arXiv subjects

Zhongzu Wu

Publications and source records attributed to Zhongzu Wu.

At least 19 recordsLinked to original sources

arXiv2026

OH Line Detections in Southern Galaxies of the IRAS Revised Bright Galaxy Sample

We present a systematic study of OH main-line emission and absorption in 186 southern galaxies from the IRAS Revised Bright Galaxy Sample, using archival MeerKAT snapshot data. OH features are detected in 38 galaxies, including eight with OH maser emission (three new) and 30 showing OH absorption, mostly unreported previously. Four absorption systems exhibit weak OH emission superposed on strong absorption. OH-emitting regions are generally more compact than the associated radio continuum. Most absorption profiles are well fit by two Gaussian components (1667 and 1665 MHz), with an average integrated line ratio of $\sim$1.5. LIRGs show an OH emission detection rate of ~13\%, versus significantly lower rates in non-LIRGs. For sources with radio continuum flux densities >20 mJy, OH absorption detection rates reach ~36\% (LIRGs) and ~27\% (non-LIRGs), while no OH absorption features were detected among sources with lower radio continuum flux densities. This suggests that sufficient background continuum is likely an important factor for the detection of OH absorption. Detected OH emitters follow the empirical $L_{\rm OH}$--$L_{\rm FIR}$ relation, consistent with far-infrared pumping, while non-detections show upper limits below the relation. No significant differences are found between OH absorbers and non-detections in infrared luminosity or radio continuum compactness. Stacked spectra of non-detections reveal no significant OH features, suggesting that sensitivity and orientation alone do not fully explain the absence of absorption. In contrast, mid-infrared colors (e.g., W2--W3) and q_TIR differ between the two populations. OH absorption galaxies occupy an intermediate regime in L_HCN/L_CO between OH megamasers and non-detections, implying that OH absorption detectability is linked to dense molecular gas conditions, with extreme star formation potentially suppressing its occurrence.

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arXiv2026

\HI 21-cm Line Properties of the Nearby LIRG IRAS 04296+2923

We present an analysis of archival Very Large Array (VLA) and Five-hundred-meter Aperture Spherical radio Telescope (FAST) \HI\ 21 cm data, together with archival multi-band radio continuum observations, of the nearby luminous infrared galaxy IRAS~04296+2923. The system, located behind the Taurus dark cloud at a distance of $\sim$29 Mpc, forms a small galaxy group consisting of five members as revealed by the \HI\ imaging. IRAS~04296+2923 has a close companion, HI~0432+2926, with a projected separation of $\sim$40 kpc, a small line-of-sight velocity difference of $Δ$ v = 26 km s$^{-1}$, and comparable total \HI\ masses of order $10^{9}$~$M_{\odot}$. Both galaxies exhibit regular \HI\ velocity fields and characteristic double-horn profiles in the VLA and FAST data, accompanied by only subtle asymmetries and extended \HI\ structures, indicating rotation-dominated kinematics with early signs of weak tidal interaction. Radio continuum emission is detected only from IRAS~04296+2923 and is confined to its nuclear region, consistent with previous studies. Modeling of its multi-band radio spectrum reveals a significant contribution from free--free emission at high frequencies ($>$30 GHz) and a high FIR-to-radio flux ratio ($q_{8.4}\simeq3.2$), implying a young, dust-obscured nuclear starburst. Taken together, the regular \HI\ kinematics, the small velocity offset, and the group-scale environment favor an interpretation in which IRAS~04296+2923 and HI~0432+2926 form a gravitationally bound, orbiting galaxy pair embedded in a small group, rather than an advanced merger. In this context, the luminous infrared galaxy (LIRG) nature of IRAS~04296+2923 is more plausibly driven by internal processes, such as bar-induced gas inflow, possibly modulated by long-timescale, low-level tidal interactions with nearby group companions.

