arXiv subjects

Shuang-Nan Zhang

Publications and source records attributed to Shuang-Nan Zhang.

At least 253 records · Page 14Linked to original sources

arXiv2015

A self-consistent interpretation of the GeV-TeV emission from a distant blazar PKS 1424+240

We propose a scenario for self-consistent interpretation of GeV - TeV spectrum of a distant blazar PKS 1424+240. In this scenario, ultra-high energy (UHE) protons are assumed to exist in the blazar jet and produce gamma rays through synchrotron emission emitted by relativistic protons and pair cascades (resulted from $pγ$ interaction); meanwhile, some of these UHE protons may escape from the jet and are injected into intergalactic space. Therefore, we assume that UHE cosmic rays (CR) originate from relativistic protons in the jet and use energy-independent escape timescale to obtain UHECR injection spectrum. Both contributions of gamma rays injected by the source and secondary gamma rays produced in interactions of UHECRs emitted by the blazar with photon background during their propagation through intergalactic space are calculated. Our results show that this scenario is able to reproduce the GeV-TeV spectrum of PKS 1424+240 self-consistently in a broad range of redshifts $0.6<z<1.3$. The required relativistic jet power $L_p\simeq3\times10^{46}\ \rm erg\ s^{-1}$ only moderately depends on the assumed source redshift, while the proton escape timescale (and the injected UHECR luminosity) strongly depends on $z$. We compare the integral TeV fluxes predicted in our scenario with the sensitivity of the planned Cherenkov Telescope Array (CTA), and discuss the implications to future observations.

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arXiv2014

Jet Properties of GeV-Selected Radio-Loud Narrow-Line Seyfert 1 Galaxies and Possible Connection to Their Disk and Corona

The observed spectral energy distributions of five GeV-selected narrow-line Seyfert 1 (NLS1) galaxies are fitted with a model including the radiation ingredients from the relativistic jet, the accretion disk, and the corona. We compare the properties of these GeV NLS1 galaxies with flat spectrum radio quasars (FSRQs), BL Lacertae objects (BL Lacs) and radio-quiet (RQ) Seyfert galaxies, and explore possible hints for jet-disk/corona connection. Our results show that the radiation physics and the jet properties of the GeV NLS1 galaxies resemble that of FSRQs. The luminosity variations of PMN J0948+0022 and 1H 0323+342 at GeV band is tightly correlated with the beaming factor (δ), similar to that observed in FSRQ 3C 279. The accretion disk luminosities and the jet powers of the GeV NLS1 galaxies cover both the ranges of FSRQs and BL Lacs. With detection of bright corona emission in 1H 0323+342, we show that the ratio of the corona luminosity (L_corona) to the accretion disk luminosity (L_d) is marginally within the high end of this ratio distribution for a RQ Seyfert galaxy sample, and the variation of jet luminosity may connect with Lcorona. However, it is still unclear whether a system with a high L_corona/L_d ratio prefers to power a jet.

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arXiv2014

Limits on the strength of individual gravitational wave sources using high-cadence observations of PSR B1937+21

We present the results of a search for gravitational waves (GWs) from individual sources using high cadence observations of PSR B1937+21. The data were acquired from an intensive observation campaign with the Lovell telescope at Jodrell Bank, between June 2011 and May 2013. The almost daily cadence achieved, allowed us to be sensitive to GWs with frequencies up to $4.98\times10^{-6}\,\rm {Hz}$, extending the upper bound of the typical frequency range probed by Pulsar Timing Arrays. We used observations taken at three different radio frequencies with the Westerbork Synthesis Radio Telescope in order to correct for dispersion measure effects and scattering variances. The corrected timing residuals exhibited an unmodeled periodic noise with an amplitude $~150\,\rm {ns}$ and a frequency of $3.4\rm {yr}^{-1}$. As the signal is not present in the entire data set, we attributed it to the rotational behaviour of the pulsar, ruling out the possibilities of being either due to a GW or an asteroid as the cause. After removing this noise component, we placed limits on the GW strain of individual sources equaling to $h_{\rm s}=1.53\times10^{-11}$ and $h_{\rm s}=4.99\times10^{-14}$ at $10^{-7}\,\rm {Hz}$ for random and optimal sources locations respectively.

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arXiv2014

A state-dependent influence of the type-I bursts on the accretion in 4U 1608--52?

