arXiv subjects

J. H. Fan

Publications and source records attributed to J. H. Fan.

At least 91 records · Page 5Linked to original sources

arXiv2008

Two types of softening detected in X-ray afterglows of Swift bursts: internal and external shock origins?

The softening process observed in the steep decay phase of early X-ray afterglows of Swift bursts has remained a puzzle since its discovery. The softening process can also be observed in the later phase of the bursts and its cause has also been unknown. Recently, it was suggested that, influenced by the curvature effect, emission from high latitudes would shift the Band function spectrum from higher energy band to lower band, and this would give rise to the observed softening process accompanied by a steep decay of the flux density. The curvature effect scenario predicts that the terminating time of the softening process would be correlated with the duration of the process. In this paper, based on the data from the UNLV GRB group web-site, we found an obvious correlation between the two quantities. In addition, we found that the softening process can be divided into two classes: the early type softening ($t_{s,max}\leq "4000"s$) and the late type softening ($t_{s,max} > "4000"s$). The two types of softening show different behaviors in the duration vs. terminating time plot. In the relation between the variation rates of the flux density and spectral index during the softening process, a discrepancy between the two types of softening is also observed. According to their time scales and the discrepancy between them, we propose that the two types are of different origins: the early type is of internal shock origin and the late type is of external shock origin. The early softening is referred to the steep decay just following the prompt emission, whereas the late decay typically conceives the transition from flat decay to late afterglow decay. We suspect that there might be a great difference of the Lorentz factor in two classes which is responsible for the observed discrepancy.

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arXiv2008

Optical Intra-day Variability in Blazars

We selected a sample of a dozen blazars which are the prime candidates for simultaneous multi-wavelength observing campaigns in their outburst phase. We searched for optical outbursts, intra-day variability and short term variability in these blazars. We carried out optical photometric monitoring of nine of these blazars in 13 observing nights during our observing run October 27, 2006 - March 20, 2007 by using the 1.02 meter optical telescope. From our observations, our data favor the hypothesis that three blazars were in the outburst state; one blazar was in the post outburst state; three blazars were in the pre/post outburst state; one blazar was in the low-state; and the state of one blazar was not known because there is not much optical data available for the blazar to compare with our observations. Out of three nights of observations of AO 0235+164, intra-day variability was detected in two nights. Out of five nights of observations of S5 0716+714, intra-day variability was detected in two nights. In one night of observations of PKS 0735+178, intra-day variability was detected. Out of six nights of observations of 3C 454.3, intra-day variability was detected in three nights. No intra-day variability was detected in S2 0109+224, OJ 287, ON 231, 3C 279 and 1ES 2344+514 in their 1, 4, 1, 2 and 1 nights of observations respectively. AO 0235+164, S5 0716+714, OJ 287, 3C 279 and 3C 454.3 were observed in more than one night and short term variations in all these blazars were also noticed. From our observations and the available data, we found that the predicted optical outburst with the time interval of ~ 8 years in AO 0235+164 and ~ 3 years in S5 0716+714 have possibly occurred.

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arXiv2007

A possible disk mechanism for the 23d QPO in Mkn~501

Optically thin two-temperature accretion flows may be thermally and viscously stable, but acoustically unstable. Here we propose that the O-mode instability of a cooling-dominated optically thin two-temperature inner disk may explain the 23-day quasi-periodic oscillation (QPO) period observed in the TeV and X-ray light curves of Mkn~501 during its 1997 high state. In our model the relativistic jet electrons Compton upscatter the disk soft X-ray photons to TeV energies, so that the instability-driven X-ray periodicity will lead to a corresponding quasi-periodicity in the TeV light curve and produce correlated variability. We analyse the dependence of the instability-driven quasi-periodicity on the mass (M) of the central black hole, the accretion rate ($\rm{\dot{M}}$) and the viscous parameter ($α$) of the inner disk. We show that in the case of Mkn~501 the first two parameters are constrained by various observational results, so that for the instability occurring within a two-temperature disk where $α=0.05-1.0$, the quasi-period is expected to lie within the range of 8 to 100 days, as indeed the case. In particular, for the observed 23-day QPO period our model implies a viscosity coefficient $α\leq 0.28$, a sub-Eddington accretion rate $\dot{M} \simeq 0.02 \dot{M}_{\rm Edd}$ and a transition radius to the outer standard disk of $r_0 \sim 60 r_g$, and predicts a period variation $δP/P \sim 0.23$ due to the motion of the instability region.