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arXiv2026

The FAST Discovery of a binary millisecond pulsar PSR~J1647-0156B (M12B) with a candidate cross matching algorithm

We propose a pulsar candidate cross matching algorithm to sift radio pulsar search candidates from repeated observations of the same sky location such as globular clusters, high energy sources, or supernova remnants. Our method uses both the candidate spin period ($P$) and dispersion measure (DM) value; if two or more candidates from different observations have similar spin periods to within 1\%, and dispersion measure values within 10\%, they are likely to correspond to the same candidate detection. We have demonstrated the effectiveness of our method through the discovery of the pulsar M12B with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). This pulsar has a spin period of 2.76\,ms and a dispersion measure of $42.70 \pm 0.05\,\mathrm{cm}^{-3}~\mathrm{pc}$. This pulsar has a profile with three peaks, being faint, showing scintillation. It is in an approximately 0.53-day orbit. Our discovery indicates that more pulsars might be effectively discovered if the algorithm is applied to the search results from other archival globular cluster observations.

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arXiv2026

A jet-gas interaction beyond the host galaxy: detection of a neutral hydrogen outflow at cosmic noon

We present upgraded Giant Metrewave Radio Telescope (uGMRT) observations of 0731+438, an \mbox{FR II} radio galaxy at a redshift of 2.429 with two lobes separated by 82 kpc. A blueshifted, faint and broad \mbox{H{\sc i}} 21 cm absorption line with velocity full width at half maximum (FWHM) $\sim 600\,\rm km\,s^{-1}$ is detected against the southern radio lobe that is 47 kpc from radio core, indicating a neutral hydrogen outflow associated with jet-gas interaction beyond the host galaxy. The outflow has a mass outflow rate of $\sim\,0.4T_{\rm s}Ω\rm\, M_\odot\,{\rm yr}^{-1}$, which could increase to $\sim\,4.0T_{\rm s}Ω\rm\,M_\odot\,{\rm yr}^{-1}$, corresponding to an energy outflow rate of $2.4T_{\rm s}Ω\times10^{40}$ -- $1.5T_{\rm s}Ω\times10^{41}\,\rm erg\,s^{-1}$, where $T_{\rm s}$ is the spin temperature and $Ω$ is the solid angle of the outflow. Previous optical observations identified an extended emission line region aligned with the radio axis, ionized by the central Active Galactic Nucleus (AGN). Within this region, a warm and ionized outflow with a mass outflow rate of $\sim\,50\rm\, M_\odot\,{\rm yr}^{-1}$ and an energy outflow rate of $\sim1.7\times10^{43}\,\rm erg\,s^{-1}$ was detected. We propose that both the extended emission line region and the optical outflow are results of synergistic effect between jet and AGN radiation. The AGN likely exerts negative feedback on the host galaxy, as evidenced by the gas expulsion by the jet and the high velocity dispersion of ionized gas observed optically. So far, detections of jet-driven neutral hydrogen outflows remain rare. The high redshift, large outflow radii, substantial mass outflow rate and energy outflow rate of the neutral hydrogen outflow in 0731+438 expand the known parameter space of such outflows.

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arXiv2026

Dust-obscured radio-emitting tidal disruption event coincident with a high-energy neutrino event

Despite the growing number of high-energy neutrinos (TeV-PeV) detected by IceCube, their astrophysical origins remain largely unidentified. Recent observations have linked a few tidal disruption events (TDEs) to the production of high-energy neutrino emission, all of which display dust-reprocessed infrared flares, indicating a dust- and gas-rich environment. By cross-matching the neutrino events and a sample of mid-infrared outbursts in nearby galaxies with transient radio flares, we uncover an optically obscured TDE candidate, SDSS J151345.75 $+$ 311125.2, which shows both spatial and temporal coincidence with the sub-PeV neutrino event IC170514B. Using a standard equipartition analysis of the synchrotron spectral evolution spanning 605 days post mid-infrared discovery, we find a little evolution in the radio-emitting region, with a kinetic energy up to $10^{51}$ erg, depending on the outflow geometry and shock acceleration efficiency assumed. High-resolution European VLBI Network imaging reveals a compact radio emission that is unresolved at a scale of $<$ 2.1 pc, with a brightness temperature of $T_b>5\times10^6$ K, suggesting that the observed late-time radio emission might originate from the interaction between a decelerating outflow and a dense circumnuclear medium. If the association is genuine, the neutrino production is possibly related to the acceleration of protons through pp collisions during the outflow expanding process, implying that the outflow-cloud interaction could provide a physical site with a high-density environment for producing the sub-PeV neutrinos. Such a scenario can be tested with future identifications of radio transients coincident with high-energy neutrinos.

