arXiv subjects

V. Chavushyan

Publications and source records attributed to V. Chavushyan.

At least 19 recordsLinked to original sources

arXiv2025

Tracking optical variability and outflows across the accretion states of the black hole transient MAXI J1820+070

We present a study of the minute-timescale optical variability and spectroscopic outflow signatures in the black hole X-ray binary MAXI J1820+070 during its 2018 outburst and re-brightenings. Minute-cadence, multi-filter optical light curves were obtained with the Las Cumbres Observatory network and the Al Sadeem Observatory (UAE) over 2018-2020, complemented by X-ray data from Swift/BAT, XRT, and MAXI. We also acquired contemporaneous low-resolution optical spectra with the 2.1 m OAN San Pedro Martir and OAGH Cananea telescopes (Mexico) and the 1.5 m G.D. Cassini telescope at Loiano (Italy). The optical fractional rms peaks in the hard state and is dominated by short-timescale flickering that strengthens toward longer wavelengths, suggesting that the variability is jet-driven. In this scenario, inflow fluctuations inject velocity perturbations at the jet base (internal shock model). The variability is quenched in the soft state, with any residual signal likely linked to accretion-flow fluctuations. This behaviour supports the accretion-ejection coupling in black hole binaries and shows that the jet's variability signature extends to optical wavelengths in all hard states. In the faintest hard states, residual optical variability may instead trace cyclo-synchrotron emission from the hot flow. The spectra show double-peaked emission lines and tentative cold-wind signatures during the hard state. Such winds were reported during the main 2018 outburst; here we find evidence of their presence also in later re-brightenings. Their absence in the soft state likely reflects stronger disc ionisation from the higher X-ray flux, suppressing low-ionisation optical features.

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arXiv2024

AT 2021hdr: A candidate tidal disruption of a gas cloud by a binary super massive black hole system

With a growing number of facilities able to monitor the entire sky and produce light curves with a cadence of days, in recent years there has been an increased rate of detection of sources whose variability deviates from standard behavior, revealing a variety of exotic nuclear transients. The aim of the present study is to disentangle the nature of the transient AT 2021hdr, whose optical light curve used to be consistent with a classic Seyfert 1 nucleus, which was also confirmed by its optical spectrum and high-energy properties. From late 2021, AT 2021hdr started to present sudden brightening episodes in the form of oscillating peaks in the Zwicky Transient Facility (ZTF) alert stream, and the same shape is observed in X-rays and UV from Swift data. The oscillations occur every about 60-90 days with amplitudes of around 0.2 mag in the g and r bands. Very Long Baseline Array (VLBA) observations show no radio emission at milliarcseconds scale. It is argued that these findings are inconsistent with a standard tidal disruption event (TDE), a binary supermassive black hole (BSMBH), or a changing-look active galactic nucleus (AGN); neither does this object resemble previous observed AGN flares, and disk or jet instabilities are an unlikely scenario. Here, we propose that the behavior of AT 2021hdr might be due to the tidal disruption of a gas cloud by a BSMBH. In this scenario, we estimate that the putative binary has a separation of about 0.83 mpc and would merge in about 70000 years. This galaxy is located at 9 kpc from a companion galaxy, and in this work we report this merger for the first time. The oscillations are not related to the companion galaxy.

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arXiv2024

Swift-XRT follow-up analysis of unidentified hard X-ray sources

It is currently established that the sources contributing to the cosmic X-ray background (CXB) emission are mainly nearby active galactic nuclei (AGN), in particular those that are obscured. Thus, it is important to fully identify the hard X-ray sky source population to accurately characterize the individual contribution of different AGNs to the overall CXB emission. We present a follow-up analysis of all the 218 sources marked as unidentified in our previous revision of the third release of the Palermo Swift-BAT hard X-ray catalog (3PBC) based on our multifrequency classification scheme. These 218 sources were classified as unidentified in our previous analyses because they lack an assigned low-energy counterpart. We searched for soft X-ray counterparts of these 218 3PBC sources in archival Swift-XRT observations obtained between 2005 January 1st and 2018 August 1st. In particular, we found 1213 archival Swift-XRT observations for 192 of the 218 unidentified sources. We found 93 possible Swift-XRT counterparts inside of the Swift-BAT positional uncertainty regions. These correspond to 73 3PBC sources, where 60 have only a single Swift-XRT detection, and 13 sources have multiple detections. We present a catalog of all the detected possible counterparts of the yet unidentified hard X-ray sources to the community as a catalog for future spectroscopic follow-up targets, together with a short catalog of our classification of the ten sources for which we found available spectra.

