Searching for γ-ray emission from LOTAAS pulsars

The LOw-Frequency ARray(LOFAR)has recently conducted a survey(LOFAR Tied-Array Allsky Survey;LOTAAS)for pulsars in the Northern hemisphere that resulted in discoveries of 73 new pulsars.For the purpose of studying the properties of these pulsars,we search for theirγ-ray counterparts using the all-sky survey data obtained with the Large Area Telescope(LAT)onboard the Fermi Gamma-Ray Space Telescope(Fermi).We analyze the LAT data for 70 LOTAAS pulsars(excluding two millisecond pulsars and one with the longest known spin period of 23.5 s).We find one candidate counterpart to PSR J1017+30,which should be searched for theγ-ray pulsation signal once its timing solution is available.For other LOTAAS pulsars,we derive their 0.3-500 GeV flux upper limits.In order to compare the LOTAAS pulsars with the knownγ-ray pulsars,we also derive the 0.3-500 GeVγ-ray fluxes for 112 of the latter contained in the Fermi LAT fourth source catalog.Based on the properties of theγ-ray pulsars,we derive upper limits on the spin-down luminosities of the LOTAAS pulsars.The upper limits are not very constraining but help suggest that most of the LOTAAS pulsars probably have<10³³ erg s^-1 spin-down luminosities and are not expected to be detectable with Fermi LAT.     

Identification of candidate millisecond pulsars from Fermi LAT observations

We report our detailed data analysis of 39 γ-ray sources selected from the 992 unassociated sources in the third Fermi Large Area Telescope Third Source Catalog.The selection criteria,which were set for finding candidate millisecond pulsars(MSPs),are non-variables with curved spectra and 5?Galactic latitudes.From our analysis,24 sources were found to be point-like sources not contaminated by background or nearby unknown sources.Three of them,J1544.6-1125,J1625.1-0021 and J1653.6-0158,have been previously studied,indicating that they are likely MSPs.The spectra of J0318.1+0252 and J2053.9+2922 do not have properties similar to known γ-ray MSPs,and we thus suggest that they are not MSPs.Analysis of archival X-ray data for most of the 24 sources was also conducted.Four sources were found with X-ray objects in their error circles,and 16 with no detection.The ratios between the γ-ray fluxes and X-ray fluxes or flux upper limits are generally lower than those of known γ-ray MSPs,suggesting that if the γ-ray sources are MSPs,none of the X-ray objects are their counterparts.Deep X-ray or radio observations of these sources are needed in order to identify their MSP nature.     

Identification of candidate millisecond pulsars from Fermi LAT observations Ⅱ

On the basis of the properties of known γ-ray millisecond pulsars(MSPs), we have selected 77un-associated sources from the Fermi Large Area Telescope(LAT) third source catalog for the purpose of finding likely candidate MSPs. Previously, detailed LAT data analysis for 39 of them was reported,and here we report the analysis for the remaining 38 sources. We identify that among the 38 sources, 28 of them are single point-like sources with clean background and their spectra show significant curvature.We also conduct an analysis of archival X-ray data available for 24 of the 28 sources. In the fields of10 sources, there is at least one X-ray object, and in those of the other 14 sources, no X-ray object is detected but this is probably due to the X-ray observations being short. We discuss the possible MSP nature for these sources. Six of them(J0514.6-4406, J1035.7-6720, J1624.2-4041, J1744.1-7619,J1946.4-5403 and J2039.6-5618) are most likely associated with pulsars because of multi-wavelength identifications including direct radio or γ-ray detection of pulsations. To firmly establish the associations or verify the MSP nature for other sources, deep X-ray and/or optical observations are needed.     

Populations of Bright X-ray Sources in the Starburst Galaxies NGC 4038/4039

Assuming a naive star formation history, we construct synthetic X-ray source pop- ulations, using a population synthesis code, for comparison with the observed X-ray lumi- nosity function (XLF) of the interacting galaxies NGC 4038/4039. We have included high- and intermediate-mass X-ray binaries, young rotation-powered pulsars and fallback disk-fed black holes in modeling the bright X-ray sources detected. We find that the majority of the X-ray sources are likely to be intermediate-mass X-ray binaries, but for typical binary evolu- tion parameters, the predicted XLF seems to be steeper than observed. We note that the shape of the XLFs depends critically on the existence of XLF break for young populations, and suggest super-Eddington accretion luminosities or the existence of intermediate-mass black holes to account for the high luminosity end and the slope of the XLF in NGC 4038/4039.     