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arXiv2025

Radio continuum and \HI 21-cm line observations of a nearby luminous infrared galaxy IRAS 17526+3253

We present results from our EVN and GMRT observations of the radio continuum and spectral line emission in IRAS 17526+3253, along with an analysis of its arcsecond-scale radio properties using archival VLA data. The EVN observations detected radio continuum emission from both the northwest (NW) and southeast (SE) nuclei. The NW nucleus shows two components with high brightness temperatures and radio luminosities, likely indicating the presence of an AGN core and jet. Meanwhile, our EVN observation failed to detect the OH line emission, possibly due to radio frequency interference and/or the emission being partly resolved out and below our detection limit. The multi-band radio spectral energy distribution (SED) deviates from a single power-law at low frequencies, suggesting low-frequency absorption. The GMRT spectral line data reveal both \HI absorption and emission. The \HI emission is diffuse and shows a velocity gradient from about 7500 \kms in the NW to 7800 \kms in the SE nucleus. On larger scales, the \HI emission extends about 4' along the NW-SE direction, with the southeastern extension matching the optical tidal tail. In addition, the weak \HI absorption features show broad line profiles, possibly due to overlapping \HI gas from the two nuclei. The aforementioned results are consistent with properties of intermediate-stage mergers reported in the literature.

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arXiv2025

Discovery of a years-delayed radio flare from an unusually slow-evolved tidal disruption event

SDSS J1115+0544 is a unique low-ionization nuclear emission-line region (LINER) galaxy with energetic ultraviolet (UV), optical and mid-infrared outbursts occurring in its nucleus. We present the results from an analysis of multi-wavelength photometric and radio follow-up observations covering a period of ~9 years since its discovery. We find that following a luminosity plateau of ~500 days, the UV/optical emission has decayed back to the pre-outburst level, suggesting that the nuclear outburst might be caused by a stellar tidal disruption event (TDE). In this case, SDSS J1115+0544 could be an unusually slow-evolved optical TDE with longest rise and decline time-scales ever found. Three years later than the optical peak, a delayed radio brightening was found with a 5.5 GHz luminosity as high as ~1.9x10^39 erg/s. Using a standard equipartition analysis, we find the outflow powering the radio emission was launched at t~1260 days with a velocity of beta<~0.1 and kinetic energy of E_K~>10^50 erg. The delayed radio brightening coupled with the disappearing plateau in the UV/optical light curves is consistent with the scenario involving delayed ejection of an outflow from a state transition in the disk. SDSS J1115+0544 is the first TDE displaying both a short-lived UV/optical plateau emission and a late-time radio brightening. Future radio observations of these TDEs in the post-plateau decay phase will help to establish the connection between outflow launching and changes in accretion rate.

