arXiv subjects

Minfeng Gu

Publications and source records attributed to Minfeng Gu.

At least 109 records · Page 6Linked to original sources

arXiv2016

An Accretion Disc-Irradiation Hybrid Model for The Optical/UV Variability in Radio-Quiet Quasars

The optical/ultraviolet (UV) variability of quasars has been discovered to be correlated with other quasar properties, such as luminosity, black hole mass and rest-frame wavelength. However,the origin of variability has been a puzzle so far. In this work, we upgrade the accretion disc model (Li & Cao 2008), which assumed the variability is caused by the change of global mass accretion rate, by constraining the disc size to match the viscous timescale of accretion disc to the variability timescale observed and by including the irradiation/X-ray reprocessing to make the emitted spectrum become steeper. We find this hybrid model can reproduce the observed bluer-when-brighter trend quite well, which is used to validate the theoretical model by several works recently. The traditional correlation between the variability amplitude and rest-frame wavelength can also be well fitted by our model. In addition, a weak positive correlation between variability amplitude and black hole mass is present, qualitatively consistent with recent observations.

astro-ph.HE↗

arXiv2016

The Optical Variability of SDSS Quasars from Multi-epoch Spectroscopy. III. A Sudden UV Cutoff in Quasar SDSS J2317+0005

We have collected near-infrared to X-ray data of 20 multi-epoch heavily reddened SDSS quasars to investigate the physical mechanism of reddening. Of these, J2317+0005 is found to be a UV cutoff quasar. Its continuum, which usually appears normal, decreases by a factor 3.5 at 3000Å, compared to its more typical bright state during an interval of 23 days. During this sudden continuum cut-off, the broad emission line fluxes do not change, perhaps due to the large size of the Broad Line Region (BLR), r > 23 / (1+z) days. The UV continuum may have suffered a dramatic drop out. However, there are some difficulties with this explanation. Another possibility is that the intrinsic continuum did not change, but was temporarily blocked out, at least towards our line of sight. As indicated by X-ray observations, the continuum rapidly recovers after 42 days. A comparison of the bright state and dim states would imply an eclipse by a dusty cloud with a reddening curve having a remarkably sharp rise shortward of 3500Å. Under the assumption of being eclipsed by a Keplerian dusty cloud, we characterized the cloud size with our observations, however, which is a little smaller than the 3000Å continuum-emitting size inferred from accretion disk models. Therefore, we speculate this is due to a rapid outflow or inflow with a dusty cloud passing through our line-of-sight to the center.

astro-ph.GA↗

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.

astro-ph.GA↗

arXiv2016

The optical variability of SDSS quasars from multi-epoch spectroscopy. II. color variation

We investigated the optical/ultraviolet (UV) color variations for a sample of 2169 quasars based on multi-epoch spectroscopy in the Sloan Digital Sky Survey (SDSS) data release seven (DR7) and data release nine (DR9). To correct the systematic difference between DR7 and DR9 due to the different instrumental setup, we produced a correction spectrum by using a sample of F-stars observed both in DR7 and DR9. The correction spectrum was then applied to quasars when comparing the spectra of DR7 with DR9. In each object, the color variation was explored by comparing the spectral index of the continuum power-law fit on the brightest spectrum with the faintest one, and also by the shape of their difference spectrum. In 1876 quasars with consistent color variations from two methods, we found that most sources (1755, $\sim 94\%$) show bluer-when-brighter (BWB) trend, and the redder-when-brighter (RWB) trend is only detected in 121 objects ($\sim 6\%$). The common BWB trend is supported by the bluer composite spectrum constructed from bright spectra than that from faint spectra, and also by the blue composite difference spectrum. The correction spectrum is proved to be highly reliable by comparing the composite spectrum from corrected DR9 and original DR7 spectra. Assuming that the optical/UV variability is triggered by fluctuations, RWB trend can likely be explained if the fluctuations occur firstly at outer disk region, and the inner disk region has not fully responded yet when the fluctuation being propagated inward. In contrast, the common BWB trend implies that the fluctuations are likely more often happening firstly in inner disk region.

astro-ph.GA↗

arXiv2015

The Radio Properties of Radio-Loud Narrow-Line Seyfert 1 Galaxies on Parsec Scales

