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Raj Prince

Publications and source records attributed to Raj Prince.

At least 37 records · Page 2Linked to original sources
arXiv2023

Detection of gamma-ray quasi-periodic oscillations in non-blazar AGN PKS 0521-36

Quasi-periodic oscillations (QPOs) have been detected in many Fermi-detected bright blazars. In this letter, we report multiple QPOs detected in a non-blazar AGN PKS 0521-36 searched over the entire 15 years of Fermi-LAT data. QPOs are detected at 268 days, at 295 days, and at 806 days timescales with more than 3$σ$ significance. The QPO detected at 806 days happens to be the third harmonic of QPO at 268 days. The time scales are consistent in both Lomb-Scargle and Wavelet analysis. Furthermore, the Gaussian Process modeling of the light curve is performed with stochastically driven damped harmonic oscillator (SHO) and damped random walk (DRW) modeling to uncover the presence of QPOs. The constructed power spectral density (PSD) exhibits two QPOs, with observed timescales of approximately 283 days and 886 days. This is the first non-blazar AGN where the long-term QPO is detected. Earlier studies show this source has a weak beamed jet. The exact cause for these QPOs remains unclear. We also assembled the $γ$-ray QPO detected in various blazar and tested the QPO time scale dependent on the black hole mass. No significant correlation is found.

astro-ph.HE↗
arXiv2023

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

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\% localization region at 2.2 deg from the best-fit IC-211208A event, was found to be flaring across all wavebands. In addition to leptonic synchrotron (SYN) and synchrotron self-Compton (SSC) emission, we invoke photohadronic ($pγ$) interactions inside the jet to model the spectral energy distribution (SED) and neutrino emission. We analyze the 100 days $γ$-ray and X-ray data and 10 days around the neutrino event is chosen to generate the broadband 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 days, 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 subdominant at the high-energy peak of the SED, suggesting a higher correlation of neutrinos flux with X-ray flux is plausible.

astro-ph.HE↗
arXiv2023

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

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. We probe the origin of the multi-wavelength emission from the transition blazar B2 1308+326 using 14-year-long gamma-ray light curve from Fermi and the quasi-simultaneous data from Swift. We used the Bayesian block algorithm to identify epochs of flaring and quiescent flux states and modelled the broadband 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 IC emission of external seed photons. We also investigated its multi-band variability properties and gamma-ray flux distribution, and the correlation between optical and gamma-ray emissions. We observed a historically bright flare from B2 1308+326 across the optical to gamma-ray bands in June and July 2022. The highest daily averaged gamma-ray flux was (14.24$\pm$2.36) $\times$ 10$^{-7}$ ph cm$^{-2}$ s$^{-1}$ and was detected on 1 July 2022. The gamma-ray flux distribution was found to be log-normal. The optical and gamma-ray emissions are well correlated with zero time lag. The synchrotron peak frequency changes from $\sim 8 \times$ 10$^{12}$ Hz (in the quiescent state) to $\sim 6 \times$ 10$^{14}$ Hz (in the flaring state), together with a decrease in the Compton dominance providing a hint that the source transitions from a LSP to an 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 $\sim$15.6 to $\sim$27 during the transition. 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.

astro-ph.HE↗
arXiv2023

Probing the disc-jet coupling in S4 0954+65, PKS 0903-57, & 4C +01.02 with $γ$-rays

