Tidal Radius Research Articles

SOFT X-RAY TEMPERATURE TIDAL DISRUPTION EVENTS FROM STARS ON DEEP PLUNGING ORBITS

One of the puzzles associated with tidal disruption event candidates (TDEs) is that there is a dichotomy between the color temperatures of ${\rm few}\times 10^4$~K for TDEs discovered with optical and UV telescopes, and the color temperatures of ${\rm few}\times 10^5 - 10^6$~K for TDEs discovered with X-ray satellites. Here we propose that high-temperature TDEs are produced when the tidal debris of a disrupted star self-intersects relatively close to the supermassive black hole, in contrast to the more distant self-intersection that leads to lower color temperatures. In particular, we note from simple ballistic considerations that greater apsidal precession in an orbit is the key to closer self-intersection. Thus larger values of $\beta$, the ratio of the tidal radius to the pericenter distance of the initial orbit, are more likely to lead to higher temperatures of more compact disks which are super-Eddington and geometrically and optically thick. For a given star and $\beta$, apsidal precession also increases for larger black hole masses, but larger black hole masses imply a lower temperature at the Eddington luminosity. Thus the expected dependence of the temperature on the mass of the black hole is non-monotonic. We find that in order to produce a soft X-ray temperature TDE, a deep plunging stellar orbit with $\beta> 3$ is needed and a black hole mass of $\lesssim 5\times 10^6 M_\odot$ is favored. Although observations of TDEs are comparatively scarce and are likely dominated by selection effects, it is encouraging that both expectations are consistent with current data.

Tidal radius estimates for three open clusters

A new method is developed for estimating tidal radii and masses of open star clusters (OCL) based on the sky-plane coordinates and proper motions and/or radial velocities of cluster member stars. To this end, we perform the correlation and spectral analysis of oscillations of absolute values of stellar velocity components relative to the cluster mass center along three coordinate planes and along each coordinate axis in five OCL models. Mutual correlation functions for fluctuations of absolute values of velocity field components are computed. The spatial Fourier transform of the mutual correlation functions in the case of zero time offset is used to compute wavenumber spectra of oscillations of absolute values of stellar velocity components. The oscillation spectra of these quantities contain series of local maxima at equidistant wavenumber k values. The ratio of the tidal radius of the cluster to the wavenumber difference Δk of adjacent local maxima in the oscillation spectra of absolute values of velocity field components is found to be the same for all five OCL models. This ratio is used to estimate the tidal radii and masses of the Pleiades, Praesepe, and M67 based on the proper motions and sky-plane coordinates of the member stars of these clusters. The radial dependences of the absolute values of the tangential and radial projections of cluster star velocities computed using the proper motions relative to the cluster center are determined, along with the corresponding autocorrelation functions and wavenumber spectra of oscillations of absolute values of velocity field components. The Pleiades virial mass is estimated assuming that the cluster is either isolated or non-isolated. Also derived are the estimates of the Pleiades dynamical mass assuming that it is non-stationary and non-isolated. The inferred Pleiades tidal radii corresponding to these masses are reported.

Strömgrenuvbyphotometry of the peculiar globular cluster NGC 2419

NGC 2419 is a peculiar Galactic globular cluster in terms of size/luminosity, and chemical abundance anomalies. Here, we present Str\"omgren $uvby$ photometry of the cluster. Using the gravity- and metallicity-sensitive $c_1$ and $m_1$ indices, we identify a sample of likely cluster members extending well beyond the formal tidal radius with an estimated contamination by non-members of only 1%. We derive photometric [Fe/H] of red giants, and depending on which literature metallicity relation we use, find reasonable to excellent agreement with spectroscopic [Fe/H]. We demonstrate explicitly that the photometric errors are not Gaussian, and using a realistic model for the photometric uncertainties, find a formal internal [Fe/H] spread of $\sigma=0.11^{+0.02}_{-0.01}$ dex. This is an upper limit to the cluster's true [Fe/H] spread and may partially/entirely reflect the limited precision of the photometric metallicity estimation and systematic effects. The lack of correlation between spectroscopic and photometric [Fe/H] of individual stars is further evidence against a [Fe/H] spread on the 0.1 dex level. Finally, the CN-sensitive $\delta_4$ anti-correlates strongly with Mg abundance, indicating that the 2nd generation stars are N-enriched. Absence of similar correlations in some other CN-sensitive indices supports the second generation being He-rich, which in these indices approximately compensates the shift due to CN. Compared to a single continuous distribution with finite dispersion, the observed $\delta_4$ distribution is slightly better fit by two discrete populations, with the N-enhanced stars accounting for 53$\pm$5%. NGC 2419 appears to be very similar to other metal-poor Galactic globular clusters with a similarly N-enhanced second generation and little or no variation in [Fe/H], which sets it apart from other suspected accreted nuclei such as {\omega}Cen. (abridged)