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arXiv2024

GMRT observations of OHM candidates from the ALFALFA survey

We present the results of our observations using the Giant Meterwave Radio Telescope (GMRT) to investigate the radio continuum and OH line emission of 10 OHM candidates from the Arecibo Legacy Fast ALFA (ALFALFA) survey. Among these candidates, we have identified two sources, AGC115713 and AGC249507, which display compact OH line emission that are spatially associated with radio continuum emission. These characteristics align with the typical properties of OHM galaxies. Furthermore, the infrared (IR) properties of these two galaxies are consistent with those of known OHM galaxies. %Importantly, these two sources have been independently confirmed by alternative methods. Of the two sources, AGC 249507 has been confirmed through optical redshift, whereas AGC 115713 meets a WISE color selection criterion in the literature, providing additional support for this source being an OHM galaxy rather than a nearby \HI galaxy. On the contrary, no significant spectral line emission were detected in the remaining eight OHM candidates using our full GMRT dataset. This suggests that the spectral line emission initially detected by the ALFALFA survey may have been significantly resolved in our high-resolution observations. Additionally, the absence of radio continuum emission in 6 candidates also distinguishes them from known OHM galaxies documented in the literature. These findings support the notion that OHM emission may be distributed on a subarcsecond scale, underscoring the utility of arcsecond-scale observations in confirming OHM candidates, particularly those lacking optical redshift data.

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arXiv2023

FAST discovery of a fast neutral hydrogen outflow

In this letter, we report the discovery of a fast neutral hydrogen outflow in SDSS J145239.38+062738.0, a merging radio galaxy containing an optical type I active galactic nuclei (AGN). This discovery was made through observations conducted by the Five-hundred-meter Aperture Spherical radio Telescope (FAST) using redshifted 21-cm absorption. The outflow exhibits a blueshifted velocity likely up to $\sim-1000\,\rm km\,s^{-1}$ with respect to the systemic velocity of the host galaxy with an absorption strength of $\sim -0.6\,\rm mJy\,beam^{-1}$ corresponding to an optical depth of 0.002 at $v=-500\,\rm km\,s^{-1}$. The mass outflow rate ranges between $2.8\times10^{-2}$ and $3.6\, \rm M_\odot \, yr^{-1}$, implying an energy outflow rate ranging between $4.2\times10^{39}$ and $9.7\times10^{40}\rm\,erg\,s^{-1}$, assuming 100 K $<T_{\rm s}<$ 1000 K. Plausible drivers of the outflow include the star bursts, the AGN radiation, and the radio jet, the last of which is considered the most likely culprit according to the kinematics. By analysing the properties of the outflow, the AGN, and the jet, we find that if the HI outflow is driven by the AGN radiation, the AGN radiation seems not powerful enough to provide negative feedback whereas the radio jet shows the potential to provide negative feedback. Our observations contribute another example of a fast outflow detected in neutral hydrogen, as well as demonstrate the capability of FAST in detecting such outflows.

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arXiv2023

Does a radio jet drive the massive multi-phase outflow in the ultra-luminous infrared galaxy IRAS 10565+2448?

We present new upgraded Giant Metrewave Radio Telescope (uGMRT) HI 21-cm observations of the ultra-luminous infrared galaxy IRAS 10565+2448, previously reported to show blueshifted, broad, and shallow HI absorption indicating an outflow. Our higher spatial resolution observations have localised this blueshifted outflow, which is $\sim$ 1.36 kpc southwest of the radio centre and has a blueshifted velocity of $\sim 148\,\rm km\,s^{-1}$ and a full width at half maximum (FWHM) of $\sim 581\,\rm km\,s^{-1}$. The spatial extent and kinematic properties of the HI outflow are consistent with the previously detected cold molecular outflows in IRAS 10565+2448, suggesting that they likely have the same driving mechanism and are tracing the same outflow. By combining the multi-phase gas observations, we estimate a total outflowing mass rate of at least $140\, \rm M_\odot \,yr^{-1}$ and a total energy loss rate of at least $8.9\times10^{42}\,\rm erg\,s^{-1}$, where the contribution from the ionised outflow is negligible, emphasising the importance of including both cold neutral and molecular gas when quantifying the impact of outflows. We present evidence of the presence of a radio jet and argue that this may play a role in driving the observed outflows. The modest radio luminosity $L_{\rm1.4GHz}$ $\sim1.3\times10^{23}\,{\rm W\,Hz^{-1}}$ of the jet in IRAS 10565+2448 implies that the jet contribution to driving outflows should not be ignored in low radio luminosity AGN.