We present a comprehensive variability study on three blazars, S4 0954+65, PKS 0903-57, and 4C +01.02 covering a mass range of log(M/M$_{\odot}$) = 8--9, by using $\sim$15 years-long $γ$-ray light curves from \textit{Fermi}-LAT. The variability level is characterized by the fractional variability amplitude which is higher for $γ$-rays compared to optical/UV and X-rays emissions. A power spectral density (PSD) study and damped random walk (DRW) modeling are done to probe the characteristic timescale. The PSD is fitted with a single power-law (PL) and bending power-law models and the corresponding success fraction was estimated. In the case of PKS 0903-57, We observed a break in the $γ$-ray PSD at 256 days which is comparable to the viscous timescale in the accretion disc suggesting a possible disk-jet coupling. The non-thermal damping timescale from the DRW modeling is compared with the thermal damping timescale for AGNs including our three sources. Our sources lie on the best-fit of the $\mathrm{τ^{rest}_{damping}} - M_{BH}$ plot derived for AGN suggesting a possible accretion disc-jet connection. If the jet's variability is linked to the disc's variability, we expect a log-normal flux distribution, often connected to the accretion disc's multiplicative processes. Our study observed a double log-normal flux distribution, possibly linked to long and short-term variability from the accretion disk and the jet. In summary, PSD and DRW modeling results for these three sources combined with blazars and AGNs studied in literature favor a disc-jet coupling scenario. However, more such studies are needed to refine this understanding.

astro-ph.HE↗
arXiv2023

Broad-line region in active galactic nuclei: Dusty or dustless?

Context. Dust in active galactic nuclei is clearly present right outside the broad-line region (BLR) in the form of a dusty molecular torus. However, some models of the BLR predict that dust may also exist within the BLR. Aims. We study the reprocessing of radiation by the BLR with the aim of observing how the presence of dust affects the reprocessed continuum and the line properties. Methods. We calculated a range of models using the CLOUDY photoionisation code for dusty and dustless plasma. We paid particular attention to the well-studied object NGC 5548, and we compared the line equivalent width predictions with the data from observations for this object. Results. We obtained a rough agreement between the expected equivalent widths of the H$β$ and Mg II lines and the observed values for NGC 5548 for the line distances implied by the time-delay measurement (for H$β)$ and the radius-luminosity relation (for Mg II) when the medium is dusty. We found the incident radiation to be consistent with the radiation seen by the observer, so no shielding between the inner disc and the BLR is required. High ionisation lines such as He II, however, clearly form in the inner dustless region. When the additional absorber is present, the H$β$ emitting region moves closer to the dustless part of the accretion disc surface.

astro-ph.GA↗
arXiv2023

Multi-wavelength temporal and spectral study of PKS 0402-362

We study the long-term behavior of the bright gamma-ray blazar PKS 0402-362. Over a span of approximately 12.5 years, from August 2008 to January 2021, we gathered Fermi-LAT temporal data and identified three distinct periods of intense $γ$-ray activity. Notably, the second period exhibited the highest brightness ever observed in this particular source. We observed most of the $γ$-ray flare peaks to be asymmetric in profile suggesting a slow cooling time of particles or the varying Doppler factor as the main cause of these flares. The $γ$-ray spectrum is fitted with power-law and log-parabola models, and in both cases, the spectral index is very steep. The lack of time lags between optical-IR and $γ$-ray emissions indicates the presence of a single-zone emission model. Using this information, we modeled the broadband SEDs with a simple one-zone leptonic model using the publicly available code `GAMERA'. The particle distribution index is found to be the same as expected in diffusive shock acceleration suggesting it as the main mechanism of particle acceleration to very high energy up to 4 - 6 GeV. During the different flux phases, we observed that the thermal disk dominates the optical emission, indicating that this source presents a valuable opportunity to investigate the connection between the disk and the jet.

astro-ph.HE↗
arXiv2023

Quasi-periodic oscillation detected in $γ$-rays in blazar PKS 0346-27

We present a variability study of the blazar PKS 0346-27 from December 2018 to January 2022 in its archival $γ$-ray observation by Fermi-LAT. We use the Lomb-Scargle periodogram and the weighted wavelet transform methods in order to detect the presence of periodicity/quasi-periodicity and localize this feature in time and frequency space. The significance of the periodicity feature has been estimated using the Monte-Carlo simulation approach. We have also determined the global significance of the periodicity to test the robustness of our claim. To explore the most probable scenario, we modeled the light curve with both a straight jet and a curved jet model. We detect a periodicity feature of $\sim$ 100 days duration for the entire period of observation with a statistical significance of $3σ$, which amounts to a 99.7\% confidence level. The global significance of this feature is found to be 96.96\%. Based on the Akaike Information Criteria, the most probable explanation is that the observed emission is enhanced due to the helical motion of a blob within a curved jet. The origin of this QPO is very likely a region of enhanced emission moving helically inside a curved jet. This work presents strong evidence for jet curvature in the source and an independent (albeit a little serendipitous) procedure to estimate the curvature in blazar jets.