Multiple tidal disruption flares in the active galaxy IC 3599

Tidal disruption events occur when a star passes too close to a massive black hole and it is totally ripped apart by tidal forces. It may also happen that the star is not close enough to the black hole to be totally disrupted and a less dramatic event might happen. If the stellar orbit is bound and highly eccentric, just like some stars in the centre of our own Galaxy, repeated flares should occur. When the star approaches the black hole tidal radius at periastron, matter might be stripped resulting in lower intensity outbursts recurring once every orbital period. We report on Swift observations of a recent bright flare from the galaxy IC 3599 hosting a middle-weight black hole, where a possible tidal disruption event was observed in the early 1990s. By light curve modelling and spectral fitting we can consistently account for the events as the non-disruptive tidal stripping of a star into a highly eccentric orbit. The recurrence time is 9.5 yr. IC 3599 is also known to host a low-luminosity active galactic nucleus. Tidal stripping from this star over several orbital passages might be able to spoon-feed also this activity.

New cluster members and halo stars of the Galactic globular cluster NGC 1851

NGC 1851 is an intriguing Galactic globular cluster, with multiple stellar evolutionary sequences, light and heavy element abundance variations and indications of a surrounding stellar halo. We present the first results of a spectroscopic study of red giant stars within and outside of the tidal radius of this cluster. Our results identify nine probable new cluster members (inside the tidal radius) with heliocentric radial velocities consistent with that of NGC 1851. We also identify, based on their radial velocities, four probable extratidal cluster halo stars at distances up to ∼3.1 times the tidal radius, which are supportive of previous findings that NGC 1851 is surrounded by an extended stellar halo. Proper motions were available for 12 of these 13 stars and all are consistent with that of NGC 1851. Apart from the cluster members and cluster halo stars, our observed radial velocity distribution agrees with the expected distribution from a Besançon disc/N-body stellar halo Milky Way model generated by the galaxia code, suggesting that no other structures at different radial velocities are present in our field. The metallicities of these stars are estimated using equivalent width measurements of the near-infrared calcium triplet absorption lines and are found, within the limitations of this method, to be consistent with that of NGC 1851. In addition we recover 110 red giant cluster members from previous studies based on their radial velocities and identify three stars with unusually high radial velocities.

The shell game: a panoramic view of Fornax

We present a panoramic study of the Fornax dwarf spheroidal galaxy, using data obtained as part of the VLT Survey Telescope (VST) ATLAS Survey. The data presented here -- a subset of the full survey -- uniformly cover a region of 25 square degrees centred on the galaxy, in $g$, $r$ and $i$-bands. This large area coverage reveals two key differences to previous studies of Fornax. First, data extending beyond the nominal tidal radius of the dwarf highlight the presence of a second distinct red giant branch population. This bluer red giant branch appears to be coeval with the horizontal branch population. Second, a shell structure located approximately 1.4 degrees from the centre of Fornax is shown to be a mis-identified background overdensity of galaxies. This last result casts further doubt on the hypothesis that Fornax underwent a gas-rich merger in its relatively recent past.

Evidences of tidal distortion and mass loss from the old open cluster NGC 6791

AbstractWe present the first evidence of clear signatures of tidal distortions in the density distribution of the fascinating open cluster NGC 6791. We find that the 2D density map shows a clear elongation and an irregular distribution starting from ~ 300″ from the cluster center and two tails extending in opposite directions beyond the tidal radius. These features are aligned to both the absolute proper motion and to the Galactic centre directions. Accordingly we find that both the surface brightness and star count density profiles reveal a departure from a King model starting from ~ 600″. These observational evidences suggest that NGC 6791 is currently undergoing mass-loss likely due to gravitational shocking and interactions with the tidal field of the Milky Way. We derive the expected mass-loss due to stellar evolution and tidal interactions and we estimate the initial cluster mass to be Mini = (1.5 − 4.0) × 105M⊙.