We investigated the possible feedback of type-I burst to the accretion process during the spectral evolution of the atoll source 4U 1608--52. By fitting the burst spectrum with a blackbody and an adjustable, persistent spectral component, we found that the latter is significant state-dependent. In the banana state the persistent flux increases along the burst evolution, while in the island state this trend holds only when the bursts are less luminous and starts to reverse at higher burst luminosities. We speculate that, by taking into account both the Poynting-Robertson drag and radiation pressure, these phenomena may arise from the interactions between the radiation field of the type-I burst and the inner region of the accretion disc.

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arXiv2014

The Magneto Hydro Dynamical Model of KHz Quasi Periodic Oscillations in Neutron Star Low Mass X-ray Binaries (II)

We study the kilohertz quasi-periodic oscillations (kHz QPOs) in neutron star low mass X-ray binaries (LMXBs) with a new magnetohydrodynamics (MHD) model, in which the compressed magnetosphere is considered. The previous MHD model (Shi \& Li 2009) is re-examined and the relation between the frequencies of the kHz QPOs and the accretion rate in LMXBs is obtained. Our result agrees with the observations of six sources (4U 0614+09, 4U 1636--53, 4U 1608--52, 4U 1915--15, 4U 1728--34, XTE 1807--294) with measured spins. In this model the kHz QPOs originate from the MHD waves in the compressed magnetosphere. The single kHz QPOs and twin kHz QPOs are produced in two different parts of the accretion disk and the boundary is close to the corotation radius. The lower QPO frequency in a frequency-accretion rate diagram is cut off at low accretion rate and the twin kHz QPOs encounter a top ceiling at high accretion rate due to the restriction of innermost stable circular orbit.

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arXiv2014

Relativistic Jet Properties of GeV-TeV Blazars and Possible Implications for the Jet Formation, Composition, and Cavity Kinematics

We fit the spectral energy distributions (SEDs) of a GeV-TeV FSRQ sample with the leptonic model. Their gamma_min of the relativistic electron distributions, which significantly affect the estimates of the jet properties, are constrained, with a typical value of 48. Their jet power, magnetized parameter, radiation efficiency, and jet production/radiation rates per central black hole (BH) mass are derived and compared to that of BL Lacs. We show that he FSRQ jets may be dominated by the Poynting flux and have a high radiation efficiency, whereas the BL Lac jets are likely dominated by particles and have a lower radiation efficiency than FSRQs. Being different from BL Lacs, the jet powers of FSRQs are proportional to their central BH masses. The jet production and radiation rates of the FSRQs distribute in narrow ranges and are correlated with each other, whereas no similar feature is found for the BL Lacs. We also show that the jet power is correlated with the cavity kinetic power, and the magnetic field energy in the jets may provide the cavity kinetic energy of FSRQs and the kinetic energy of cold protons in the jets may be crucial for cavity kinetic energy of BL Lacs. We suggest that the dominating formation mechanism of FSRQ jets may be the BZ process, but BL Lac jets may be produced via the BP and/or BZ processes, depending on the structures and accretion rates of accretion disks.

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arXiv2014

Interpreting the Spin-down Evolutions of Isolated Neutron Stars with Hall Effects

The observed long-term spin-down evolution of isolated radio pulsars cannot be explained by the standard magnetic dipole radiation with a constant braking torque. However, how and why the torque varies still remains controversial, which is a major issue in understanding neutron stars. Many pulsars have been observed with significant long-term changes of their spin-down rates modulated by quasi-periodic oscillations. Applying the phenomenological model of pulsar timing noise we developed recently to the observed precise pulsar timing data of isolated neutron stars, here we show that the observed long-term evolutions of their spin-down rates and quasi-periodic modulations can be explained by Hall effects in their crusts. Therefore the evolution of their crustal magnetic fields, rather than that in their cores, dominates the observed long term spin-down evolution of these young pulsars. Understanding of the nature of pulsar timing noise not only reveals the interior physics of neutron stars, but also allows physical modeling of pulsar spin-down and thus improves the sensitivity of gravitational wave detections with pulsars.