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arXiv2022

Radio continuum and OH line emission of high-z OH megamaser galaxies

We present the study of arcsecond scale radio continuum and OH line emission of a sample of known OH megamaser galaxies with $z \geq$ 0.15 using archival Very Large Array (VLA) data. And also the results of our pilot Five hundred meter aperture spherical radio telescope (FAST) observations of 12 of these OHM galaxies. The arcsecond-scale resolution images show that the OH emission is distributed in one compact structure and spatially associated with radio continuum emission. Furthermore, nearly all the fitted components are likely smaller than the beam size ($\sim$ 1.4"), which indicates that the broad OH line profiles of these sources originated from one masing region or that more components are distributed in sub-arcsec scales. The radio parameters, including brightness temperature, spectral index, and q-index, show no significant differences with the low-redshift OHM galaxies, which have significantly lower OH line luminosities. Because these parameters are indicators of the central power sources (AGN, starburst, or both), our results indicate that the presence of radio AGN in the nuclei may not be essential for the formation of OH emission. Over 1/3 of OHMs in this sample (6/17) show possible variable features likely caused by interstellar scintillation due to small angular sizes. We might underestimate this value because these sources are associated with this sample's highest OH line flux densities. Those with low OH line flux densities might need higher sensitivity observations to study the variabilities. These results support the compact nature of OH maser emission and a starburst origin for the OHMs in our selected sample.

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arXiv2022

Radio properties of the OH megamaser galaxy IIZw 096

Based on the two epochs EVN archive data from OH line observations of IIZw 096, we confirm that the high-resolution OH emission in this source mainly comes from two spots (OH1 and OH2) of comp D1 of this merging system. We found no significant variations in the OH line emission. The OH 1665 MHz line emission is detected at about 6 $σ$ level in the OH1 region by combining two epoch EVN observations. We found that the comp D1 shows the brightest CO, HCO+ line emission, as well as multi-band radio continuum emission. The environment around D1 shows no clear velocity structure associated with circular motions, making it different from most other OHMs in the literature, which might have been caused by an effect during the merger stage. Meanwhile, we found that the CO emission shows three velocity structures around D1, including the central broad FWHM region, the double peak region where the CO line profile shows two separated peaks, and the region of the high-velocity clouds where the CO line peaks at a high velocity ($\sim$ 11000 \kms). \HI in absorption also show high-velocity clouds around the D1 region, which might be due to inflows caused by the merging of two or more galaxy components. Based on the high-resolution K-band VLA and L-band VLBA observations of the radio continuum emission, we derived the brightness temperature in the range $10^{5}$ K to $10^{6}$ K, which is consistent with other starburst dominant OHM sources in the literature. The multi-band VLA observations show that the radio continuum emission of comp D might also have contributions from free-free emission, besides synchrotron emission. As a concenquence, these results support a starburst origin for the OHMs, without the presence of an AGN.

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arXiv2020

The radio properties of the OH megamaser galaxy IRAS 02524+2046

We present results from VLBI observations of continuum and OH line emission in IRAS 02524+2046 and also arcsecond-scale radio properties of this galaxy using VLA archive data. We found that there is no significant detection of radio continuum emission from VLBI observations. The arcsecond-scale radio images of this source show no clear extended emission, the total radio flux density at L and C band are around 2.9 mJy and 1.0 mJy respectively, which indicate a steep radio spectral index between the two band. Steep spectral index, low brightness temperature and high $q$-ratio (the FIR to the radio flux density), which are three critical indicators in classification of radio activity in the nuclei of galaxies, are all consistent with the classification of this source as a starburst galaxy from its optical spectrum. The high-resolution line profile show that both of \textbf{the 1665 and 1667 MHz OH maser} line have been detected which show three and two clear components respectively. The channel maps show that the maser emission are distributed in a region $\sim$ 210 pc $\times$ 90 pc, the detected maser components at different region show similar double spectral feature, which might be an evidence that this galaxy is at a stage of major merger as seen from the optical morphology.