THE STRUCTURE AND DARK HALO CORE PROPERTIES OF DWARF SPHEROIDAL GALAXIES

The structure and dark matter halo core properties of dwarf spheroidal galaxies (dSphs) are investigated. A double-isothermal (DIS) model of an isothermal, non selfgravitating stellar system, in gravitational equilibrium and embedded in an isothermal dark halo core provides an excellent fit to the various observed stellar surface density distributions �∗(r). Despite its constant velocity dispersion, the stellar system can be well characterised by King profiles (King 1966) with a broad distribution of concentration parameters c = log(r∗,t/r∗,c), with r∗,t and r∗,c the stellar tidal and core radius, respectively. The DIS model confirms the suggestion of Kormendy & Freeman (2014) that the core scale length of the stellar system, defined as a∗ = −(dln�∗/dr 2 ) −1/2 , is sensitive to the central dark matter density �0. In contrast to single-component systems, r∗,t however does not trace the tidal radius of the galaxy but the core radius rc of its dark matter halo. c is therefore sensitive to the ratio �∗/�0 with �∗ and �0 the stellar and dark matter velocity dispersion, respectively. Simple empirical relationships are derived that allow to calculate the dark halo core parameters �0, rc and �0, given the observable quantities �∗, a∗ and c. The DIS model is applied to the Milky Way’s dSphs. Their halo velocity dispersions lie in a narrow range of 10 km/s ≤ �0 ≤ 18 km/s with halo core radii of 280 pc ≤ rc ≤ 1.3 kpc and rc ≈ 2a∗. All dSphs follow closely the same universal scaling relations h�0rci ≡ �0 × rc = 75 +8545 M⊙ pc −2 and � 2 0 ×r −1 c = 0.45 +0.51 −0.27 (km/s) 2 pc −1 that characterise the cores of more massive galaxies over a range of 18 magnitudes in blue magnitude MB. For given h�0rcithe core mass is a strong function of core radius, Mc ∼ r 2 . Inside a fixed radius ru, with ru the logarithmic mean of the dSph’s core radii, the total mass Mu = 2.17h�0rcir 2 u is however roughly constant. Outliers with smaller masses are expected for dSphs with core radii that are much larger or smaller than ru. For the Milky Way’s dSphs we find ru = 400 ± 100 pc and Mu = 2.6 ± 1.4 × 10 7 M⊙, in agreement with Strigari et al. (2008). Due to their small rc, the core densities of the Galaxy’s dSphs are very higher, with �0 = 0.03 - 0.3 M⊙ pc −3 . The dSphs would have to be on galactic orbits with pericenters smaller than a few kpc in order for their stellar systems to be affected by Galactic tides which is very unlikely. dSphs should therefore be tidally undisturbed. Observational evidence for tidal effects might then provide a serious challenge for the cold dark matter scenario.

BATC 15 BAND PHOTOMETRY OF THE OPEN CLUSTER NGC 188

This paper presents CCD multicolour photometry for the old open cluster NGC 188. The observations were carried out as a part of the Beijing--Arizona--Taiwan--Connecticut Multicolour Sky Survey from 1995 February to 2008 March, using 15 intermediate-band filters covering 3000--10000 \AA. By fitting the Padova theoretical isochrones to our data, the fundamental parameters of this cluster are derived: an age of $t=7.5\pm 0.5$ Gyr, a distant modulus of $(m-M)_0=11.17\pm0.08$, and a reddening of $E(B-V)=0.036\pm0.010$. The radial surface density profile of NGC 188 is obtained by star count. By fitting the King model, the structural parameters of NGC 188 are derived: a core radius of $R_{c}=3.80'$, a tidal radius of $R_{t}=44.78'$, and a concentration parameter of $C_{0}=\log(R_{t}/R_{c})=1.07$. Fitting the mass function to a power-law function $\phi(m) \propto m^{\alpha}$, the slopes of mass functions for different spatial regions are derived. We find that NGC 188 presents a slope break in the mass function. The break mass is $m_{\rm break}=0.885~M_{\odot}$. In the mass range above $m_{\rm break}$, the slope of the overall region is $\alpha=-0.76$. The slope of the core region is $\alpha=1.09$, and the slopes of the external regions are $\alpha=-0.86$ and $\alpha=-2.15$, respectively. In the mass range below $m_{\rm break}$, these slopes are $\alpha=0.12$, $\alpha=4.91$, $\alpha=1.33$, and $\alpha=-1.09$, respectively. The mass segregation in NGC 188 is reflected in the obvious variation of the slopes in different spatial regions of this cluster.