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arXiv2014

X-ray Scattered Halo around IGR J17544-2619

X-ray photons coming from an X-ray point source not only arrive at the detector directly, but also can be strongly forward-scattered by the interstellar dust along the line of sight (LOS), leading to a detectable diffuse halo around the X-ray point source. The geometry of small angle X-ray scattering is straightforward, namely, the scattered photons travel longer paths and thus arrive later than the unscattered ones; thus the delay time of X-ray scattered halo photons can reveal information of the distances of the interstellar dust and the point source. Here we present a study of the X-ray scattered around IGR J17544-2619, which is one of the so-called supergiant fast X-ray transients. IGR J17544-2619 underwent a striking outburst when observed with Chandra on 2004 July 3, providing a near delta-function lightcurve. We find that the X-ray scattered halo around IGR J17544-2619 is produced by two interstellar dust clouds along the LOS. The one which is closer to the observer gives the X-ray scattered at larger observational angles; whereas the farther one, which is in the vicinity of the point source, explains the halo with a smaller angular size. By comparing the observational angle of the scattered halo photons with that predicted by different dust grain models, we are able to determine the normalized dust distance. With the delay times of the scattered halo photons, we can determine the point source distance, given a dust grain model. Alternatively we can discriminate between the dust grain models, given the point source distance.

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arXiv2014

Common Mistakes in Writing Astronomy and Physics Literature in English

This is the 3rd version with major updates and revisions to the 2nd version of the same title. In this article we have collected and corrected some common mistakes made by Chinese students in writing astronomy and physics literature in English. Brief explanations of these mistakes are given in Chinese. We plan to continue to update this collection periodically. Comments, suggestions and criticisms are welcome.

astro-ph.IM↗

arXiv2014

Possible hard X-ray shortages in bursts from KS 1731-260 and 4U 1705-44

Aims: A hard X-ray shortage, implying the cooling of the corona, was observed during bursts of IGR J17473-272, 4U 1636-536, Aql X-1, and GS 1826-238. Apart from these four sources, we investigate here an atoll sample, in which the number of bursts for each source is larger than 5, to explore the possible additional hard X-ray shortage during {\it Rossi X-ray timing explorer (RXTE)} era. Methods: According to the source catalog that shows type-I bursts, we analyzed all the available pointing observations of these sources carried out by the {\it RXTE} proportional counter array (PCA). We grouped and combined the bursts according to their outburst states and searched for the possible hard X-ray shortage while bursting. Results: We found that the island states of KS 1731-260 and 4U 1705-44 show a hard X-ray shortage at significant levels of 4.5 and 4.7 $σ$ and a systematic time lag of $0.9 \pm 2.1$ s and $2.5 \pm 2.0$ s with respect to the soft X-rays, respectively. While in their banana branches and other sources, we did not find any consistent shortage.

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arXiv2014

Rotational Behaviors and Magnetic Field Evolution of Radio Pulsars

The observed long-term spin-down evolution of isolated radio pulsars cannot be explained by the standard magnetic dipole radiation with a constant braking torque. However how and why the torque varies still remains controversial, which is an outstanding problem in our understanding of neutron stars. We have constructed a phenomenological model of the evolution of surface magnetic fields of pulsars, which contains a long-term decay modulated by short-term oscillations; a pulsar's spin is thus modified by its magnetic field evolution. The predictions of this model agree with the precisely measured spin evolutions of several individual pulsars; the derived parameters suggest that the Hall drift and Hall waves in the NS crusts are probably responsible for the long-term change and short-term quasi-periodical oscillations, respectively. Many statistical properties of the timing noise of pulsars can be well re-produced with this model, including correlations and the distributions of the observed braking indices of the pulsars, which span over a range of more than 100 millions. We have also presented a phenomenological model for the recovery processes of classical and slow glitches, which can successfully model the observed slow and classical glitch events without biases.

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arXiv2013

The hard X-ray behavior of Aql X-1 during type-I bursts

We report the discovery of an anti-correlation between the soft and the hard X-ray lightcurves of the X-ray binary Aql X-1 when bursting. This behavior may indicate that the corona is cooled by the soft X-ray shower fed by the type-I X-ray bursts, and that this process happens within a few seconds. Stacking the Aql X-1 lightcurves of type-I bursts, we find a shortage in the 40--50 keV band, delayed by 4.5$\pm$1.4 s with respect to the soft X-rays. The photospheric radius expansion (PRE) bursts are different in that neither a shortage nor an excess shows up in the hard X-ray lightcurve.