We present the detection of compact radio structures of fourteen radio-loud narrow line Seyfert 1 (NLS1) galaxies from Very Long Baseline Array observations at 5 GHz, which were performed in 2013. While 50\% of the sources of our sample show a compact core only, the remaining 50\% exhibit a core-jet structure. The measured brightness temperatures of the cores range from $10^{8.4}$ to $10^{11.4}$ K with a median value of $10^{10.1}$ K, indicating that the radio emission is from non-thermal jets, and that, likely, most sources are not strongly beamed, then implying a lower jet speed in these radio-loud NLS1 galaxies. In combination with archival data taken at multiple frequencies, we find that seven sources show flat or even inverted radio spectra, while steep spectra are revealed in the remaining seven objects. Although all these sources are very radio-loud with $R > 100$, their jet properties are diverse, in terms of their milli-arcsecond (mas) scale (pc scale) morphology and their overall radio spectral shape. The evidence for slow jet speeds (i.e., less relativistic jets), in combination with the low kinetic/radio power, may offer an explanation for the compact VLBA radio structure in most sources. The mildly relativistic jets in these high accretion rate systems are consistent with a scenario, where jets are accelerated from the hot corona above the disk by the magnetic field and the radiation force of the accretion disk. Alternatively, a low jet bulk velocity can be explained by low spin in the Blandford-Znajek mechanism.

astro-ph.GA↗

arXiv2015

Optical and Radio Variability of BL Lacertae

We observed the prototype blazar, BL Lacertae, extensively in optical and radio bands during an active phase in the period 2010--2013 when the source showed several prominent outbursts. We searched for possible correlations and time lags between the optical and radio band flux variations using multifrequency data to learn about the mechanisms producing variability. During an active phase of BL Lacertae, we searched for possible correlations and time lags between multifrequency light curves of several optical and radio bands. We tried to estimate any possible variability timescales and inter-band lags in these bands. We performed optical observations in B, V, R and I bands from seven telescopes in Bulgaria, Georgia, Greece and India and obtained radio data at 36.8, 22.2, 14.5, 8 and 4.8 GHz frequencies from three telescopes in Ukraine, Finland and USA. Significant cross-correlations between optical and radio bands are found in our observations with a delay of cm-fluxes with respect to optical ones of ~250 days. The optical and radio light curves do not show any significant timescales of variability. BL Lacertae showed many optical 'mini-flares' on short time-scales. Variations on longer term timescales are mildly chromatic with superposition of many strong optical outbursts. In radio bands, the amplitude of variability is frequency dependent. Flux variations at higher radio frequencies lead the lower frequencies by days or weeks. The optical variations are consistent with being dominated by a geometric scenario where a region of emitting plasma moves along a helical path in a relativistic jet. The frequency dependence of the variability amplitude supports an origin of the observed variations intrinsic to the source.

astro-ph.HE↗

arXiv2015

Nature of Intra-night Optical Variability of BL Lacertae

We present the results of extensive multi-band intra-night optical monitoring of BL Lacertae during 2010--2012. BL Lacertae was very active in this period and showed intense variability in almost all wavelengths. We extensively observed it for a total for 38 nights; on 26 of them observations were done quasi-simultaneously in B, V, R and I bands (totaling 113 light curves), with an average sampling interval of around 8 minutes. BL Lacertae showed significant variations on hour-like timescales in a total of 19 nights in different optical bands. We did not find any evidence for periodicities or characteristic variability time-scales in the light curves. The intranight variability amplitude is generally greater at higher frequencies and decreases as the source flux increases. We found spectral variations in BL Lacertae in the sense that the optical spectrum becomes flatter as the flux increases but in several flaring states deviates from the linear trend suggesting different jet components contributing to the emission at different times.

astro-ph.HE↗

arXiv2014

The optical variability of SDSS quasars from multi-epoch spectroscopy: I. Results from 60 quasars with $\geq$ six-epoch spectra

In a sample of 60 quasars selected from Sloan Digital Sky Survey (SDSS) with at least six-epoch spectroscopy, we investigate the variability of emission lines and continuum luminosity at various aspects. A strong anti-correlation between the variability and continuum luminosity at 2500 Å is found for the sample, which is consistent with previous works. In individual sources, we find that half of the sample objects have a trend of being bluer-when-brighter trend (BWB), while the remaining half exhibit redder-when-brighter trend (RWB). Although the mechanism for RWB is unclear, the effects of host galaxy contribution due to seeing variations can not be completely ruled out. As expected from photoionization model, the positive correlations between the broad emission line and continuum luminosity are found in most individual sources, as well as for the whole sample. We confirm the Baldwin effect in most individual objects and the whole sample, while a negative Baldwin effect is also found in several quasars, which can be at least partly (if not all) due to the host galaxy contamination. We find positive correlations between the broad emission line luminosity and line width in most individual quasars, as well as the whole sample, implying a more variable line base than the line core.