Possible distance indicators in gamma-ray pulsars

Distance measurement of gamma-ray pulsars is a current challenge in pulsar studies.The Large Area Telescope(LAT) aboard the Fermi gamma-ray observatory discovered more than 70 gamma-ray pulsars including 24 new gamma-selected pulsars with almost no distance information.We study the relation between gammaray emission efficiency(η=Lγ/E˙) and pulsar parameters for young radio-selected gamma-ray pulsars with known distance information in the first gamma-ray pulsar catalog reported by Fermi/LAT.We have introduce...     

An Arecibo follow-up study of seven pulsars discovered by Five-hundred-meter Aperture Spherical radio Telescope(FAST)

We present Arecibo 327 MHz confirmation and follow-up studies of seven new pulsars discovered by the Five-hundred-meter Aperture Spherical radio Telescope (FAST).These pulsars are discovered in a pilot program of the Commensal Radio Astronomy FAST Survey (CRAFTS) with the ultra-widebandwidth commissioning receiver.Five of them are normal pulsars and two are extreme nulling slow pulsars.PSR J2111+2132’s dispersion measure (DM:78.5 pc cm~(-3)) is above the upper limits of the two Galactic free electron density models,NE2001 and YMW16,and PSR J2057+2133’s position is out of the Scutum-Crux Arm,making them uniquely useful for improving the Galactic free electron density model in their directions.We present a detailed single pulse analysis for the slow nulling pulsars.We show evidence that PSR J2323+1214’s main pulse component follows a non-Poisson distribution and marginal evidence for a sub-pulse-drift or recurrent period of 32.3±0.4 rotations from PSR J0539+0013.We discuss the implication of our finding to the pulsar radiation mechanism.     

Long term optical variability of bright X-ray point sources in elliptical galaxies

We present long term optical variability studies of bright X-ray sources in four nearby elliptical galaxies with the Chandra Advanced CCD Imaging Spectrometer array(ACIS-S) and observations from the Hubble Space Telescope(HST) Advanced Camera for Surveys.Out of the 46 bright(X-ray counts 60)sources that are in the common field of view of the Chandra and HST observations,34 of them have potential optical counterparts,while the rest of them are optically dark.After taking into account systematic errors,estimated using optical sources in the field as a reference,we find that four of the X-ray sources(three in NGC 1399 and one in NGC 1427) have variable optical counterparts at a high level of significance.The X-ray luminosities of these sources are ~ 10~(38) erg s~(-1) and are also variable on similar time scales.The optical variability implies that the optical emission is associated with the X-ray source itself rather than being the integrated light from a host globular cluster.For one source,the change in optical magnitude is 0.3,which is one of the highest reported for this class of X-ray sources and this suggests that the optical variability is induced by the X-ray activity.However,the optically variable sources in NGC 1399 have been reported to have blue colors(g- z 1).All four sources have been detected in the infrared(IR) by Spitzer as point sources,and their ratios of 5.8 to 3.6 μm flux are 0.63,indicating that their IR spectra are like those of Active Galactic Nuclei(AGNs).While spectroscopic confirmation is required,it is likely that all four sources are background AGNs.We find none of the X-ray sources having optical/IR colors different from AGNs to be optically variable.     

The Great Pretenders Among the ULX Class

The recent discoveries of pulsed X-ray emission from three ultraluminous X-ray(ULX)sources have finally enabled us to recognize a subclass within the ULX class:the great pretenders,neutron stars(NSs) that appear to emit X-ray radiation at isotropic luminosities L~(39)X= 7 × 10 erg s~(-1)-1 × 10~(41) erg s~(-1) only because their emissions are strongly beamed toward our direction and our sight lines are offset by only a few degrees from their magnetic-dipole axes.The three known pretenders appear to be stronger emitters than the presumed black holes of the ULX class,such as Holmberg II IX X-1,IC10 X-1 and NGC 300 X-1.For these three NSs,we have adopted a single reasonable assumption,that their brightest observed outbursts unfold at the Eddington rate,and we have calculated both their propeller states and their surface magnetic-field magnitudes.We find that the results are not at all different from those recently obtained for the Magellanic Be/X-ray pulsars:the three NSs reveal modest magnetic fields of about 0.3–0.4 TG and beamed propeller-line X-ray luminosities of ~10~(36)-10~(37) erg s~(-1),substantially below the Eddington limit.     

