processes. The CNN is capable of reproducing the radiative signatures of blazars with high accuracy. This approach significantly reduces the computational time, thereby enabling real-time fitting to multiwavelength data sets. As a demonstration, we used the trained CNN with MultiNest to fit the broadband SEDs of Mrk 421 and 1ES 1959+650, successfully obtaining their parameter posterior distributions. This novel framework for fitting the SEDs of blazars will be further extended to incorporate more sophisticated models based on external Compton and hadronic scenarios, allowing for multimessenger constraints in the analysis. The models will be made publicly available via a web interface at the Markarian Multiwavelength Data Center to facilitate self-consistent modeling of multimessenger data from blazar observations.

Broad-band spectral and temporal study of Ton 599 during the brightest 2023 January flare

ABSTRACT In this work, we provide a detailed analysis of the broad-band temporal and spectral properties of the blazar Ton 599 by using observations from the Fermi Large Area Telescope (LAT) and Swift X-Ray Telescope (XRT)/Ultraviolet–Optical Telescope (UVOT), during its brightest γ-ray flaring. The one-day bin γ-ray light curve exhibits multiple substructures with asymmetric and symmetric profiles. Notably, the γ-ray light curve shows a maximum flux of $\rm 3.63 \times 10^{-6}\, photon\, cm^{-2}\, s^{-1}$ on MJD 59954.50, which is the highest flux ever observed from this source. The correlation between the γ-ray flux and γ-ray spectral indices suggests a moderate ‘harder when brighter’ trend. Taking the γ-ray light curve as the reference, a strong correlation is observed with X-ray, optical, and UV energies. Additionally, the γ-rays and optical/UV emission exhibit higher variability compared with X-rays. To understand the parameter variation during the active state of the source, we conducted a statistical broad-band spectral modelling of the source in 10 flux intervals of equal duration. A one-zone leptonic model involving synchrotron, synchrotron-self-Compton, and external Compton processes successfully reproduces the broad-band spectral energy distribution (SED) in each of these flux intervals. We observed that flux variation during the active state is associated mainly with variation in the magnetic field and particle spectral indices.

A Unified Model for Multiepoch Neutrino Events and Broadband Spectral Energy Distribution of TXS 0506+056

The blazar TXS 0506+056 has been proposed as a high-energy neutrino emitter. However, it has been shown that the standard one-zone model cannot produce sufficiently high neutrino flux due to constraints from the X-ray data, implying more complex properties of the radiation zones in the blazar than that described by the standard one-zone model. In this work, we investigate multiepoch high-energy muon-neutrino events associated with the blazar TXS 0506+056 that occurred in 2014–2015, 2017–2018, 2021–2022, and 2022–2023, respectively. We applied the so-called “stochastic dissipation model” to account for the neutrino-blazar associations detected in the four epochs simultaneously. This model describes a scenario in which the emission of the blazar arises from the superimposition of two components: a persistent component related to the quasi-stable state of the blazar and a transient component responsible for the sudden enhancement of the blazar’s flux, either in electromagnetic radiation or in neutrino emission. The latter component could form at a random distance along the jet by a strong energy dissipation event. Under such an assumption, the multiepoch broadband spectral energy distribution can be well explained, and the expected number of high-energy neutrino events is statistically realistic. The expected number of neutrino events in half year is around 8.2, 0.07, 0.73, and 0.41, corresponding to the epoch in 2014–2015, 2017–2018, 2021–2022, and 2022–2023, respectively. Hence, our model self-consistently explains the episodic neutrino emission from TXS 0506+056.

Bjet_MCMC: A New Tool to Automatically Fit the Broadband Spectral Energy Distributions of Blazars

Multiwavelength observations are now the norm for studying blazars’ various states of activity, classifying them, and determining the possible underlying physical processes driving their emission. Broadband emission models became unavoidable tools for testing emission scenarios and setting the values of physical quantities such as the magnetic field strength, Doppler factor, or shape of the particle distribution of the emission zone(s). We announce here the first public release of a new tool, Bjet_MCMC, that can automatically fit the broadband spectral energy distributions (SEDs) of blazars. The complete code is available on GitHub and allows for testing leptonic synchrotron self-Compton models with or without external inverse-Compton processes from the thermal environment of supermassive black holes (accretion disk and broad-line region). The code is designed to be user-friendly and computationally efficient. It contains a core written in C++ and a fully parallelized SED fitting method. The original multi-SSC zone model of Bjet is also available on GitHub but is not included in the Markov Chain Monte Carlo fitting process at the moment. We present the features, performance, and results of Bjet_MCMC, as well as user advice.

