arXiv subjects

Jin H. An

Publications and source records attributed to Jin H. An.

17 recordsLinked to original sources

arXiv2013

Degeneracies in triple gravitational microlensing

We study microlensing light curves by a triple lens, in particular, by a primary star plus two planets. A four-fold degeneracy is confirmed in the light curves, similar to the close and wide degeneracy found in a double lens. Furthermore, we derive a set of equations for triple-lens in the external shear approximation. By using these external shear equations, we identify two kinds of continuous degeneracies which may confused double and triple lenses, i.e. the continuous external shear degeneracy among triple-lens systems and the double-triple lens degeneracy. These degeneracies are particularly important in high magnification events, and thus some caution needs to be applied when one infers the fraction of stars hosting multiple planets from microlensing. We study the dependence of the degeneracies on the lensing parameters (e.g., source trajectory) and give recipes about how the degeneracies should be explored with real data.

astro-ph.EP↗

arXiv2013

Three-dimensional Microlensing

We study three-dimensional microlensing where two lenses are located at different distances along the line of sight. We formulate the lens equation in complex notations and recover several previous results. There are in total either 4 or 6 images, with an equal number of images with positive and negative parities. We find that the sum of signed magnifications for six image configurations is unity. Furthermore, we show that the light curves can be qualitatively different from those for binary lensing in a single plane. In particular, the magnifications between a `U'-shaped caustic crossing can be close to unity, rather than having a minimum magnification of 3 as in the single plane binary lensing. There is only a small probability three-dimensional microlensing events will be seen in microlensing toward the Galactic centre. It is more likely they will be first seen in cosmological microlensing.

astro-ph.GA↗

arXiv2009

A theorem on central velocity dispersions

It is shown that, if the tracer population is supported by a spherical dark halo with a core or a cusp diverging more slowly than that of a singular isothermal sphere, the logarithmic cusp slope 'g' of the tracers must be given exactly by g=2b where b is their velocity anisotropy parameter at the center unless the same tracers are dynamically cold at the center. If the halo cusp diverges faster than that of the singular isothermal sphere, the velocity dispersion of the tracers must diverge at the center too. In particular, if the logarithmic halo cusp slope is larger than two, the diverging velocity dispersion also traces the behavior of the potential. The implication of our theorem on projected quantities is also discussed. We argue that our theorem should be understood as a warning against interpreting results based on simplifying assumptions such as isotropy and spherical symmetry.

astro-ph.GA↗

arXiv2009

Dark Matter Angular Momentum Profile from the Jeans Equation

Cosmological simulations of dark matter structures have shown that the equilibrated dark matter structures have a fairly small angular momentum. It appears from these N-body simulations that the radial profile of the angular momentum has an almost universal behavior, even if the different dark matter structures have experienced very different formation and merger histories. We suggest a perturbed Jeans equation, which includes a rotational term. This is done under a reasonable assumed form of the change in the distribution function. By conjecturing that the (new) subdominant rotation term must be proportional to the (old) dominant mass term, we find a clear connection, which is in rather good agreement with the results of recent high resolution simulations. We also present a new connection between the radial profiles of the angular momentum and the velocity anisotropy, which is also in fair agreement with numerical findings. Finally we show how the spin parameter $λ$ increases as a function of radius.

astro-ph.CO↗

arXiv2007

Gravitational lensing by point masses on regular grid points

It is shown that gravitational lensing by point masses arranged in an infinitely extended regular lattice can be studied analytically using the Weierstrass functions. In particular, we draw the critical curves and the caustic networks for the lenses arranged in regular-polygonal -- square, equilateral triangle, regular hexagon -- grids. From this, the mean number of positive parity images as a function of the average optical depth is derived and compared to the case of the infinitely extended field of randomly distributed lenses. We find that the high degree of the symmetry in the lattice arrangement leads to a significant bias towards canceling of the shear caused by the neighboring lenses on a given lens position and lensing behaviour that is qualitatively distinct from the random star field. We also discuss some possible connections to more realistic lensing scenarios.

astro-ph↗

arXiv2005

Simple Models for the Distribution of Dark Matter

We introduce a simple family of models for representing the dark matter in galaxies. The potential and phase space distribution function are all elementary, while the density is cusped. The models are all hypervirial, that is, the virial theorem holds locally, as well as globally. As an application to dark matter studies, we compute some of the properties of gamma-ray sources caused by neutralino self-annihilation in dark matter clumps.

astro-ph↗

arXiv2005

Gravitational lens under perturbations: Symmetry of perturbing potentials with invariant caustics

When the gravitational lensing potential can be approximated by that of circularly symmetric system affected by weak perturbations, it is found that the shape of the resulting (tangential) caustics is entirely specified by the local azimuthal behaviour of the affecting perturbations. This provides a common mathematical groundwork for understanding problems such as the close-wide (d <-> 1/d) separation degeneracy of binary lens microlensing lightcurves and the shear-ellipticity degeneracy of quadruple image lens modelling.