Does black hole spin play a key role in the FSRQ/BL Lac dichotomy?

Blazars are characterized by large intensity and spectral variations across the electromagnetic spectrum It is believed that jets emerging from them are almost aligned with the line-of-sight. The majority of identified extragalactic sources in γ-ray catalogs of EGRET and Fermi are blazars. Observationally,blazars can be divided into two classes: flat spectrum radio quasars(FSRQs) and BL Lacs. BL Lacs usually exhibit lower γ-ray luminosity and harder power law spectra at γ-ray energies than FSRQs. We attempt to explain the high energy properties of FSRQs and BL Lacs from Fermi γ-ray space telescope observations. It was argued previously that the difference in accretion rates is mainly responsible for the large mismatch in observed luminosity in γ-ray. However, when intrinsic luminosities are derived by correcting for beaming effects, this difference in γ-ray luminosity between the two classes is significantly reduced. In order to explain this difference in intrinsic luminosities, we propose that spin plays an important role in the luminosity distribution dichotomy of BL Lacs and FSRQs. As the outflow power of a blazar increases with increasing spin of a central black hole, we suggest that the spin plays a crucial role in making BL Lac sources low luminous and slow rotators compared to FSRQ sources.     

Similarity of jet radiation between flat spectrum radio quasars and GeV narrow-line Seyfert 1 galaxies: a universal δ-L_c correlation

By modeling the broadband spectral energy distributions(SEDs) of a typical flat spectrum radio quasar(FSRQ, 3C 279) and two GeV narrow-line Seyfert 1 galaxies(NLS1s, PMN J0948+0022 and 1H0323+342) in different flux stages with one-zone leptonic models, we find a universal correlation between their Doppler factors(δ) and peak luminosities(L_c) of external Compton scattering bumps. Compiling a combined sample of FSRQs and GeV NLS1s, it is found that both FSRQs and GeV NLS1s in different stages and in different sources follow the same δ-L_c correlation well. This indicates that the variations of observed luminosities may be essentially due to the Doppler boosting effect. The universal δ-L_c relation between FSRQs and GeV NLS1s in different stages may be further evidence that the particle acceleration and radiation mechanisms for the two kinds of sources are similar. In addition, by replacing L_c with the observed luminosity in the Fermi/LAT band(L_(LAT)), this correlation holds and it may serve as an empirical indicator of δ. We estimate the δ values with L_(LAT) for 484 FSRQs in the Fermi/LAT Catalog and they range from 3 to 41, with a median of 16, which are statistically consistent with the values derived by other methods.     

How can FAST improve study of the pulsar emission mechanism and magnetospheric dynamics?

The Five-hundred-meter Aperture Spherical radio Telescope(FAST) has the potential to discover many new pulsars and new phenomena. In this paper we mainly concentrate on how FAST can impact study of the pulsar emission mechanism and magnetospheric dynamics. Several observational programs heading to this direction are reviewed. To make full use of the superior performance of FAST and maximize the scientific outcome, these programs can be arranged in different phases of FAST according to their demands for observational conditions. We suggest that programs can be performed following the test phase, which are observations of multifrequency mean pulse profiles, anomalous X-ray pulsars(AXPs)/soft gamma-ray repeaters(SGRs), mode changing, drifting subpulse and nulling. The long-term monitoring can be carried out for mode changing, AXPs/SGRs and precessional pulsars. Others programs, including polarization observations of radio and γ-ray pulsars, searching for weak pulse components, and multifrequency observations of subpulse drifting, microstructure and giant pulses, can be conducted in all the normal operating phases(the first and second phases). These programs will push forward the frontier in this field in different respects. The search for sub-millisecond pulsars and follow-up observations of their emission properties are very important projects for FAST, but they may be covered by other papers in this mini-volume; therefore,they are not discussed here.     