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arXiv2020

Radio/X-ray Correlation in the Mini-Outburts of Black Hole X-ray Transient GRS 1739--278

We present quasi-simultaneous radio and X-ray observations of the black hole X-ray binary GRS 1739--278 of its 2015-2016 mini-outbursts, i.e. between 2015 June 10 and 2016 October 31, with the X-ray-to-radio time interval being less than one day. The monitor champaign was run by \swift\, in the X-rays and by JVLA in the radio (at both 5 GHz and 8 GHz). We find the brightest radio emission is actually achieved during the soft sate, where radio spectrum is unexpectedly flat with a spectral index $α\approx -0.2$ (flux $F_ν\propto ν^α$). For the radio emission in hard state, we also find a large diversity in the spectral index, i.e. a majority of radio spectra are optically thick with $-0.5 < α< 0.5$, while a few are optically thin with $α< -1$. We also investigate the correlation between the luminosities in radio (monochromatic at 5 GHz, $L_R$) and 1-10 keV X-rays ($L_X$) during the hard state. We find this source does not follow the standard correlation whose $p\approx 0.6$ (in the form of $L_R\propto L_X^p$), but instead belongs to the "outlier" category that may follow a hybrid correlation. For more than orders of magnitude variation in the X-ray luminosity this source exhibits a flat correlation with $p\approx 0.16$. Both the slope and the corresponding luminosity range agree well with those in H1743--322, the prototype of the hybrid correlation.

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arXiv2016

The core dominance parameter and \emph{Fermi} detection of extragalactic radio sources

In this paper, by cross-correlating an archive sample of 542 extragalactic radio sources with the \emph{Fermi}-LAT Third Source Catalog(3FGL), we have compiled a sample of 80 $γ$-ray sources and 462 non-\emph{Fermi} sources with available core dominance parameter($R_{CD}$), core and extended radio luminosity; all the parameters are directly measured or derived from available data in the literature. We found that $R_{CD}$ have significant correlations with radio core luminosities, $γ$-ray luminosity and $γ$-ray flux respectively; the \emph{Fermi} sources have on average higher $R_{CD}$ than non-\emph{Fermi} sources. These results indicate that the \emph{Fermi} sources should be more compact, and beaming effect should play a crucial role for the detection of $γ$-ray emission. Moreover, our results also show \emph{Fermi} sources have systematically larger radio flux than non-\emph{Fermi} sources at fixed $R_{CD}$, indicating larger intrinsic radio flux in \emph{Fermi} sources. These results show a strong connection between radio and $γ$-ray flux for the present sample and indicate that the non-\emph{Fermi} sources is likely due to low beaming effect, and/or the low intrinsic $γ$-ray flux, support a scenario in the literature: a co-spatial origin of the activity for the radio and $γ$-ray emission, suggesting that the origin of the seed photons for the high-energy $γ$-ray emission is within the jet.

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arXiv2014

350 $μ$m map of the Ophiuchus molecular cloud: core mass function

Stars are born in dense cores of molecular clouds. The core mass function (CMF), which is the mass distribution of dense cores, is important for understanding the stellar initial mass function (IMF). We obtained 350 $μ$m dust continuum data using the SHARC-II camera at the Caltech Submillimeter Observatory (CSO) telescope. A 350 $μ$m map covering 0.25 ${deg}^{2}$ of the Ophiuchus molecular cloud was created by mosaicing 56 separate scans. The CSO telescope had an angular resolution of 9 $^{\prime\prime}$, corresponding to $1.2\times {10}^{3}\ $AU at the distance of the Ophiuchus molecular cloud (131 pc). The data was reduced using the Comprehensive Reduction Utility for SHARC-II (CRUSH). The flux density map was analyzed using the GaussClumps algorithm, within which 75 cores has been identified. We used the Spitzer c2d catalogs to separate the cores into 63 starless cores and 12 protostellar cores. By locating Jeans instabilities, 55 prestellar cores (a subcategory of starless cores) were also identified. The excitation temperatures, which were derived from FCRAO ${}^{12}$CO data, help to improve the accuracy of the masses of the cores. We adopted a Monte Carlo approach to analyze the CMF with two types of functional forms; power law and log-normal. The whole and prestellar CMF are both well fitted by a log-normal distribution, with $μ=-1.18\pm0.10,\ σ=0.58\pm0.05$ and $μ=1.40\pm0.10,\ σ=0.50\pm0.05$ respectively. This finding suggests that turbulence influences the evolution of the Ophiuchus molecular cloud.