Warping and tearing of misaligned circumbinary disks around eccentric supermassive black hole binaries

We study the warping and tearing of a geometrically thin, non-self-gravitating disk surrounding binary supermassive black holes on an eccentric orbit. The circumbinary disk is significantly misaligned with the binary orbital plane, and is subject to the time-dependent tidal torques. In principle, such a disk is warped and precesses, and is torn into mutually misaligned rings in the region, where the tidal precession torques are stronger than the local viscous torques. We derive the tidal-warp and tearing radii of the misaligned circumbinary disks around eccentric SMBH binaries. We find that in disks with the viscosity parameter α larger than a critical value depending on the disk aspect ratio, the disk warping appears outside the tearing radius. This condition is expressed for small amplitude warps as α > √H/(3r) for H/r≲0.1, where H is the disk scale height. If α < √H/(3r), only the disk tearing occurs because the tidal warp radius is inside the tearing radius, where most of disk material is likely to rapidly accrete onto SMBHs. In warped and torn disks, both the tidal-warp and the tearing radii most strongly depend on the binary semi-major axis, although they also mildly depend on the other orbital and disk parameters. This strong dependence enables us to estimate the semi-major axis, once the tidal warp or tearing radius is determined observationally: for the tidal warp radius of 0.1 pc, the semi-major axis is estimated to be ∼10−2 pc for 107 M⊙ black hole with typical orbital and disk parameters. We also briefly discuss the possibility that central objects of observed warped maser disks in active galactic nuclei are supermassive black hole binaries.

GEMINI/GeMS OBSERVATIONS UNVEIL THE STRUCTURE OF THE HEAVILY OBSCURED GLOBULAR CLUSTER LILLER 1.

By exploiting the exceptional high-resolution capabilities of the near-IR camera GSAOI combined with the multi-conjugate adaptive optics system GeMS at the GEMINI South Telescope, we investigated the structural and physical properties of the heavily obscured globular cluster Liller 1 in the Galactic bulge. We have obtained the deepest and most accurate color-magnitude diagram published so far for this cluster, reaching Ks ~ 19 (below the main sequence turn-off level). We used these data to re-determine the center of gravity of the system, finding that it is located about 2.2" south-east from the literature value. We also built new star density and surface brightness profiles for the cluster, and re-derived its main structural and physical parameters (scale radii, concentration parameter, central mass density, total mass). We find that Liller 1 is significantly less concentrated (concentration parameter c=1.74) and less extended (tidal radius r_t=298" and core radius r_c=5.39") than previously thought. By using these newly determined structural parameters we estimated the mass of Liller 1 M_tot = 2.3 x 10^6 Msun (Mtot = 1.5 x 10^6 Msun for a Kroupa IMF), which is comparable to that of the most massive clusters in the Galaxy (omega Centauri and Terzan 5). Also Liller 1 has the second highest collision rate (after Terzan 5) among all star clusters in the Galaxy, thus confirming that it is an ideal environment for the formation of collisional objects (such as millisecond pulsars).

A YOUNG WHITE DWARF WITH AN INFRARED EXCESS

Using observations of Spitzer/IRAC, we report the serendipitous discovery of excess infrared emission from a single white dwarf PG 0010+280. At a temperature of 27,220 K and a cooling age of 16 Myr, it is the hottest and youngest white dwarf to display an excess at 3-8 $\mu$m. The infrared excess can be fit by either an opaque dust disk within the tidal radius of the white dwarf or a 1300 K blackbody, possibly from an irradiated substellar object or a re-heated giant planet. PG 0010+280 has two unique properties that are different from white dwarfs with a dust disk: (i) relatively low emission at 8 $\mu$m and (ii) non-detection of heavy elements in its atmosphere from high-resolution spectroscopic observations with Keck/HIRES. The origin of the infrared excess remains unclear.