astro-ph.HE↗
arXiv2023

Multi-wavelength study of TeV blazar 1ES 1218+304 using gamma-ray, X-ray and optical observations

We report the multi-wavelength study for a high-synchrotron-peaked BL Lac 1ES 1218+304 using near-simultaneous data obtained during the period from January 1, 2018, to May 31, 2021 (MJD 58119-59365) from various instruments including Fermi-LAT, Swift-XRT, AstroSat, and optical from Swift-UVOT $\&$ TUBITAK observatory in Turkey. The source was reported to be flaring in TeV $γ$-ray band during 2019, but no significant variation is observed with Fermi-LAT. A sub-hour variability is seen in the SXT light curve, suggesting a compact emission region for their variability. However, hour scale variability is observed in the $γ$-ray light curve. A "softer-when-brighter" trend is observed in $γ$-rays, and an opposite trend is seen in X-rays suggesting both emissions are produced via two different processes as expected from an HBL source. We have chosen the two epochs in January 2019 to study and compare their physical parameters. A joint fit of SXT and LAXPC provides a constraint on the synchrotron peak, roughly estimated to be $\sim$1.6 keV. A clear shift in the synchrotron peak is observed from $\sim$1 keV to above 10 keV revealing its extreme nature or behaving like an EHBL-type source. The optical observation provides color-index variation as "blue-when-brighter". The broadband SED is fitted with a single-zone SSC model, and their parameters are discussed in the context of a TeV blazar and the possible mechanism behind the broadband emission.

astro-ph.HE↗
arXiv2023

Wavelength-resolved reverberation mapping of intermediate redshift quasars HE 0413-4031 and HE 0435-4312: Dissecting Mg II, optical Fe II, and UV Fe II emission regions

We present the wavelength-resolved reverberation mapping (RM) of combined MgII and UV FeII broad-line emissions for two intermediate redshifts (z$\sim$1), luminous quasars - HE 0413-4031 and HE 0435-4312, monitored by the SALT and 1-m class telescopes between 2012-2022. Through this technique, we aim to disentangle the Mg II and FeII emission regions and to build a radius-luminosity relation for UV FeII emission, which has so far remained unconstrained. Several methodologies have been applied to constrain the time delays for total MgII and FeII emissions. In addition, this technique is performed to quantify the inflow or outflow of broad-line region gas around the supermassive black hole and to disentangle the emission/emitting regions from lines produced in proximity to each other. The mean total FeII time delay is nearly equal to the mean total Mg II time delay for HE 0435-4312 suggesting a co-spatiality of their emissions. However, in HE 0413-4031, the mean FeII time delay is found to be longer than the mean MgII time delay, suggesting that FeII is produced at longer distances from the black hole. The UV Fe II R-L relation is updated with these two quasars and compared with the optical FeII relation, which suggests that the optical FeII region is located further than the UV FeII by a factor of 1.7-1.9 i.e. $R_{\rm FeII-opt}\sim(1.7-1.9)R_{\rm FeII-UV}$. We detected a weak pattern in the time delay vs. wavelength relation, suggesting that the MgII broad-line originates a bit closer to the SMBH than the UV FeII, however, the difference is not very significant. Comparison of MgII, UV, and optical FeII R-L relations suggests that the difference may be larger for lower-luminosity sources, possibly with the MgII emission originating further from the SMBH. In the future, more RM data will be acquired to put better constraints on these trends, in particular the UV FeII R-L relation.

astro-ph.GA↗
arXiv2023

Expectations for time-delay measurements in active galactic nuclei with the Vera Rubin Observatory