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arXiv2013

Evolution of oxygen and nitrogen abundances and nitrogen production mechanism in massive star-forming galaxies

Utilizing the observational data of 55,318 star-forming galaxies (SFGs) selected from the catalog of MPA-JHU emission-line measurements for the SDSS DR8, we investigate the galaxy downsizing effect of their O and N enrichments, and the nitrogen production mechanism in them. We show the redshift evolution of O and N abundances and specific star formation rates for different galaxy mass ranges, demonstrating the galaxy downsizing effect caused by less massive progenitors of less massive galaxies. The O and N abundances do not remain constant for different galaxy mass ranges, and the enrichment (and hence star formation) decreases with increasing galaxy stellar mass. We find evidence of the O enrichment for galaxies with stellar masses $M_{*}>10^{11.0} $ (in units of $M_{\odot}$), i.e. $Δ({\log}({\rm O/H})) \sim 0.10$ and $Δ({\log}({\rm N/H})) \sim 0.28$ from redshift 0.023 to 0.30. Based on the evolutionary schematic model of N/O ratios in Coziol et al., who proposed the scheme that the production of nitrogen is the consequence of a sequence of bursts in SFGs, we conclude that the nitrogen production is dominated by the intermediate-mass stars, which dominate the secondary synthesis in SFGs. However, for galaxies with $M_{*}>10^{10.35} $ we find evidence of enhanced N/O abundance ratios, which are significantly above the secondary synthesis line. This suggests that outflows of massive stars, which deplete oxygen efficiently, are more important in massive galaxies. Finally we find an excellent linear relation between $M_{*}$ and log(N/O), indicating that the N/O abundance ratio is a good indicator of the stellar mass in a SFG and may be used as a standard candle for studying cosmology, if confirmed with further studies.

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arXiv2013

On the Relaxation Behaviors of Slow and Classical Glitches: Observational Biases and Their Opposite Recovery Trends

We study the pulsar timing properties and the data analysis methods during glitch recoveries. In some cases one first fits the time-of-arrivals (TOAs) to obtain the "time-averaged" frequency $ν$ and its first derivative $\dotν$, and then fits models to them. However, our simulations show that $ν$ and $\dotν$ obtained this way are systematically biased, unless the time intervals between the nearby data points of TOAs are smaller than about $10^4$ s, which is much shorter than typical observation intervals. Alternatively, glitch parameters can be obtained by fitting the phases directly with relatively smaller biases; but the initial recovery timescale is usually chosen by eyes, which may introduce a strong bias. We also construct a phenomenological model by assuming a pulsar's spin-down law of $\dotνν^{-3} =-H_0 G(t)$ with $G(t)=1+κe^{-t/τ}$ for a glitch recovery, where $H_0$ is a constant and $κ$ and $τ$ are the glitch parameters to be found. This model can reproduce the observed data of slow glitches from B1822--09 and a giant classical glitch of B2334+61, with $κ<0$ or $κ>0$, respectively. We then use this model to simulate TOA data and test several fitting procedures for a glitch recovery. The best procedure is: 1) use a very high order polynomial (e.g. to 50th order) to precisely describe the phase; 2) then obtain $ν(t)$ and $\dotν(t)$ from the polynomial; and 3) the glitch parameters are obtained from $ν(t)$ or $\dotν(t)$. Finally, the uncertainty in the starting time $t_0$ of a classical glitch causes uncertainties to some glitch parameters, but less so to a slow glitch and $t_0$ of which can be determined from data.

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arXiv2013

Modeling the Evolution and Distribution of the Frequency's Second Derivative and Braking Index of Pulsar Spin with Simulations

We model the evolution of spin frequency's second derivative $\ddotν$ and braking index $n$ of radio pulsars with simulations within the phenomenological model of their surface magnetic field evolution, which contains a long-term decay modulated by short-term oscillations. For the pulsar PSR B0329+54, the model can reproduce the main characteristics of its $\ddotν$ variation with oscillation periods, predicts another $\sim 50$ yr oscillation component and another recent swing of the sign of $\ddotν$. We show that the "averaged" $n$ is different from the instantaneous $n$, and its oscillation magnitude decreases abruptly as the time span increases, due to the "averaging" effect. The simulation predicted timing residuals agree with the main features of the reported data. We further perform Monte Carlo simulations for the distribution of the reported data in $|\ddotν|$ versus characteristic age $τ_{\rm c}$ diagram. The model with a power law index $α=0.5$ can reproduce the slope of the linear fit to pulsars' distributions in the diagrams of $\log|\ddotν|-\logτ_{\rm c}$ and $\log|n|-\logτ_{\rm c}$, but the oscillations are responsible for the almost equal number of positive and negative values of $\ddotν$, in agreement with our previous analytical studies; an oscillation period of about several decades is also preferred. However the range of the oscillation amplitudes is $-11.4\lesssim\log f\lesssim-10.2$, slightly lager than the analytical prediction, $\log f\simeq-11.85$, because the "averaging" effect was not included previously.