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arXiv2023

Powerful Radio Sources in the Southern Sky. II. A SWIFT X-Ray Perspective

We recently constructed the G4Jy-3CRE, a catalog of extragalactic radio sources based on the GLEAM 4-Jy (G4Jy) sample, with the aim of increasing the number of powerful radio galaxies and quasars with similar selection criteria to those of the revised release of the Third Cambridge catalog (3CR). The G4Jy-3CRE consists of a total of 264 radio sources mainly visible from the Southern Hemisphere. Here, we present an initial X-ray analysis of 89 G4Jy-3CRE radio sources with archival X- ray observations from the Neil Gehrels Swift Observatory. We reduced a total of 615 Swift observations, for about 0.89 Msec of integrated exposure time, we found X-ray counterparts for 61 radio sources belonging to the G4Jy-3CRE, 11 of them showing extended X-ray emission. The remaining 28 sources do not show any X-ray emission associated with their radio cores. Our analysis demonstrates that X-ray snapshot observations, even if lacking uniform exposure times, as those carried out with Swift, allow us to (i) verify and/or re ne the host galaxy identi cation; (ii) discover the extended X-ray emission around radio galaxies of the intracluster medium when harbored in galaxy clusters, as the case of G4Jy 1518 and G4Jy 1664, and (iii) detect X-ray radiation arising from their radio lobes, as for G4Jy 1863.

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arXiv2023

Powerful Radio Sources in the Southern Sky. I. Optical Identifications

Since the early sixties, our view of radio galaxies and quasars has been drastically shaped by discoveries made thanks to observations of radio sources listed in the Third Cambridge catalog and its revised version (3CR). However, the largest fraction of data collected to date on 3CR sources was performed with relatively old instruments, rarely repeated and/or updated. Importantly, the 3CR contains only objects located in the Northern Hemisphere thus having limited access to new and innovative astronomical facilities. To mitigate these limitations we present a new catalog of powerful radio sources visible from the Southern Hemisphere, extracted from the GLEAM 4-Jy (G4Jy) catalog and based on equivalent selection criteria as the 3CR. This new catalog, named G4Jy- 3CRE, where the E stands for "equivalent", lists a total of 264 sources at declination below -5 degrees and with 9 Jy limiting sensitivity at ~178 MHz. We explored archival radio maps obtained with different surveys and compared then with optical images available in the Pan-STARRS, DES and DSS databases to search for optical counterparts of their radio cores. We compared mid-infrared counterparts, originally associated in the G4Jy, with the optical ones identified here and we present results of a vast literature search carried out to collect redshift estimates for all G4Jy-3CRE sources resulting in a total of 145 reliable z measurements.

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arXiv2023

Multiwavelength monitoring of the nucleus in PBC J2333.9-2343: the giant radio galaxy with a blazar-like core

PBC J2333.9-2343 is a giant radio galaxy at z = 0.047 with a bright central core associated to a blazar nucleus. If the nuclear blazar jet is a new phase of the jet activity, then the small orientation angle suggest a dramatic change of the jet direction. We present observations obtained between September 2018 and January 2019 (cadence larger than three days) with Effeslberg, SMARTS-1.3m, ZTF, ATLAS, Swift, and Fermi-LAT, and between April-July 2019 (daily cadence) with SMARTS-1.3m and ATLAS. Large (>2x) flux increases are observed on timescales shorter than a month, which are interpreted as flaring events. The cross correlation between the SMARTS-1.3m monitoring in the NIR and optical shows that these data do not show significant time lag within the measured errors. A comparison of the optical variability properties between non-blazars and blazars AGN shows that PBC J2333.9-2343 has properties more comparable to the latter. The SED of the nucleus shows two peaks, that were fitted with a one zone leptonic model. Our data and modelling shows that the high energy peak is dominated by External Compton from the dusty torus with mild contribution from Inverse Compton from the jet. The derived jet angle of 3 degrees is also typical of a blazar. Therefore, we confirm the presence of a blazar-like core in the center of this giant radio galaxy, likely a Flat Spectrum Radio Quasar with peculiar properties.