The Vera Rubin Observatory will provide an unprecedented set of time-dependent observations of the sky. The planned Legacy Survey of Space and Time (LSST) operating for 10 years will provide dense lightcurves for thousands of active galactic nuclei (AGN) in Deep Drilling Fields (DDFs) and less dense lightcurves for millions of AGN. We model the prospects for measuring time delays for emission lines with respect to the continuum, using these data. We model the artificial lightcurves using Timmer-Koenig algorithm, we use the exemplary cadence to sample them, we supplement lightcurves with the expected contamination by the strong emission lines (Hbeta, Mg II and CIV as well as with Fe II pseudo-continuum and the starlight). We choose the suitable photometric bands appropriate for the redshift and compare the assumed line time delay with the recovered time delay for 100 statistical realizations of the light curves. We show that time delays for emission lines can be well measured from the Main Survey for the bright tail of the quasar distribution (about 15% of all sources) with the accuracy within 1 sigma error, for DDFs results for fainter quasars are also reliable when all 10 years of data are used. There are also some prospects to measure the time delays for the faintest quasars at the smallest redshifts from the first two years of data, and eventually even from the first season. The entire quasar population will allow obtaining results of apparently high accuracy but in our simulations, we see a systematic offset between the assumed and recovered time delay depending on the redshift and source luminosity which will not disappear even in the case of large statistics. Such a problem might affect the slope of the radius-luminosity relation and cosmological applications of quasars if simulations correcting for such effects are not performed.

astro-ph.GA↗
arXiv2023

The detection of possible transient Quasi-Periodic Oscillations in the $γ$-ray light curve of PKS 0244-470 and 4C+38.41

The continuous monitoring capability of Fermi-LAT has enabled the exploration of Quasi-Periodic Oscillations (QPOs) in the $γ$-ray light curve of blazar that has given a new perspective to probe these sources over a wide range of time scales. We report the presence of transient QPOs in the long-term $γ$-ray light curve of blazars PKS 0244-470 and 4C +38.41. We first identified different flux states using the Bayesian Block algorithm and then explored the possible transient QPOs in the segments of each flux phase where the flux level changes over fairly regular intervals. Combining this with the source's intrinsic variance, we identified two flux phases for PKS 0244-470: one activity (AP-1) and one quiescent phase (QP-1). For 4C+38.41, we similarly identified four activity (AP-1, AP-2, AP-3, AP-4) and two quiescent (QP-1, QP-2) phases. AP-1 phase of PKS 0244-470 shows QPO of $\sim$225 days persisting for 8 cycles ($\sim$ 4.1$σ$). In 4C+38.41, AP-1 and AP-2 phases show QPO-like behavior of $\sim$110 days and $\sim$ 60 days, respectively, persisting for 5 cycles. In AP-3, we identified three sub-phases, and all show a $\sim$ week scale possible recurrent rise with five complete cycles, while in QP-1, we could identify 2 sub-phases (Q1, and Q2). Q1 phase shows a period of $\sim$ 104 days with six complete cycles. Q2 phase also shows QPO but with only $\sim$3.7 cycles. We discuss the possible origin and argue that the current driven kink instability and curved jet model seem the most likely cause for shorter and longer QPOs.

astro-ph.HE↗
arXiv2023

Quasar UV/X-ray relation luminosity distances are shorter than reverberation-measured radius-luminosity relation luminosity distances

We use measurements of 59/58 quasars (QSOs), over a redshift range $0.0041\leq z \leq 1.686$, to do a comparative study of the radius--luminosity ($R-L$) and X-ray$-$UV luminosity ($L_X-L_{UV}$) relations and the implication of these relations for cosmological parameter estimation. By simultaneously determining $R-L$ or $L_X-L_{UV}$ relation parameters and cosmological parameters in six different cosmological models, we find that both $R-L$ and $L_X-L_{UV}$ relations are standardizable but provide only weak cosmological parameter constraints, with $L_X-L_{UV}$ relation data favoring larger current non-relativistic matter density parameter $Ω_{m0}$ values than $R-L$ relation data and most other available data. We derive $L_X-L_{UV}$ and $R-L$ luminosity distances for each of the sources in the six cosmological models and find that $L_X-L_{UV}$ relation luminosity distances are shorter than $R-L$ relation luminosity distances as well as standard flat $Λ$CDM model luminosity distances. This explains why $L_X-L_{UV}$ relation QSO data favor larger $Ω_{m0}$ values than do $R-L$ relation QSO data or most other cosmological measurements. While our sample size is small and only spans a small $z$ range, these results indicate that more work is needed to determine whether the $L_X-L_{UV}$ relation can be used as a cosmological probe.