astro-ph.HE↗

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.

astro-ph.HE↗

arXiv2013

The evolution of dusty torus covering factor in quasars

We have assembled a large sample of 5996 quasars at redshift 2.0=< z <= 2.4 (high-z) or 0.7=< z <= 1.1 (low-z) from SDSS data release nine and seven quasar catalogs. The spectral energy distribution (SED) of quasars were constructed by collecting WISE, UKIDSS, and GALEX photometric data in addition to SDSS, from which the IR luminosity at 1-7 μm and bolometric luminosity at 1100 Å- 1 μm were calculated. A red tail is clearly seen in the distributions of the spectral index in 1100 Å- 1 μm both for high-z and low-z sources, which is likely due to red or reddened quasars. The covering factor of dusty torus is estimated as the ratio of the IR luminosity to the bolometric luminosity. We found significant anti-correlations between the covering factor and bolometric luminosity, in both high-z and low-z quasars, however they follow different tracks. At overlapped bolometric luminosity, the covering factor of high-z quasars are systematically larger than those of low-z quasars, implying an evolution of covering factor with redshift.

astro-ph.CO↗

arXiv2013

The Ultraviolet/optical variability of steep-spectrum radio quasars: the change in accretion rate ?

Context. The steep-spectrum radio quasars (SSRQs) are powerful radio sources, with thermal emission from accretion disk and jet nonthermal emission likely both contributing in the Ultraviolet (UV)/optical luminosity, however the former may play a dominant role. While the UV/optical variability of SSRQs has been poorly studied, little is known on the mechanism of their variability. Aims. We investigate the mechanism of the UV/optical variability of SSRQs. Methods. A sample of eighteen SSRQs has been established in SDSS Stripe 82 region in our previous works, in which the flux and spectral variability have been studied. In this work, we construct the flux-flux diagram using SDSS u and i multi-epoch data for these eighteen SSRQs. The standard accretion disk model is used to fit the flux-flux variations, in order to explore the variability mechanism. Results. The model fit to flux-flux diagram are tuned with fixed black hole mass and varying accretion rate. We found that the flux-flux diagram of all our SSRQs can be qualitatively described by the standard accretion disk model with change of the accretion rate. Although non-thermal jet power-law emission can also qualitatively reproduce the variability, the reasonable accretion rates and black hole masses required to fit the flux-flux variations suggest that a disk emission with variable accretion rate is a plausible description of the data.

astro-ph.CO↗

arXiv2013

The spectral variability of quasar SDSS J030639.57+000343.1

We compiled a sample of 60 quasars with spectroscopy on at least six epochs from the Sloan Digital Sky Survey (SDSS) to study the variabilities of the spectral shape, the continuum and the emission lines luminosity. In this paper, we present the results of SDSS J030639.57+000343.1. We found a strong anti-correlation between the continuum luminosity at $5100 Å$ and the spectral index, implying a bluer-when-brighter trend. The luminosity of the broad $\rm H_α$ line is proportion to the continuum luminosity at $5100 Å$. Correspondingly, we did not find strong correlation between the equivalent width of broad $\rm Hα$ and the continuum luminosity, i.e. no baldwin effect of broad $\rm Hα$ in this source.

astro-ph.CO↗

arXiv2013

The spectral variability in radio-loud quasars

The spectral variability of a sample of 44 flat-spectrum radio quasars (FSRQs) and 18 steep-spectrum radio quasars (SSRQs) in the SDSS stripe 82 region is investigated. Twenty-five of 44 FSRQs show a bluer-when-brighter trend (BWB), while only one FSRQ shows a redder-when-brighter trend, which is in contrast to our previous results. Eight of 18 SSRQs display a BWB. We found an anti-correlation between the Eddington ratio and the variability amplitude in r band for SSRQs, which is similar to that in radio-quiet AGNs. This implies that the thermal emission from the accretion disk may be responsible for the variability in SSRQs. The spectral variability from the SDSS multi-epoch spectroscopy also shows BWB for several SSRQs, consistent with that from photometry.

astro-ph.CO↗

arXiv2012

What governs the bulk velocity of the jet components in active galactic nuclei?