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arXiv2023

New Spectroscopy of U Gem

We present new optical spectroscopic observations of U Geminorum obtained during a quiescent stage. We performed a radial velocity analysis of three Balmer emission lines yielding inconsistent results. Assuming that the radial velocity semi amplitude accurately reflects the motion of the white dwarf, we arrive at masses for the primary which are in the range of M_wd= 1.21 - 1.37 M_Sun. Based on the internal radial velocity inconsistencies and results produced from the Doppler tomography -- wherein we do not detect emission from the hot spot, but rather an intense asymmetric emission overlaying the disc, reminiscent of spiral arms -- we discuss the possibility that the overestimation of the masses may be due to variations of gas opacities and a partial truncation of the disc.

astro-ph.SR↗

arXiv2022

Disentangling the nature of the prototype radio weak BL Lac: Contemporaneous multifrequency observations of WISE J141046.00 + 740511.2

Context. The gamma-ray emitting source WISE J141046.00+740511.2 has been associated with a Fermi-LAT detection by crossmatching with Swift/XRT data. It has shown all the canonical observational characteristics of a BL Lac source, including a power-law, featureless optical spectrum. However, it was only recently detected at radio frequencies and its radio flux is significantly low. Aims. Given that a radio detection is fundamental to associate lower-energy counterparts to Fermi-LAT sources, we aim to unambiguously classify this source by performing a multiwavelength analysis based on contemporaneous data. Methods. By using multifrequency observations at the Jansky Very Large Array, Giant Metrewave Radio Telescope, Gran Telescopio Canarias, Gemini, William Herschel Telescope and Liverpool observatories, together with Fermi-LAT and Swift data, we carried out two kinds of analyses. On one hand, we studied several known parameters that account for the radio loudness or weakness characterization and their application to blazars (in general) and to our source (in particular). And, on the other hand, we built and analyzed the observed spectral energy distribution (SED) of this source to try to explain its peculiar characteristics. Results. The multiwavelength analysis indicates that WISE J141046.00+740511.2 is a blazar of the high-frequency peaked (HBL) type that emits highly polarized light and that is likely located at a low redshift. In addition, the one-zone model parameters that best fit its SED are those of an extreme HBL (EHBL); this blazar type has been extensively predicted in theory to be lacking in the radio emission that is otherwise typical of canonical gamma-ray blazars. Conclusions. We confirm that WISE J141046.00+740511.2 is indeed a highly polarized BL Lac of the HBL type. Further studies will be conducted to explain the atypical low radio flux detected for this source.

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arXiv2022

Long Term Multi-band Near Infra-Red Variability of the Blazar OJ 287 during 2007--2021

We present the most extensive and well-sampled long-term multi-band near-infrared (NIR) temporal and spectral variability study of OJ 287, considered to be the best candidate binary supermassive black hole blazar. These observations were made between December 2007 and November 2021. The source underwent ~ 2 -- 2.5 magnitude variations in the J, H, and Ks NIR bands. Over these long-term timescales there were no systematic trends in either flux or spectral evolution with time or with the source's flux states. However, on shorter timescales, there are significant variations in flux and spectra indicative of strong changes during different activity states. The NIR spectral energy distributions show diverse facets at each flux state, from the lowest to the highest. The spectra are, in general, consistent with a power-law spectral profile (within 10%) and many of them indicate minor changes (observationally insignificant) in the shift of the peak. The NIR spectra generally steepens during bright phases. We briefly discuss these behaviors in the context of blazar emission scenarios/mechanisms, OJ 287's well-known traditional behavior, and implications for models of the source central engine invoked for its long-term optical semi-periodic variations.

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arXiv2021

Evidence of galaxy interaction in the Narrow-line Seyfert 1 galaxy IRAS17020+4544 seen by NOEMA