astro-ph↗

arXiv2004

Microlensing Characterization of Wide-Separation Planets

With their excellent photometric precision and dramatic increase in monitoring frequency, future microlensing survey experiments are expected to be sensitive to very short time-scale, isolated events caused by free-floating and wide-separation planets with mass as low as a few lunar masses. We estimate the probability of measuring the Einstein radius θ_E for bound and free-floating planets. We carry out detailed simulations of the planetary events expected in next-generation surveys and estimate the resulting uncertainty in θ_E for these events. We show that, for main-sequence sources and Jupiter-mass planets, the caustic structure of wide-separation planets with projected separations of < 20 AU substantially increases the probability of measuring the dimensionless source size and thus determining θ_E compared to the case of unbound planets. In this limit where the source is much smaller than the caustic, the effective cross-section to measure θ_E to 10% is ~25% larger than the full width of the caustic. Measurement of the lens parallax is possible for low-mass planetary events by combined observations from the ground and a satellite located in an L2 orbit; this would complete the mass measurements for such wide-separation planets. Finally, short-duration events caused by bound planets can be routinely distinguished from those caused by free-floating planets for planet-star separations < 20 AU from either the deviations due to the planetary caustic or (more often) the low-amplitude bump from the magnification due to the parent star.

astro-ph↗

arXiv2004

The POINT-AGAPE Survey I: The Variable Stars in M31

The POINT-AGAPE collaboration has been monitoring M31 for three seasons with the Wide Field Camera on the Isaac Newton Telescope. In each season, data are taken for one hour per night for roughly sixty nights during the six months that M31 is visible. The two fields of view straddle the central bulge, northwards and southwards. We have calculated the locations, periods and amplitudes of 35414 variable stars in M31 as a by-product of our microlensing search. The variables are classified according to their period and amplitude of variation. They are classified into population I and II Cepheids, Miras and semi-regular long-period variables. The population I Cepheids are associated with the spiral arms, while the central concentration of the Miras and long-period variables varies noticeably, the stars with brighter (and shorter) variations being much more centrally concentrated. A crucial role in the microlensing experiment is played by the asymmetry signal. It was initially assumed that the variable stars would not be a serious problem as their distributions would be symmetric. We demonstrate that this assumption is not correct. We find that differential extinction associated with the dust lanes causes the variable star distributions to be asymmetric. The size and direction of the asymmetry of the variable stars is measured as a function of period and amplitude of variation. The implications of this discovery for the successful completion of the microlensing experiments towards M31 are discussed. (Abridged)

astro-ph↗

arXiv2003

Spatial Separation of the 3.29 micron Emission Feature and Associated 2 micron Continuum in NGC 7023

We present a new 0.9" resolution 3.29 micron narrowband image of the reflection nebula NGC 7023. We find that the 3.29 micron IEF in NGC 7023 is brightest in narrow filaments NW of the illuminating star. These filaments have been seen in images of K', molecular hydrogen emission lines, the 6.2 and 11.3 micron IEFs, and HCO+. We also detect 3.29 micron emission faintly but distinctly between the filaments and the star. The 3.29 micron image is in contrast to narrowband images at 2.09, 2.14, and 2.18 micron, which show an extended emission peak midway between the filaments and the star, and much fainter emission near the filaments. The [2.18]-[3.29] color shows a wide variation, ranging from 3.4-3.6 mag at the 2 micron continuum peak to 5.5 mag in the filaments. We observe [2.18]-[3.29] to increase smoothly with increasing distance from the star, up until the filament, suggesting that the main difference between the spatial distributions of the 2 micron continuum and the the 3.29 micron emission is related to the incident stellar flux. Our result suggests that the 3.29 micron IEF carriers are likely to be distinct from, but related to, the 2 micron continuum emitters. Our finding also imply that, in NGC 7023, the 2 micron continuum emitters are mainly associated with HI, while the 3.29 micron IEF carriers are primarily found in warm molecular hydrogen, but that both can survive in HI or molecular hydrogen. (abridged)

astro-ph↗

arXiv2002

First microlens mass measurement: PLANET photometry of EROS BLG-2000-5

We analyze PLANET photometric observations of the caustic-crossing binary-lens microlensing event, EROS BLG-2000-5, and find that modeling the observed light curve requires incorporation of the microlens parallax and the binary orbital motion. The projected Einstein radius (~r_E = 3.61 +/- 0.11 AU) is derived from the measurement of the microlens parallax, and we are also able to infer the angular Einstein radius (theta_E = 1.38 +/- 0.12 mas) from the finite source effect on the light curve, combined with an estimate of the angular size of the source given by the source position in a color-magnitude diagram. The lens mass, M = 0.612 +/- 0.057 M_sun, is found by combining these two quantities. This is the first time that parallax effects are detected for a caustic-crossing event and also the first time that the lens mass degeneracy has been completely broken through photometric monitoring alone. The combination of the projected Einstein radius and the angular Einstein radius also allows us to conclude that the lens lies in the near side of the disk, within 2.6 kpc of the Sun, while the radial velocity measurement indicates that the source is a Galactic bulge giant.