Searching for AGN and pulsar candidates in 4FGL unassociated sources using machine learning

In the fourth Fermi Large Area Telescope source catalog(4 FGL),5064γ-ray sources are reported,including 3207 active galactic nuclei(AGNs),239 pulsars,1336 unassociated sources,92 sources with weak association with blazars at low Galactic latitudes and 190 other sources.We employ two different supervised machine learning classifiers,combined with the direct observation parameters given by the 4 FGL fits table,to search for sources potentially classified as AGNs and pulsars in the 1336 unassociated sources.In order to reduce the error caused by the large difference in the sizes of samples,we divide the classification process into two separate steps in order to identify the AGNs and the pulsars.First,we select the identified AGNs from all of the samples,and then select the identified pulsars from the remaining cases.Using the 4 FGL sources associated or identified as AGNs,pulsars and other sources with the features selected through the K-S test and the random forest(RF)feature importance measurement,we trained,optimized and tested our classifier models.Then,the models are applied to classify the 1336 unassociated sources.According to the calculation results of the two classifiers,we report the sensitivity,specificity,accuracy in each step and the class of unassociated sources given by each classifier.The accuracy obtained in the first step is approximately 9595%;in the second step,the obtained overall accuracy is approximately 80%.Combining the results of the two classifiers,we predict that there are 583 AGN-type candidates,115 pulsar-type candidates,154 other types ofγ-ray candidates and 484 of uncertain types.     

Variability Analysis of EGRET Gamma-Ray Sources

The variability of γ-ray sources listed in the third EGRET catalog is studied using three variability indices. These indices are found to be statistically equivalent if the observed data are sufficiently accurate. Using the three indices,30 EGRET point sources which are positionally coincident with pulsars and 40 persistent unidentified sources at low latitudes are analyzed for their variability status. It is found that 14 of the 30 point sources may have genuine or plausible associations with pulsars, and 16 of the 40 persistent unidentified sources are possible pulsar candidates.     

The core dominance parameter and Fermi detection of extragalactic radio sources

By cross-correlating an archive sample of 542 extragalactic radio sources with the Fermi-LAT Third Source Catalog(3FGL), we have compiled a sample of 80 γ-ray sources and 462 non-Fermi sources with available core dominance parameter(RCD), and core and extended radio luminosity; all the parameters are directly measured or derived from available data in the literature. We found that RCD has significant correlations with radio core luminosity, γ-ray luminosity and γ-ray flux; the Fermi sources have on average higher RCD than non-Fermi sources. These results indicate that the Fermi sources should be more compact,and the beaming effect should play a crucial role in the detection of γ-ray emission. Moreover, our results also show Fermi sources have systematically larger radio flux than non-Fermi sources at fixed RCD, indicating larger intrinsic radio flux in Fermi sources. These results show a strong connection between radio andγ-ray flux for the present sample and indicate that the non-Fermi sources are likely due to the low beaming effect, and/or the low intrinsic γ-ray flux. This supports a scenario that has been published 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.     

Non-thermal emissions from accreting X-ray binary pulsars

We study non-thermal emissions from cascade processes in accreting X-ray binary pulsars.In the framework of the magnetospheric gap model,we consider three photon fields,which are respectively from the polar cap of a pulsar,its surrounding accretion disk and a massive companion star with a circumstellar disk,to shield the gap.The gap-accelerated ultra-relativistic electrons emit high-energy photons via curvature radiation and an inverse Compton scattering process,in which part of these high-energy photons absorbed by interactions with the surrounding photon fields can facilitate the following electromagnetic cascades.We first carry out numerical calculations of the cascade processes in order to obtain the predicted emission spectra.As an example,we subsequently apply this model to reproduce observations of LS I +61?303.We find that the results can fit observations ranging from hard X-ray to γ-ray bands.In particular,they can explain the spectral cutoff feature at a few GeV.Finally,we suggest that the emissions detected by the Fermi Large Area Telescope from X-ray binary pulsars originate in the magnetosphere region of the pulsar.     