astro-ph.CO↗
arXiv2023

Dusty plasma in active galactic nuclei

Since many years we know that dust in the form of the dusty-molecular torus is responsible for the obscuration in active galactic nuclei (AGN) at large viewing angles and thus for the classification of AGN. Recently, we gained some observational and theoretical insight into geometry of the region and the role of the dust in the dynamics of the outflow and failed winds. We will briefly touch on all these aspects, including our dust-based model (FRADO - Failed Radiatively Accelerated Dusty Outflow) of the formation of the Balmer lines in AGN.

astro-ph.GA↗
arXiv2023

Accretion disks, quasars and cosmology: meandering towards understanding

As Setti & Woltjer noted back in 1973, quasars could be used to construct the Hubble diagram but the actual application was not that straightforward. It took years to implement the idea successfully. Most of the ways to use quasars for cosmology now require an advanced understanding of their structure, step by step. We briefly review this progress, with unavoidable personal bias, and concentrate on bright unobscured sources. We will mention the problem of the gas flow character close to the innermost stable circular orbit close to the black hole, discussed 50 years ago, which later led to the development of the slim disk scenario, but was recently revived in the context of Magnetically Arrested Disks (MAD) and Standard and Normal Evolution (SANE) disk models. We also discuss the hot/warm corona issue, which is still under discussion and complicates the analysis of the X-ray reflection. We present the scenario of the formation of the low ionization part of the Broad Line Region as a mostly failed wind powered by radiation pressure acting on dust (FRADO - Failed Radiatively Driven Dusty Outflow model). Next, we discuss the cosmological constraints that are currently achievable with quasars, mostly concentrating on light echo methods (continuum time delays and spectral line time delays with respect to the continuum) which are (or should be) incorporating the progress mentioned above. Finally, we briefly mention future prospects in this direction.

astro-ph.GA↗
arXiv2023

Modelling time delays from two reprocessors in active galactic nuclei

Context. Continuum time delays from accretion disks in active galactic nuclei (AGN) has been proposed long time ago as a tool for measuring distances to the monitored sources. However, the method faces serious problems as a number of effects must be taken into account, including the contribution from the Broad Line Region (BLR). Aims. In this paper, we model the expected time delays when both the disk reprocessing of the incident X-ray flux and further reprocessing by the BLR are included, with the aim to see if the two effects can be disentangled. Methods. We use simple response function for the accretion disk, without relativistic effects, and we use a parametric description to account for the BLR contribution. We include only scattering of the disk emission by the BLR inter-cloud medium. We also use artificial lightcurves with 1-day sampling to check if the effects are likely to be seen in real data. Results. We show that the effect of the BLR scattering on the predicted time delay is very similar to the effect of rising height of the X-ray source, without any BLR contribution. This brings additional degeneracy if we want in the future to recover the parameters of the system from the observed time delays in a specific object. Both effects, however, modify the slope of the delay versus wavelength curve when plotted in log space which opens a way to obtaining bare disk time delay needed for cosmology. In addition, when the disk irradiation is strong, the modification of the predicted delay by the BLR scattering and by X-ray source height become considerably different. However, the required data quality must be high since our artificial curves with 1-day sampling give rather noisy results.