Lobe-dominated γ-Ray Emission of Compact Symmetric Objects

The γ-ray emitting compact symmetric objects (CSOs) PKS 1718–649, NGC 3894, and TXS 0128+554 are lobe-dominated in the radio emission. In order to investigate their γ-ray radiation properties, we analyze the ∼14 yr Fermi/LAT observation data of the three CSOs. They all show the low luminosity (1041–1043 erg s−1) and no significant variability in the γ-ray band. Their γ-ray average spectra can be well fitted by a power-law function. These properties of γ-rays are clearly different from the γ-ray emitting CSOs CTD 135 and PKS 1413+135, for which the γ-rays are produced by a restarted aligned jet. In the L γ − Γ γ plane, the three CSOs are also located at the region occupied by radio galaxies (RGs) while CTD 135 and PKS 1413+135 display a similar feature to blazars. Together with a similar radio emission property to γ-ray emitting RGs Cen A and Fornax A, we speculate that the γ-rays of the three CSOs stem from their extended mini-lobes. The broadband spectral energy distributions of the three CSOs can be well explained by the two-zone leptonic model, where their γ-rays are produced by the inverse Compton process of the relativistic electrons in extended regions. By extrapolating the observed Fermi/LAT spectra to the very high energy band, we find that TXS 0128+554 among the three CSOs may be detected by the Cherenkov Telescope Array in the future.

Stellar Population Properties in the Stellar Streams around SPRC047

We have investigated the properties (e.g., age, metallicity) of the stellar populations of a ringlike tidal stellar stream (or streams) around the edge-on galaxy SPRC047 (z = 0.031) using spectral energy distribution (SED) fits to integrated broadband aperture flux densities. We used visual images in six different bands and Spitzer/IRAC 3.6 μm data. We have attempted to derive best-fit stellar population parameters (metallicity, age) in three noncontiguous segments of the stream. Due to the very low surface brightness of the stream, we have performed a deconvolution with a Richardson–Lucy–type algorithm of the low spatial resolution 3.6 μm IRAC image, thereby reducing the effect of the point-spread function aliased emission from the bright edge-on central galaxy at the locations of our three stream segments. Our SED fits that used several different star formation (SF) history priors, from an exponentially decaying SF burst to continuous SF, indicate that the age–metallicity–dust degeneracy is not resolved, most likely because of inadequate wavelength coverage and low signal-to-noise ratios of the low surface brightness features. We also discuss how future deep visual–near-infrared observations, combined with absolute flux calibration uncertainties at or below the 1% level, complemented by equally well absolute flux-calibrated observations in ultraviolet and mid-infrared bands, would improve the accuracy of broadband SED fitting results for low surface brightness targets, such as stellar streams around nearby galaxies that are not resolved into stars.

Origin of the broadband emission from the transition blazar B2 1308+326

Context. Transition blazars exhibit a shift from one subclass to the next during different flux states. It is therefore crucial to study them to understand the underlying physics of blazars. Aims. We probe the origin of the multi-wavelength emission from the transition blazar B2 1308+326 using the ∼14-year long γ-ray light curve from Fermi and the quasi-simultaneous data from Swift. Methods. We used the Bayesian block algorithm to identify epochs of flaring and quiescent flux states and modelled the broadband spectral energy distributions (SEDs) for these epochs. We employed the one-zone leptonic model in which the synchrotron emission causes the low-energy part of the SED and the high-energy part is produced by the inverse-Compton (IC) emission of external seed photons. We also investigated its multi-band variability properties and γ-ray flux distribution, and the correlation between optical and γ-ray emissions. Results. We observed a historically bright flare from B2 1308+326 across the optical to γ-ray bands in June and July 2022. The highest daily averaged γ-ray flux was (14.24 ± 2.36) × 10−7 ph cm−2 s−1 and was detected on 1 July 2022. For the entire period, the observed variability amplitude was higher at low (optical/UV) energies than at high (X-ray/γ-ray) energies. The γ-ray flux distribution was found to be log-normal. The optical and γ-ray emissions are well correlated with zero time lag. The synchrotron peak frequency changes from ∼8 × 1012 Hz (in the quiescent state) to ∼6 × 1014 Hz (in the flaring state), together with a decrease in the Compton dominance (the ratio of IC to the synchrotron peak luminosities), providing a hint that the source transitions from a low-synchrotron peaked blazar (LSP) to an intermediate-synchrotron peaked blazar (ISP). The SEDs for these two states are well fitted by one-zone leptonic models. The parameters in the model fits are essentially consistent between both SEDs, except for the Doppler-beaming factor, which changes from ∼15.6 to ∼27 during the transition. Conclusions. An increase in the Doppler factor might cause both the flare and the transition of B2 1308+326 from an LSP to an ISP blazar.