The narrow-line Seyfert 1 galaxy IRAS17020+4544 is one of the few sources where both an X-ray ultra-fast outflow and a molecular outflow were observed to be consistent with energy conservation. However, IRAS17020+4544 is less massive and has a much more modest active galactic nucleus (AGN) luminosity than the other examples. Using recent CO(1-0) observations with the NOrthern Extended Millimeter Array (NOEMA), we characterised the molecular gas content of the host galaxy for the first time. We found that the molecular gas is distributed into an apparent central disc of 1.1x10^9 Msun, and a northern extension located up to 8 kpc from the centre with a molecular gas mass M_H2~10^8 Msun. The molecular gas mass and the CO dynamics in the northern extension reveal that IRAS 17020+4544 is not a standard spiral galaxy, instead it is interacting with a dwarf object corresponding to the northern extension. This interaction possibly triggers the high accretion rate onto the super massive black hole. Within the main galaxy, which hosts the AGN, a simple analytical model predicts that the molecular gas may lie in a ring, with less molecular gas in the nuclear region. Such distribution may be the result of the AGN activity which removes or photodissociates the molecular gas in the nuclear region (AGN feedback). Finally, we have detected a molecular outflow of mass M_H2=(0.7-1.2)x10^7 Msun in projection at the location of the northern galaxy, with a similar velocity to that of the massive outflow reported in previous millimeter data obtained by the Large Millimeter Telescope.

astro-ph.GA↗

arXiv2020

Optical spectroscopic observations of low-energy counterparts of Fermi-LAT gamma-ray sources

A significant fraction of all $γ$-ray sources detected by the Large Area Telescope aboard the \fer\ satellite is still lacking a low-energy counterpart. In addition, there is still a large population of $γ$-ray sources with associated low-energy counterparts that lack firm classifications. In the last 10 years we have undertaken an optical spectroscopic campaign to address the problem of unassociated/unidentified $γ$-ray sources (UGSs), mainly devoted to observing blazars and blazar candidates because they are the largest population of $γ$-ray sources associated to date. Here we describe the overall impact of our optical spectroscopic campaign on sources associated in \fer-LAT catalogs, coupled with objects found in the literature. In the literature search, we kept track of efforts by different teams that presented optical spectra of counterparts or potential counterparts of \fer-LAT catalog sources. Our summary includes an analysis of an additional 30 newly-collected optical spectra of counterparts or potential counterparts of \fer-LAT sources of previously unknown nature.New spectra were acquired at the Blanco 4-m and OAN-SPM 2.1-m telescopes, and those available in the Sloan Digital Sky Survey (data release 15) archive. All new sources with optical spectra analyzed here are classified as blazars. Thanks to our campaign, we altogether discovered and classified 394 targets with an additional 123 objects collected from a literature search. We began our optical spectroscopic campaign between the release of the second and third \fer-LAT source catalogs (2FGL and 3FGL, respectively), and classified about 25\% of the sources with uncertain nature and discovered a blazar-like potential counterpart for $\sim$10\% of UGSs listed therein. In the 4FGL catalog, about 350 \fer-LAT sources are classified to date thanks to our campaign. [incomplete abstract]

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arXiv2020

Optical spectroscopic classification of 35 hard X-ray sources from the Swift-BAT 70-month catalogue

The nature of a substantial percentage (about one fifth) of hard X-ray sources discovered with the BAT instrument onboard the Neil Gehrels Swift Observatory (hereafter Swift) is unknown because of the lack of an identified longer-wavelength counterpart. Without such follow-up, an X-ray catalogue is of limited astrophysical value: we therefore embarked, since 2009, on a long-term project to uncover the optical properties of sources identified by Swift by using a large suite of ground-based telescopes and instruments. In this work, we continue our programme of characterization of unidentified or poorly studied hard X-ray sources by presenting the results of an optical spectroscopic campaign aimed at pinpointing and classifying the optical counterparts of 35 hard X-ray sources taken from the 70-month BAT catalogue. (...) With the use of optical spectra taken at six different telescopes we were able to identify the main spectral characteristics (continuum type, redshift, and emission or absorption lines) of the observed objects, and determined their nature. We identify and characterize a total of 41 optical candidate counterparts corresponding to 35 hard X-ray sources given that, because of positional uncertainties, multiple lower energy counterparts can sometimes be associated with higher energy detections. We discuss which ones are the actual (or at least most likely) counterparts based on our observational results. In particular, 31 sources in our sample are active galactic nuclei: 16 are classified as Type 1 (with broad and narrow emission lines) and 13 are classified as Type 2 (with narrow emission lines only); two more are BL Lac-type objects. We also identify one LINER, one starburst, and 3 elliptical galaxies. The remaining 5 objects are galactic sources: we identify 4 of them as cataclysmic variables, whereas one is a low mass X-ray binary.