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arXiv2007

Radio variability properties for radio sources

In this paper, we used the database of the university of Michigan Radio Astronomy Observatory (UMRAO) at three (4.8 GHz, 8.0 GHZ, and 14.5 GHz) radio frequency to analyze the radio light curves by the power spectral analysis method in search of possible periodicity. The analysis results showed that the radio sources display astrophysically meaningful periodicity ranging from 2.2 to 20.8 years in their light curves at the three frequencies. We also calculated the variability parameters and investigated the correlations between the variability parameter and the flux density. For the variability parameters, we found that the parameters at higher frequency are higher than those in the lower frequency. In addition, the variability parameters of BL Lacertae objects are larger than those of flat-spectrum radio quasars. suggesting that they are more variable than flat spectrum radio quasars.

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arXiv2007

Hardness ratio evolutionary curves of gamma-ray bursts expected by the curvature effect

We have investigated the gamma-ray bursts pulses assumed to arise from relativistically expending fireballs. We find, due to the curvature effect, the evolutionary curve of the raw hardness ratio (when the background count is included) varies significantly, depending on the hardness of a burst. For a soft burst, an upside-down pulse of its raw hardness ratio curve (HRC) would be observed; for a hard burst, its raw HRC shows a pulse-like profile with a sinkage in its decaying phase; for a very hard burst, the raw HRC possesses a pulse-like profile without a sinkage in its decaying phase. For a pulse-like raw HRC as shown in the case of the hard and very hard bursts, its peak would appear in advance of that of the corresponding light curve. We have studied here the HRC of GRB 920216, GRB 920830 and GRB 990816 in detail. The features of the raw HRC expected in the hard burst are observed in these bursts. A fit to the three bursts shows that the curvature effect alone could indeed account for the predicted characteristics of HRCs. In addition, we find that the observed hardness ratio tends to be harder at the beginning of the pulses than what the curvature effect could predict and be softer at the late time of the pulses. We believe this is an evidence showing the existence of intrinsic hard-to-soft radiation.

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arXiv2006

Optical Photometrical Observations and Variability for Quasar 4C 29.45

We reported the result of long term optical variability of the blazar 4C 29.45 (QSO 1156+295, Ton 599), carried out optical photometric observations in Johnson V, Cousins RI passbands during April 1997 to March 2002 using the 1.56 meter telescope of the Shanghai Astronomical Observatory (SHAO) at Sheshan, China, compiled the post-1974 optical photometric data of the blazar by combining our new observations with the published optical data, and found maximum variations in different passbands: $Δ$U = 4.41 mag, $Δ$B = 5.55 mag, $Δ$V = 4.53 mag, $Δ$R = 5.80 mag, and $Δ$I = 5.34 mag. The average color indices are: U$-$B = $-0.54\pm$0.18 mag, B$-$V = 0.56$\pm$0.21 mag, B$-$R = 0.93$\pm$0.18 mag, B$-$I = 1.51$\pm$0.24 mag, V$-$R = 0.44$\pm$0.15 mag and V$-$I = 1.03$\pm$0.23 mag. The post-1974 data give us an excellent opportunity to search for the existence of possible periodicity in the light curve. In search for periodicity in the R passband light curve, we performed Jurkevich test and power spectral (Fourier) analysis methods, and CLEANest algorithms to remove false signals. We found possible periods of 3.55 and 1.58 years. The possible mechanism for the periodic variability was discussed.

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arXiv2005

The basic parameters of gamma-ray-loud blazars

We determined the basic parameters, such as the central black hole mass ($M$), the boosting factor (or Doppler factor) ($δ$), the propagation angle ($Φ$) and the distance along the axis to the site of $γ$-ray production ($d$) for 23 $γ$-ray-loud blazars using their available variability timescales. In this method, the absorption effect depends on the $γ$-ray energy, emission size and property of the accretion disk. Using the intrinsic $γ$-ray luminosity as a fraction $λ$ of the Eddington luminosity, $L^{in}_γ=λL_{Ledd.}$ and the optical depth equal to unity, we can determine the upper limit of the central black hole masses. We found that the black hole masses range between $10^{7}M_{\odot}$ and $10^{9}M_{\odot}$ when $λ$ = 0.1 and 1.0 are adopted. Since this method is based on gamma-ray emissions and the short time-scale of the sources, it can also be used for central black hole mass determination of high redshift gamma-ray sources. In the case of the upper limit of black hole mass there is no clear difference between BLs and FSRQs, which suggests that the central black hole masses do not play an important role in the evolutionary sequence of blazars.