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arXiv2013

Radiation Mechanism and Jet Composition of Gamma-Ray Bursts and GeV-TeV selected Radio Loud Active Galactic Nuclei

Gamma-ray bursts (GRBs) and GeV-TeV selected radio loud Active Galactic Nuclei (AGNs) are compared based on our systematic modeling of the observed spectral energy distributions of a sample of AGNs with a single-zone leptonic model. We show that the correlation between the jet power (P_{jet}) and the prompt gamma-ray luminosity (L_{jet}) of GRBs is consistent, within the uncertainties, with the correlation between jet power and the synchrotron peak luminosity (L_{s, jet}) of flat spectrum radio quasars (FSRQs). Their radiation efficiencies (\varepsilon) are also comparable (>10% for most sources), which increase with the bolometric jet luminosity (L_{bol,jet}) for FSRQs and with the L_{jet} for GRBs with similar power-law indices. BL Lacs do not follow the P_{jet}-L_{s, jet} relation of FSRQs. They have lower \varepsilon and L_{bol, jet} values than FSRQs, and a tentative L_{bol, jet}-\varepsilon relation is also found, with a power-law index being different from that of the FSRQs. The magnetization parameters (σ) of FSRQs are averagely larger than that of BL Lacs. They are anti-correlated with $\varepsilon$ for the FSRQs, but positive correlated with \varepsilon for the BL Lacs. GeV Narrow-line Seyfert 1 galaxies potentially share similar properties with FSRQs. Based on the analogy between GRBs and FSRQs, we suggest that the prompt gamma-ray emission of GRBs is likely produced by synchrotron process in a magnetized jet with high radiation efficiency, similar to FSRQs. The jets of BL Lacs, on the other hand, are less efficient and are likely more matter dominated.

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arXiv2013

Direct measurement of evolving dark energy density and super-accelerating expansion of the universe

A higher value of Hubble constant has been obtained from measurements with nearby Type Ia supernovae, than that obtained at much higher redshift. With the peculiar motions of their hosts, we find that the matter content at such low redshift is only about 10% of that at much higher redshifts; such a low matter density cannot be produced from density perturbations in the background of the ΛCDM expansion. Recently the Planck team has reported a lower Hubble constant and a higher matter content. We find that the dark energy density increases with cosmic time, so that its equation-of-state parameter decreases with cosmic time and is less than -1 at low redshift. Such dark energy evolution is responsible for driving the super-accelerating expansion of the universe. In this extended ΛCDM model, the cosmological redshift represents time rather than radial coordinate, so that the universe complies to the Copernican Principle.

astro-ph.CO↗

arXiv2013

Wavelength Measurements of K Transitions of Oxygen, Neon, and Magnesium with X-ray Absorption Lines

Accurate atomic transition data are important in many astronomical research areas especially in line spectroscopy study. Whereas transition data of He-like and H-like ions (i.e., ions at high-charge states) are accurately calculated, that of K transitions of neutral or low-ionized metal elements are still very uncertain. Spectroscopy of absorption lines produced in the interstellar medium (ISM) has been proven to be an effective way to measure the central wavelengths of these atomic transitions. In this work we analyze 36 Chandra High Energy Transmission Grating observations and search for and measure the ISM absorption lines along sight lines of 11 low-mass X-ray binaries. We correct the Galactic rotation velocity to the rest frame for every observation and then use two different methods to merge all the corrected spectra to a co-added spectrum. However the co-added spectra obtained by these methods exhibit biases, either to the observations of high counts or high signal-to-noise ratios of the lines. We make Bayesian analysis to several significantly detected lines to obtain the systematic uncertainty and the bias correction of other lines. Compared to previous studies (e.g., Yao et al. 2009), our results improve the accuracy of wavelengths by a factor from two to five and significantly reduce the systematic uncertainties and biases. Several weak transitions (e.g., 1s--2p of Mg IV and Mg V; 1s--3p of Mg III and Mg V) are also detected for the first time, albeit with low significance; future observations with improved accuracy are required to confirm these detections.

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