astro-ph↗

arXiv2002

Effect of a wide binary companion to the lens on the astrometric behavior of gravitational microlensing events

In this paper, we investigate the effect of a wide binary companion of the lens on the astrometric behavior of Galactic gravitational microlensing events and compare it to the effect on the photometric behavior. We find that the wide binary companion of the lens can affect the centroid motion of images substantially even if the corresponding light curve appears to be the one of a standard single point-mass lens event. The relatively significant effect of the wide binary lens on the astrometric lensing behavior, on one side, calls for careful consideration of the lens binarity in analyzing the future astrometric lensing data. On the other side, larger astrometric effect of the companion makes astrometric lensing an efficient method to detect binary lenses over a broad range of separations.

astro-ph↗

arXiv2001

Resolving Microlens Blends Using Image Subtraction

Blended light is an important source of degeneracy in the characterization of microlensing events, particularly in binary-lens and high magnification events. We show how the techniques of image subtraction can be applied to form an image of the blend with the source removed. In many cases, it should be possible to construct images with very high signal-to-noise ratio. Analysis of these images can help distinguish between competing models that have different blend fractions, and in some cases should allow direct detection of the lens.

astro-ph↗

arXiv2001

Microlens Mass Measurement using Triple-Peak Events

We show that one can measure the effects of microlens parallax for binary microlensing events with three well-measured peaks -- two caustic crossings plus a cusp approach, and hence derive the projected Einstein radius. Since the angular Einstein radius is measurable from finite source effects for almost any well-observed caustic crossing, triple-peak events can yield the determination of the lens mass. We note that, to a certain extent, rotation of the binary can mimic the effects of parallax, but it should often be possible to disentangle parallax from rotation by making use of the late-time light curve.

astro-ph↗

arXiv2001

Hubble Space Telescope Observations of the CfA Seyfert 2s: Near-infrared Surface Photometry and Nuclear Bars

We present near-infrared J and H surface photometry of 24 of the nearby Seyfert 1.8, 1.9 and 2 galaxies from the CfA Seyfert sample. The excellent angular resolution of HST probes spatial scales as small as tens of parsecs in most of these AGN and is sensitive to the presence of nuclear bars and other potential signatures of the AGN fueling process that channels host galaxy gas and dust to the nuclear region. We have used elliptical isophote techniques to search for nuclear bars in all of these galaxies and have employed a two-dimensional fitting technique to model the nuclear point source and surface brightness distribution of a bright subsample of these galaxies in an attempt to alleviate the impact of the nuclear point source on our sensitivity to nuclear bars. We find stellar nuclear bar candidates in four of these galaxies: Mrk 471, Mrk 270, Mrk 573, and NGC 5929, nearly 20% of the total sample. The percentage rises to ~30% when systems with disturbed morphologies or high inclinations are excluded. The nuclear bars in Mrk 573 and Mrk 270 exhibit some evidence for dust lanes along their leading edges, analogous to the structures seen in host galaxy bars, while the dust lanes in Mrk 471 and NGC 5929 exhibit a more complex morphology. The fact that most of these AGN do not appear to contain stellar nuclear bars suggests that they are not the fueling mechanism for most low-luminosity AGN.

astro-ph↗

arXiv2001

Properties of Microlensing Light Curve Anomalies Induced by Multiple Planets

In this paper, we show that the pattern of microlensing light curve anomalies induced by multiple planets are well described by the superposition of those of the single-planet systems where the individual planet-primary binary pairs act as independent lens systems. Since the outer deviation regions around the planetary caustics of the individual planets occur in general at different locations, we find that the pattern of anomalies in these regions are hardly affected by the existence of other planet(s). This implies that even if an event is caused by a multiple planetary system, a simple single-planet lensing model is good enough for the description of most anomalies caused by the source passage of the outer deviation regions. Detection of the anomalies resulting from the source trajectory passing both the outer deviation regions caused by more than two planets will provide a new channel of detecting multiple planets.

astro-ph↗

arXiv2000

Microlensing Constraints on the Frequency of Jupiter Mass Planets

Microlensing is the only technique likely, within the next 5 years, to constrain the frequency of Jupiter-analogs. The PLANET collaboration has monitored nearly 100 microlensing events of which more than 20 have sensitivity to the perturbations that would be caused by a Jovian-mass companion to the primary lens. No clear signatures of such planets have been detected. These null results indicate that Jupiter mass planets with separations of 1.5-3 AU occur in less than 1/3 of systems. A similar limit applies to planets of 3 Jupiter masses between 1-4 AU.

astro-ph↗