Investigation of the correlation between optical and γ-ray flux variations in the blazar Ton 599

ABSTRACT The correlation between optical and γ-ray flux variations in blazars reveals a complex behaviour. In this study, we present our analysis of the connection between changes in optical and γ-ray emissions in the blazar Ton 599 over a span of approximately 15 yr, from 2008 August to 2023 March. Ton 599 reached its highest flux state across the entire electromagnetic spectrum during the second week of 2023 January. To investigate the connection between changes in optical and γ-ray flux, we have designated five specific time periods, labelled as epochs A, B, C, D, and E. During periods B, C, D, and E, the source exhibited optical flares, while it was in its quiescent state during period A. The γ-ray counterparts to these optical flares are present during periods B, C, and E; however, during period D, the γ-ray counterpart is either weak or absent. We conducted a broad-band spectral energy distribution (SED) fitting by employing a one-zone leptonic emission model for these epochs. The SED analysis unveiled that the optical–ultraviolet emission primarily emanated from the accretion disc in quiescent period A, whereas synchrotron radiation from the jet dominated during periods B, C, D, and E. Diverse correlated patterns in the variations of optical and γ-ray emissions, like correlated optical and γ-ray flares, could be accounted for by changes in factors such as the magnetic field, bulk Lorentz factor, and electron density. On the other hand, an orphan optical flare could result from increased magnetic field and bulk Lorentz factor.

Can FSRQ 3C 345 be a very high energy blazar candidate?

ABSTRACT The recent detection of very high energy (VHE) emissions from flat spectrum radio quasars (FSRQs) at high redshifts has revealed that the universe is more transparent to VHE γ-rays than it was expected. It has also questioned the plausible VHE emission mechanism responsible for these objects. Particularly for FSRQs, the γ-ray emission is attributed to the external Compton (EC) process. We perform a detailed spectral study of Fermi-detected FSRQ 3C 345 using synchrotron, synchrotron self-Compton, and EC emission mechanisms. The simultaneous data available in optical, ultraviolet, X-ray, and γ-ray energy bands is statistically fitted under these emission mechanisms using the χ2-minimization technique. Three high flux states and one low flux state are chosen for spectral fitting. The broad-band spectral energy distribution during these flux states is fitted under different target photon temperatures, and the model VHE flux is compared with the 50 h Cherenkov Telescope Array sensitivity. Our results indicate a significant VHE emission could be attained during the high flux state from MJD 59635−59715 when the target photon temperature is within 900–1200 K. Furthermore, our study shows a clear trend of variation in the bulk Lorentz factor of the emission region as the source transits through different flux states. We also note that during high γ-ray flux states, an increase in external photon temperature demands high bulk Lorentz factors, while this behaviour reverses in case of low γ-ray flux state.

Dissecting the broad-band emission from γ-ray blazar PKS 0735+178 in search of neutrinos

ABSTRACT The origin of the diffuse flux of TeV–PeV astrophysical neutrinos is still unknown. The γ-ray blazar PKS 0735+178, located outside the 90 percent localization region at 2.2° from the best-fitting IC-211208A event, was found to be flaring across all wavebands. In addition to leptonic synchrotron (SYN) and SYN self-Compton (SSC) emission, we invoke photohadronic (pγ) interactions inside the jet to model the spectral energy distribution (SED) and neutrino emission. We analyse the 100 d γ-ray and X-ray data and 10 d around the neutrino event is chosen to generate the broad-band SED. The temporal light curve indicates that the source was in a high state in optical, UV, γ-ray, and X-ray frequencies during the neutrino detection epoch. In the one-zone lepto-hadronic model, the SSC photons do not provide enough seed photons for pγ interactions to explain the neutrino event. However, including an external photon field yields a neutrino event rate of 0.12 in 100 d, for the IceCube detector, using physically motivated values of the magnetic field, an external photon field peaking at optical wavelength, and other jet parameters. The radiation from secondary electrons at X-ray energies severely constrains the neutrino flux to a lower value than found in previous studies. Moreover, the flux of high-energy γ-rays at GeV energies from the decay of neutral pions is sub-dominant at the high-energy peak of the SED, suggesting a higher correlation of neutrinos flux with X-ray flux is plausible.

Hadronic Processes at Work in 5BZB J0630−2406

Recent observations are shedding light on the important role that active galactic nuclei play in the production of high-energy neutrinos. In this study, we focus on one object, 5BZB J0630−2406, which is among the blazars recently proposed as associated with neutrino emission during the first 7 yr of IceCube observations. Modeling the quasi-simultaneous, broadband spectral energy distribution, we explore various scenarios from purely leptonic to leptohadronic models, testing the inclusion of external photon fields. This theoretical study provides a complementary testing ground for the proposed neutrino–blazar association. Despite being historically classified as a BL Lac, our study shows that 5BZB J0630−2406 belongs to the relatively rare subclass of high-power flat-spectrum radio quasars. Our results indicate that interactions between protons and external radiation fields can produce a neutrino flux that is within the reach of the IceCube detector. Furthermore, the spectral shape of the X-ray emission suggests the imprint of hadronic processes related to very energetic protons.