The updated Bulk Lorentz Factors of Gamma-Ray Burst X-Ray Flares

X-ray flares are the most common phenomena in the afterglow phase of gamma-ray bursts(GRBs) in the Swift era,and are known as a canonical component in X-ray afterglows.In this work,we constrain the Lorentz factor of X-ray flares with an updated sample.We extensively search for X-ray light curves showing flare and jet break simultaneously.A smooth broken power law function is used to fit the jet breaks in 11 GRBs.We also use a smooth broken power law function to fit the profile of X-ray flares,and the total number of the flares is 20.We obtain the lower and upper limits of Lorentz factor(Γ_X) with the timescale,half-opening angle and mean luminosity of the X-ray flares for interstellar medium(ISM) and wind cases.The lower limits on Γ_X range from tens to a few hundred,and the upper limits are mainly about a few hundred.We also apply the limited Lorentz factor to test correlations of Γ_0-E_(γ,iso) and Γ_0-L_(γ,iso) for GRBs,and find X-ray flares in the ISM case are much more consistent with those of prompt emission than the wind case in a statistical sense for both correlations.X-ray flares are almost consistent with the trend in the correlations of Γ_0-E__(γ,iso)(L_(γ,iso)) for prompt GRBs,indicating X-ray flares and prompt bursts may have the same physical origin.     

Possible modulated γ-ray emission from the transitional millisecond pulsar binary PSR J1023+0038

We report the results from our analysis of Fermi Large Area Telescope(LAT) data for the transitional millisecond pulsar binary PSR J1023+0038. The time period of the data is nearly 9 yr, and that after the source's transition, in June 2013 from the disk-free state to the active state of having an accretion disk, is approximately 4 yr. We identify a high-energy 5.5 GeV component in the source's spectrum in the active state, and find this component is only significantly detected in half of the orbital phase centered at the descending node(when the pulsar is moving towards the Earth). Considering the pulsar scenario proposed for multi-frequency emission from the source, in which the pulsar is still active and a cold-relativistic pulsar wind inverse-Compton scatters the photons from the accretion disk, we discuss the origin of the high-energy component. In order to explain the observed spectrum, a power-law distribution of particles, with an index of ～3, in the pulsar wind is required, while the orbital variations are possibly due to changes in power-law index as a function of orbital phase.     

Effects of dynamo magnetic fields on observational properties of accreting millisecond X-ray pulsars

In this paper,we have investigated accreting millisecond X-ray pulsars,which are rapidly rotating neutron stars in low-mass X-ray binaries.These systems exhibit coherent X-ray pulsations that arise when the accretion flow is magnetically channeled to the stellar surface.Here,we have developed the fundamental equations for an accretion disk around accreting millisecond X-ray pulsars in the presence of a dynamo generated magnetic field in the inner part of the disk.We have also formulated the numerical method for the structure equations in the inner region of the disk and the highest accretion rate is enough to form the inner region of the disk,which is overpowered by radiation pressure and electron scattering.Finally,we have examined our results with the effects of dynamo magnetic fields on accreting millisecond X-ray pulsars.     

The lower limit of the Doppler factor for a Fermi blazar sample

We selected 457 blazars(193 flat spectrum radio quasars, 61 lowsynchrotron peaked blazars, 69 intermediate-synchrotron peaked blazars and 134high-synchrotron peaked blazars) from the second Fermi-LAT catalog(2FGL) of γ-ray sources, which have X-ray observations. We calculated the lower limits for their Doppler factors, δγ, and compared the lower limits with the available Doppler factors and the apparent superluminal velocities in the literature.     

Revisiting pulsar velocities using Gaia Data Release 2

Precise measurements of neutron star(NS) velocities provide critical clues in regard to the supernova physics and evolution of binary systems. Based on Gaia Data Release 2(DR2), we selected a sample of 24 young( 3 Myr) pulsars with precise parallax measurements and measured the velocity of their local standard of rest(LSR) and the velocity dispersion among their respective local stellar groups. The median velocity difference between thus calculated LSRs and the Galactic rotation model is ～ 7.6 km s~(-1),small compared to the typical velocity dispersion of ～ 27.5 km s~(-1). For pulsars off the Galactic plane,such differences grow significantly to as large as ～ 40 km s~(-1). More importantly, the velocity dispersion of stars in the local group of low-velocity pulsars can be comparable to their transverse velocities, suggesting that the intrinsic velocities of NS progenitors should be taken into account when we consider their natal kicks and binary evolution. We also examined the double NS system J0737-3039 A/B, and measured its transverse velocity to be 26_(-13)~(+18) km s~(-1) assuming nearby Gaia sources are representative of their birth environment. This work demonstrated the feasibility and importance of using Gaia data to study the velocity of individual systems and velocity distribution of NSs.