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arXiv2020

Optical spectroscopic observations of gamma-ray blazar candidates. IX. Optical archival spectra and further observations from SOAR and OAGH

Nearly one-third of the sources in the $Fermi$-LAT catalogs lack a lower energy counterpart, hence being referred as unidentified/unassociated gamma-ray sources (UGSs). In order to firmly classify them, dedicated multifrequency follow-up campaigns are necessary. These will permit to unveil their nature and identify the fraction that could belong to the class of active galaxies known as blazars that is the largest population of extragalactic $γ$-ray sources. In $Fermi$-LAT catalogs there are also gamma-ray sources associated with multifrequency blazar-like objects known as Blazars Candidates of Uncertain type (i.e., BCUs) for which follow up spectroscopic campaigns are mandatory to confirm their blazar nature. Thus, in 2013 we started an optical spectroscopic campaign to identify blazar-like objects potential counterparts of UGSs and BCUs. Here we report the spectra of 31 additional targets observed as part of our follow up campaign. Thirteen of them are BCUs for which we acquired spectroscopic observations at Observatorio Astrofísico Guillermo Haro (OAGH) and at Southern Astrophysical Research Observatory (SOAR) telescopes, while the rest has been identified thanks to the archival observations available from the Sloan Digital Sky Survey (SDSS). We confirm the blazar nature of all BCUs: three of them are in blazar of quasar type (BZQs) while the remaining ones can be spectroscopically classified as BL Lac objects (BZBs). Then we also discovered 18 BL Lac objects lying within the positional uncertainty regions of UGSs that could be their potential counterparts.

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arXiv2020

Baldwin Effect and Additional BLR Component in AGN with Superluminal Jets

We study the Baldwin Effect (BE) in 96 core-jet blazars with optical and ultraviolet spectroscopic data from a radio-loud AGN sample obtained from the MOJAVE 2 cm survey. A statistical analysis is presented of the equivalent widths ($W_λ$) of emission lines H$β\,λ$4861, Mg II\,$λ$2798, C IV\,$λ$1549, and continuum luminosities at 5100\,Å, 3000\,Å, and 1350\,Å. The BE is found statistically significant (with confidence level \textit{c.l.} $\geq\,$ 95\%) in H$β$ and C IV emission lines, while for Mg II the trend is slightly less significant (\textit{c.l.} = 94.5\%). The slopes of the BE in the studied samples for H$β$ and Mg II are found steeper and with statistically significant difference than those of a comparison radio-quiet sample. We present simulations of the expected BE slopes produced by the contribution to the total continuum of the non-thermal boosted emission from the relativistic jet, and by variability of the continuum components. We find that the slopes of the BE between radio-quiet and radio-loud AGN should not be different, under the assumption that the broad line is only being emitted by the canonical broad line region around the black hole. We discuss that the BE slope steepening in radio AGN is due to a jet associated broad-line region.

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arXiv2019

A young and obscured AGN embedded in the giant radio galaxy Mrk 1498

Mrk 1498 is part of a sample of galaxies with extended emission line regions (extended outwards up to a distance of $\sim$7 kpc) suggested to be photo-ionized by an AGN that has faded away or that is still active but heavily absorbed. Interestingly, the nucleus of Mrk 1498 is at the center of two giant radio lobes with a projected linear size of 1.1 Mpc. Our multi-wavelength analysis reveals a complex nuclear structure, with a young radio source (Giga-hertz Peaked Spectrum) surrounded by a strong X-ray nuclear absorption, a mid-infrared spectrum that is dominated by the torus emission, plus a circum-nuclear extended emission in the [OIII] image (with radius of $\sim$ 1 kpc), most likely related to the ionization of the AGN, aligned with the small and large scale radio jet and extended also at X-rays. In addition a large-scale extended emission (up to $\sim$ 10 kpc) is only visible in [OIII]. These data show conclusive evidence of a heavily absorbed nucleus and has recently restarted its nuclear activity. To explain its complexity, we propose that Mrk 1498 is the result of a merging event or secular processes, such as a minor interaction, that has triggered the nuclear activity and produced tidal streams. The large-scale extended emission that gives place to the actual morphology could either be explained by star formation or outflowing material from the AGN.