astro-ph.GA↗
arXiv2023

Multi-frequency Variability Study of Flat-Spectrum Radio Quasar PKS 0346-27

We have presented a multiwavelength temporal and spectral study of the Blazar PKS 0346-27 for the period 2019 January-2021 December (MJD 58484-59575) using data from Fermi-LAT (gamma-rays), Swift-XRT (X-rays) and Swift-UVOT (ultra-violet and optical). We identified multiple flaring episodes by analyzing the gamma-ray light curve generated from the Fermi-LAT data over a two-year period. The light curves of these individual gamma-ray flares with one-day binning were then modeled using a sum-of-exponentials fit. We found the minimum variability times for the gamma-ray light curve to be 1.34 +\- 0.3 days and a range of 0.1-3.2 days for the Swift wavelengths suggesting the compactness of the source. The broadband emission mechanism was studied by modeling the simultaneous multi-waveband Spectral Energy Distributions (SED) using the one-zone leptonic emission mechanism. We found that the optical-UV and X-ray data can be explained by the synchrotron and Synchrotron Self-Compton (SSC) emissions. However, the disk component of the External Compton radiation is dominant at higher energies with contributions from the EC broad line region component and SSC. Further, we performed a power spectral density (PSD) analysis with data from the gamma-ray light curve using the power spectrum response (PSRESP) method. With the power law model, a best-fit slope of 2.15 +\- 0.87 was found. This source could be a promising target for upcoming CTA for its harder spectrum at lower energies (tens of GeV).

astro-ph.HE↗
arXiv2022

Temporal and spectral study of PKS 0208-512 during the 2019-2020 flare

We present the temporal and spectral study of blazar PKS 0208-512, using recent flaring activity from November 2019 to May 2020, as detected by the Fermi-LAT observatory. The contemporaneous X-ray, optical/UV observations from Swift-XRT/UVOT are also used. During the activity state, 2-days binned $γ$-ray lightcurve shows multiple peaks indicating sub-flares. To understand the possible physical mechanisms behind flux enhancement, we divided the activity state of the source into several flux-states and carried out detailed temporal and spectral studies. Timing analysis of lightcurves suggest that peaks of sub-flares have rise and decay time on the order of days, with flux-doubling time $\sim$ 2-days. The 2-days binned $γ$-ray lightcurve shows double-lognormal flux distribution. The broadband spectral-energy-distribution (SED) for three selected flux states can be well fitted under synchrotron, synchrotron self-Compton (SSC) and external-Compton (EC) emission mechanisms. We obtained the physical parameters of the jet by the SED modeling and their confidence intervals through $χ^2$-statistics. Our SED modeling results suggest that during quiescent-state, $γ$-ray spectrum can be well explained by considering the EC-scattering of infra-red (IR) photons from dusty-torus. However, $γ$-ray spectra corresponding to flares demand additional target photons from broad-line-region (BLR) along with IR. These suggest that during flares, the emission-region is close to the edge of BLR, while for quiescent-state the emission-region is away from BLR. The best fit results suggest that marginal increase in magnetic-field during the flaring-episode can result in the flux enhancement. This is possibly associated with the efficiency of particle acceleration during flaring-states as compared to quiescent-state.

astro-ph.HE↗
arXiv2022

Multiwavelength temporal and spectral study of TeV blazar 1ES 1727+502 during 2014 to 2021

One of the most important questions in blazar physics is the origin of broadband emission and fast-flux variation. In this work, we studied the broadband temporal and spectral properties of a TeV blazar 1ES 1727+502 and explore the one-zone synchrotron-self Compton (SSC) model to fit the broadband spectral energy distribution (SED). We collected the long-term (2014-2021) multiband data which includes both the low and high flux states of the source. The entire light curve is divided into three segments of different flux states and the best-fit parameters obtained by broadband SED modeling corresponding to three flux states were then compared. The TeV blazar 1ES 1727+502 has been observed to show the brightest flaring episode in X-ray followed by optical-UV and gamma-ray. The fractional variability estimated during various segments behaves differently in multiple wavebands, suggesting a complex nature of emission in this source. This source has shown a range of variability time from days scale to month scale during this long period of observations between 2014-2021. A "harder-when-brighter" trend is not prominent in X-ray but seen in optical-UV and an opposite trend is observed in gamma-ray. The complex nature of correlation among various bands is observed. The SED modeling suggests that the one-zone SSC emission model can reproduce the broadband spectrum in the energy range from optical-UV to very high energy gamma-ray.

astro-ph.HE↗