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arXiv2003

The binary black hole scenario for the BL Lacertae object AO 0235+16

Recent analysis of the long term radio lightcurve of the extremely variable BL Lacertae object AO 0235+16 by Raiteri et al. (2001) have revealed the presence of recurrent outbursts in this source with a period of ~ 5.7 +/- 0.5 yr. Periodicity analysis of the optical lightcurve also show evidence for a shorter period. Here we discuss whether such a behavior can be explained by a binary black hole model where the accretion disk of one of the supermassive black holes is precessing due to the tidal effects of the companion. We estimate the mass of the accreting hole and analyze under what constraints onto the secondary mass and the orbital parameters of the system it is possible to provide a viable interpretation of the available multiwavelength data.

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arXiv2001

Optical observations of BL Lacertae from 1997 to 1999

We present the optical (V, R, and I) photometry for BL Lacertae, which was observed from 1997 through 1999, with the 1.56-m telescope at the Shanghai astronomical observatory (SHAO). After the 1997 outburst, it dimmed to a low state and then brightened again. During the period JD 2450701 to JD 2450701.5, variations of 0.40mag, 0.27mag, and 0.21mag over a time scale of 100 minutes were found for V, R, and I bands, suggesting that the variations were decreasing with wavelength. The correlation between V, R, and I is also analyzed using the DCF (Discrete Correlation Function) method. This shows that the variability in the V, R, and I bands are correlated with no time delay longer than 0.2 day.

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arXiv2001

Polarization and Variations of BL Lacertae Objects

BL Lacertae objects are an extreme subclass of AGNs showing rapid and large-amplitude variability, high and variable polarization, and core-dominated radio emissions. If a strong beaming effect is the cause of the extreme observation properties, one would expect that these properties would be correlated with each other. Based on the relativistic beaming model, relationships between the polarization and the magnitude variation in brightness, as well as the core- dominance parameter are derived and used statistically to compare with the observational data of a BL Lacertae object sample. The statistical results are consistent with these correlations, which suggests that the polarization, the variation, and the core-dominance parameter are possible indications of the beaming effect.

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arXiv2001

The Radio and Gamma-Ray Luminosities of Blazars

Based on the $γ$-ray data of blazars in the third EGRET catalog and radio data at 5 GHz, we studied the correlation between the radio and $γ$-ray luminosities using two statistical methods. The first method was the partial correlation analysis method, which indicates that there exist correlations between the radio and $γ$-ray luminosities in both high and low states as well as in the average case. The second method involved a comparison of expected $γ$-ray luminosity distribution with the observed data using the Kolmogorov-- Smirnov (KS) test. In the second method, we assumed that there is a correlation between the radio and $γ$-ray luminosities and that the $γ$-ray luminosity function is proportional to the radio luminosity function. The KS test indicates that the expected gamma-ray luminosity distributions are consistent with the observed data in a reasonable parameter range. Finally, we used different $γ$-ray luminosity functions to estimate the possible 'observed' $γ$-ray luminosity distributions by GLAST.

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arXiv2000

Relation between $γ$-rays and emission lines for the $γ$-ray loud blazars

The relation between the $γ$-ray and the emission line luminosities for a sample of 36 $γ$-ray loud blazars is investigated; an apparent correlation between them, $L_γ \propto L_{Line}^{0.69\pm0.11}$, with a correlation coefficient $r=0.741$ and a chance probability of $p = 1.9\times10^{-6}$, is found. It is found, however, that there is no intrinsic correlation between them: the apparent correlation is due to the redshift dependence in a flux-limited sample. Thus no evidence is found to support the argument that the up-scattered soft photons are from the broad emission lines. Our analysis does not conflict with the SSC model. The disk-jet symbiosis and radio/$γ$-ray correlation found in the literature are also discussed. The radio/$γ$-ray correlation may be an apparent correlation caused by the boosting effect since both bands are strongly beamed.

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arXiv2000

The variability analysis of PKS 2155-304

In this paper, the post-1977 photometric observations of PKS 2155-304 are compiled and used to discuss the variation periodicity. Largest amplitude variations ($ΔU = 1^{m}.5$; $ΔB = 1^{m}.65$; $ΔV = 1^{m}.85 $; $ΔR = 1^{m}.25$; $ΔI = 1^{m}.14 $) and color indices ($(B-V) = 0.30\pm 0.06$; $(U-B) = -0.72\pm 0.08$; $(B-R) = 0.62\pm 0.07$; $(V-R) = 0.32\pm 0.04$) are found. The Jurkevich's method and DCF (Discrete Correlation Function) method indicate possible periods of 4.16-year and 7.0-year in the V light curve.