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arXiv2014

Why are some BL Lacs detected by \fermi, but others not ?

By cross-correlating an archival sample of 170 BL Lacs with 2 year \fermilat AGN sample, we have compiled a sample of 100 BL Lacs with \fermi detection (FBLs), and a sample of 70 non-\fermi BL Lacs (NFBLs). We compared various parameters of FBLs with those of NFBLs, including the redshift, the low frequency radio luminosity at 408 MHz ($L_{\rm 408MHz}$), the absolute magnitude of host galaxies ($M_{\rm host}$), the polarization fraction from NVSS survey ($P_{\rm NVSS}$), the observed arcsecond scale radio core flux at 5 GHz ($F_{\rm core}$) and jet Doppler factor; all the parameters are directly \textbf{measured} or derived from available data from literatures. We found that the Doppler factor is on average larger in FBLs than in NFBLs, and the $Fermi~ γ$-ray detection rate is higher in sources with higher Doppler factor. In contrast, there are no significant differences in terms of the intrinsic parameters of redshift, $ L_{\rm 408MHz}$, $ M_{\rm host}$ and $ P_{\rm NVSS}$. FBLs seem to have a higher probability of exhibiting measurable proper motion. These results strongly indicate a higher beaming effect in FBLs compared to NFBLs. The radio core flux is found to be strongly correlated with $γ$-ray flux, which remains after excluding the common dependence of the Doppler factor. At the fixed Doppler factor, FBLs have systematically larger radio core flux than NFBLs, implying lower $γ$-ray emission in NFBLs since the radio and $γ$-ray flux are significantly correlated. Our results indicate that the Doppler factor is an important parameter of $γ$-ray detection, the non-detection of $γ$-ray emission in NFBLs is likely due to low beaming effect, and/or low intrinsic $γ$-ray flux, and the gamma-rays are likely produced co-spatially with the arcsecond-scale radio core radiation and mainly through the SSC process.

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arXiv2012

The radio structure of ultra-high-energy synchrotron peak BL Lacs

We present the results of EVN and MERLIN 5 GHz observations of nine ultra-high-energy synchrotron peak BL Lacs (UHBLs) selected as all BL Lacs with \textbf{log ($ν_{\rm peak}/ \rm Hz)>20$} from Nieppola et al.. The radio structure was investigated for these sources, in combination with the available VLBA archive data. We found that the core-jet structure is detected in five sources, while four sources only have a compact core on pc scale. The core of all sources shows high brightness temperature (with mean and median values \textbf{log ($T_{\rm b} / {\rm K}) \sim11$}, which implies that the beaming effect likely present in all sources. When the multi-epoch VLBI data are available, we found no significant variations either for core or total flux density in two sources (2E 0414+0057 and EXO 0706.1+5913), and no evident proper motion in 2E 0414+0057, while the superluminal motion is likely detected in EXO 0706.1+5913. Our sources are found to be less compact than the typical HBLs in Giroletti et al, by comparing the ratio of the VLBI total flux to the core flux at arcsec scale. Combining all our results, we propose that the beaming effect might be present in the jets of UHBLs, however, it is likely weaker than that of typical HBLs. Moreover, we found that UHBLs could be less Doppler beamed versions of HBLs with similar jet power, by comparing the distribution of redshift, and radio luminosities. The results are in good consistence with the expectations from our previous work.

astro-ph.HE↗