We use a sample of radio-loud active galactic nuclei (AGNs) with measured black hole masses to explore the jet formation mechanisms in these sources. Based on the Königl's inhomogeneous jet model, the jet parameters, such as the bulk motion Lorentz factor, magnetic field strength, and electron density in the jet, can be estimated with the very long-baseline interferometry and X-ray data. We find a significant correlation between black hole mass and the bulk Lorentz factor of the jet components for this sample, while no significant correlation is present between the bulk Lorentz factor and the Eddington ratio. The massive black holes will be spun up through accretion, as the black holes acquire mass and angular momentum simultaneously through accretion. Recent investigation indeed suggested that most supermassive black holes in elliptical galaxies have on average higher spins than the black holes in spiral galaxies, where random, small accretion episodes (e.g., tidally disrupted stars, accretion of molecular clouds) might have played a more important role. If this is true, the correlation between black hole mass and the bulk Lorentz factor of the jet components found in this work implies that the motion velocity of the jet components is probably governed by the black hole spin. No correlation is found between the magnetic field strength at $10R_{\rm S}$ ($R_{\rm S}=2GM/c^2$ is the Schwarzschild radius) in the jets and the bulk Lorentz factor of the jet components for this sample. This is consistent with the black hole spin scenario, i.e., the faster moving jets are magnetically accelerated by the magnetic fields threading the horizon of more rapidly rotating black holes. The results imply that the Blandford-Znajek (BZ) mechanism may dominate over the Blandford-Payne (BP) mechanism for the jet acceleration at least in these radio-loud AGNs.

astro-ph.CO↗

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↗

arXiv2011

The optical variability of flat-spectrum radio quasars in the SDSS stripe 82 region

Context. Although a bluer-when-brighter trend is commonly observed in blazars, the opposite trend of redder-when-brighter has also been found in some blazars. Aims. We investigate the frequency of the redder-when-brighter trend in flat-spectrum radio quasars (FSRQs). Methods. We investigate the optical variability of 29 FSRQs in the SDSS Stripe 82 region using SDSS DR7 released multi-epoch data covering about nine years. We determined the spectral index by fitting a powerlaw to SDSS ugriz photometric data, and explored the relationship between the spectral index and source brightness. Results. For all FSRQs studied, we detect variations in r band flux of overall amplitude between 0.24 mag and 3.46 mag in different sources. Fourteen of 29 FSRQs display a bluer-when-brighter trend. However, only one source exhibits a redder-when-brighter trend, which implies that this behavior is rare in our FSRQ sample. In this source, the thermal emission from the accretion disk may be responsible for the redder-when-brighter trend.

astro-ph.CO↗

arXiv2011

The optical variability of steep-spectrum radio quasars in the SDSS Stripe 82 region

While there are a lot of investigations on the optical variability of flat-spectrum radio quasars (FSRQs), not much work has been done on the optical variability of steep-spectrum radio quasars (SSRQs). We investigate the optical variability of 18 SSRQs and 15 FSRQs in the SDSS Stripe 82 region using SDSS DR7 released multi-epoch data covering about nine years. For all SSRQs studied, we detect variations in r band flux of overall amplitude between 0.22 mag and 0.92 mag in different sources. Eight of 18 SSRQs display a bluer-when-brighter (BWB) trend. In contrast, the variability amplitude of 15 FSRQs in r band ranges from 0.18 to 0.97 mag, and 11 of 15 sources show a BWB trend. We found an anti-correlation between the Eddington ratio and the variability amplitude in r band for SSRQs, which is similar to that in radio quiet AGNs. This implies that the thermal emission from accretion disk may be responsible for the variability in SSRQs, although the jet nonthermal emission cannot be excluded. In contrast, there is no correlation in FSRQs, which implies that the mechanisms of variability in FSRQs may be different from that in SSRQs. The optical continuum variability of radio loud broad absorption line quasars (BALQs) are investigated for the first time here on two sources with steep radio spectrum. Both radio loud BALQs show variations with amplitude of about 0.3 mag at r band. Their spectral variability all show a BWB trend. In a combined sample (18 SSRQs, and 44 FSRQs) of our sample with the FSRQs in Gu & Ai (2011), we found a trend of a broader line width of broad Mg II emission line with steeper radio spectral index. It implies a disc-like broad line region (BLR) geometry may present in these quasars. In these 62 quasars, we found a ~ 57% source percentage showing BWB trend in FSRQs, whereas it is ~ 44% in SSRQs.

astro-ph.CO↗

arXiv2011

N(HI) and jet power/emission in AGNs

Neutral hydrogen (HI) 21 cm absorption has been detected against more and more powerful radio jets. In this work, based on the Guppta et al. 2006a sample, we present our preliminary study of the correlations between the HI column density N(HI) and the jet power, N(HI) versus the low frequency luminosity at 408MHz, and N(HI) versus the radio luminosity at 1400MHz.

astro-ph.HE↗