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arXiv2000

Long-term Variability Properties and Periodicity Analysis for Blazars

In this paper, the compiled long-term optical and infrared measurements of some blazars are used to analyze the variation properties and the optical data are used to search for periodicity evidence in the lightcurve by means of the Jurkevich technique and the discrete correlation function (DCF) method. Following periods are found: 4.52-year for 3C 66A; 1.56 and 2.95 years for AO 0235+164; 14.4, 18.6 years for PKS 0735+178; 17.85 and 24.7 years for PKS 0754+100; 5.53 and 11.75 for OJ 287. 4.45, and 6.89 years for PKS 1215; 9 and 14.84 years for PKS 1219+285; 2.0, 13.5 and 22.5 for 3C273; 7.1 year for 3C279; 6.07 for PKS 1308+326; 3.0 and 16.5 years for PKS 1418+546; 2.0 and 9.35 years for PKS 1514-241; 18.18 for PKS 1807+698; 4.16 and 7.0 for 2155-304; 14 and 20 years for BL Lacertae. Some explanations have been discussed.

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arXiv1999

The polarization and beaming effect for GRBs

Both observations and theoretical models suggest that the emissions in gamma-ray bursts (GRBs) and the afterglows are beamed. We argue that the recent polarization measured in the afterglows gives further evidence of beaming in GRBs. In this approach, we adopted the polarization-magnitude relation of BL Lacertae objects to 4 GRBs with available polarization measurements and found that the data of the 4 GRBs are consistent with the relation of BL Lacertae objects. This result suggests that the emissions in GRBs are beamed and there are similarities between GRBs and BL Lacertae objects (or generally blazars). We suggest that the weak polarization of GRBs may result from low percentage of orderly magnetic field in the shock front.

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arXiv1999

Basic properties of Gamma-ray loud blazars

In this paper, a method is proposed to determine the basic properties of $γ$-ray loud blazars, among them the central black hole mass, M, the Doppler factor, $δ$, the propagation angle of the $γ$-rays with respect to the symmetric axis of a two-temperature accretion disk, $Φ$, and the distance (i.e. the height above the accretion disk), d at which the $γ$-rays are created, for seven $γ$-ray loud blazars with available GeV variability timescales and in which the absorption effect of a $γ$-ray and the beaming effect have been taken into account. Our results indicate that, if we take the intrinsic $γ$-ray luminosity to be $λ$ times the Eddington luminosity, $L_γ^{in} = λL_{Edd.}$, the masses of the blazars are in the range of $(4 \sim 131)\times 10^{7}M_{\odot}$, the Doppler factors ($δ$) lie in the range of 0.57 to 5.33 the angle ($Φ$) is in the range of $13^{\circ}$ to 43$^{\circ}$ and the distance (d) is in the range of 26R_{g} to 411R_{g}. Our model results are independent of $γ$-ray emission mechanisms but they do depend on the X-ray emission mechanism of the accretion disk.

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arXiv1999

The central black hole masses and Doppler factors of the $γ$-ray loud blazars

In this paper, The central black hole masses and the Doppler factors are derived for PKS 0528+134, PKS 0537-441, 3C279, PKS 1406-074, PKS 1622-297, Q1633+382, Mkn 501, and BL Lacertae. The masses obtained are in the range of (1 -7)$\times 10^{7}M_{\odot}$ and compared with that obtained with the Klein-Nishina cross section considered (Dermer & Gehrels 1995). If we considered only the Thomson cross section, the masses are in the range of 2.6$\times 10^{6}M_{\odot}$ - 2$\times 10^{11}M_{\odot}$. The masses obtained from our method are less sensitive to the flux than those obtained from Dermer & Gehrels (1995) method. The masses obtained from two flares (1991 and 1996 flares) of 3C279 are almost the same. For 3C279 and BL Lacertae, viewing angle, $θ$, and Lorentz factor, $Γ$, are estimated from the derived Doppler factor and the measured superluminal velocity. For 3C279, $θ= 10^{\circ}.9-15^{\circ}.6$, $Γ$ = 2.4-14.4 for $δ$ = 3.37; $θ= 8^{\circ}.45-9^{\circ}.7$, $Γ$ = 2.95-11.20 for $δ$ = 4.89; For BL Lacertae, $θ= 25^{\circ}-29^{\circ}.4$, $Γ$ = 2.0-4.0.

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arXiv1999

Infrared Variation of Blazars

In this paper, the historical infrared (JHK) data compiled from the published literature are presented here for 30 blazars. Maximum near-IR variations are found and compared with the optical ones. For the color-magnitude relation, some objects show that the color index increases with magnitude while 1253-055 shows complex behaviour, which perhaps suggests that the emission mechanism consists of, at least, two parts. The spectral indexes are in the range of $α_{IR}$ = 0.77 to 2.37.

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