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arXiv2019

From outburst to quiescence: spectroscopic evolution of V1838 Aql imbedded in a bow-shock nebula

We analyse new optical spectroscopic, direct-image and X-ray observations of the recently discovered a high proper motion cataclysmic variable V1838 Aql. The data were obtained during its 2013 superoutburst and its subsequent quiescent state. An extended emission around the source was observed up to 30 days after the peak of the superoutburst, interpreted it as a bow--shock formed by a quasi-continuous outflow from the source in quiescence. The head of the bow--shock is coincident with the high--proper motion vector of the source ($v_{\perp}=123\pm5$ km s$^{-1}$) at a distance of $d=202\pm7$ pc. The object was detected as a weak X-ray source ($0.015\pm0.002$ counts s$^{-1}$) in the plateau of the superoutburst, and its flux lowered by two times in quiescence (0.007$\pm$0.002 counts s$^{-1}$). Spectroscopic observations in quiescence we confirmed the orbital period value $P_{\rm{orb}}=0.0545\pm 0.0026$ days, consistent with early-superhump estimates, and the following orbital parameters: $γ= -21\pm3$ km s$^{-1}$ and $K_1 = 53\pm3$ km s$^{-1}$. The white dwarf is revealed as the system approaches quiescence, which enables us to infer the effective temperature of the primary $T_{eff}=11,600\pm400$K. The donor temperature is estimated $\lesssim 2200$K and suggestive of a system approaching the period minimum. Doppler maps in quiescence show the presence of the hot spot in HeI line at the expected accretion disc-stream shock position and an unusual structure of the accretion disc in H$α$.

astro-ph.SR↗

arXiv2018

Early Science with the Large Millimeter Telescope: an energy-driven wind revealed by massive molecular and fast X-ray outflows in the Seyfert Galaxy IRAS 17020+4544

We report on the coexistence of powerful gas outflows observed in millimeter and X-ray data of the Radio-Loud Narrow Line Seyfert 1 Galaxy IRAS 17020+4544. Thanks to the large collecting power of the Large Millimeter Telescope, a prominent line arising from the 12CO(1-0) transition was revealed in recent observations of this source. The complex profile is composed by a narrow double-peak line and a broad wing. While the double-peak structure may be arising in a disk of molecular material, the broad wing is interpreted as the signature of a massive outflow of molecular gas with an approximate bulk velocity of -660 km/s. This molecular wind is likely associated to a multi-component X-ray Ultra-Fast Outflow with velocities reaching up to ~0.1c and column densities in the range 10^{21-23.9} cm^-2 that was reported in the source prior to the LMT observations. The momentum load estimated in the two gas phases indicates that within the observational uncertainties the outflow is consistent with being propagating through the galaxy and sweeping up the gas while conserving its energy. This scenario, which has been often postulated as a viable mechanism of how AGN feedback takes place, has so far been observed only in ULIRGs sources. IRAS 17020+4544 with bolometric and infrared luminosity respectively of 5X10^{44} erg/s and 1.05X10^{11} L_sun appears to be an example of AGN feedback in a NLSy1 Galaxy (a low power AGN). New proprietary multi-wavelength data recently obtained on this source will allow us to corroborate the proposed hypothesis.

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arXiv2018

Multiwavelength Photometric and Spectropolarimetric Analysis of the FSRQ 3C 279

In this paper, we present light curves for 3C 279 over a time period of six years; from 2008 to 2014. Our multiwavelength data comprise 1 mm to gamma-rays, with additional optical polarimetry. Based on the behaviour of the gamma-ray light curve with respect to other bands, we identified three different activity periods. One of the activity periods shows anomalous behaviour with no gamma-ray counterpart associated with optical and NIR flares. Another anomalous activity period shows a flare in gamma-rays, 1 mm and polarization degree, however, it does not have counterparts in the UV continuum, optical and NIR bands. We find a significant overall correlation of the UV continuum emission, the optical and NIR bands. This correlation suggests that the NIR to UV continuum is co-spatial. We also find a correlation between the UV continuum and the 1 mm data, which implies that the dominant process in producing the UV continuum is synchrotron emission. The gamma-ray spectral index shows statistically significant variability and an anti-correlation with the gamma-ray luminosity. We demonstrate that the dominant gamma-ray emission mechanism in 3C 279 changes over time. Alternatively, the location of the gamma-ray emission zone itself may change depending on the activity state of the central engine.

astro-ph.HE↗