Circumstellar discs in Galactic centre clusters: Disc-bearing B-type stars in the Quintuplet and Arches clusters

We investigate the circumstellar disc fraction as determined from L-band excess observations of the young, massive Arches and Quintuplet clusters residing in the central molecular zone of the Milky Way. The Quintuplet cluster was searched for L-band excess sources for the first time. We find a total of 26 excess sources in the Quintuplet cluster and 21 in the Arches cluster, of which 13 are new detections. With the aid of proper motion membership samples, the disc fraction of the Quintuplet cluster was derived for the first time to be 4.0 +/- 0.7%. There is no evidence for a radially varying disc fraction in this cluster. In the case of the Arches cluster, a disc fraction of 9.2 +/- 1.2% approximately out to the cluster's predicted tidal radius, r < 1.5 pc, is observed. This excess fraction is consistent with our previously found disc fraction in the cluster in the radial range 0.3 < r < 0.8 pc. In both clusters, the host star mass range covers late A- to early B-type stars, 2 < M < 15 Msun, as derived from J-band photospheric magnitudes. We discuss the unexpected finding of dusty circumstellar discs in these UV intense environments in the context of primordial disc survival and formation scenarios of secondary discs. We consider the possibility that the L-band excess sources in the Arches and Quintuplet clusters could be the high-mass counterparts to T Tauri pre-transitional discs. As such a scenario requires a long pre-transitional disc lifetime in a UV intense environment, we suggest that mass transfer discs in binary systems are a likely formation mechanism for the B-star discs observed in these starburst clusters.

STELLAR SUBSTRUCTURES AROUND THE HERCULES DWARF SPHEROIDAL GALAXY

We present deep $g,i$-band DECam stellar photometry of the Hercules Milky Way satellite galaxy, and its surrounding field, out to a radial distance of 5.4 times the tidal radius. We have identified nine extended stellar substructures associated with the dwarf; preferentially distributed along the major axis of the galaxy. Two significant over-densities lie outside the 95\% confidence band for the likely orbital path of the galaxy and appear to be free-floating tidal debris. We estimate the luminosity of the new stellar substructures, and find that approximately the same amount of stellar flux is lying in these extended structures as inside the main body of Hercules. We also analyse the distribution of candidate blue-horizontal-branch stars and find agreement with the alignment of the substructures at a confidence level greater than 98\%. Our analysis provides a quantitative demonstration that Hercules is a strongly tidally disrupted system, with noticeable stellar features at least 1.9 kpc away from the galaxy.

G2C2 – III. Structural parameters for Galactic globular clusters in SDSS passbands

We use our Galactic Globular Cluster Catalog (G2C2) photometry for 111 Galactic globular clusters (GC) in g and z, as well as r and i photometry for a subset of 60 GCs and u photometry for 22 GCs, to determine the structural parameters assuming King (1962) models. In general, the resulting core radii are in good comparison with the current literature values. However, our half-light radii are slightly lower than the literature. The concentrations (and therefore also the tidal radii) are poorly constrained mostly because of the limited radial extent of our imaging. Therefore, we extensively discuss the effects of a limited field-of-view on the derived parameters using mosaicked SDSS data, which do not suffer from this restriction. We also illustrate how red giant branch (RGB) stars in cluster cores can stochastically induce artificial peaks in the surface brightness profiles. The issues related to these bright stars are scrutinised based on both our photometry and simulated clusters. We also examine colour gradients and find that the strongest central colour gradients are caused by central RGB stars and thus not representative for the cluster light or colour distribution. We recover the known relation between the half-light radius and the Galactocentric distance in the g-band, but find a lower slope for redder filters. We did not find a correlation between the scatter on this relation and other cluster properties. We find tentative evidence for a correlation between the half-light radii and the [Fe/H], with metal-poor GCs being larger than metal-rich GCs. However, we conclude that this trend is caused by the position of the clusters in the Galaxy, with metal-rich clusters being more centrally located.

GENERAL RELATIVISTIC HYDRODYNAMIC SIMULATION OF ACCRETION FLOW FROM A STELLAR TIDAL DISRUPTION

We study how the matter dispersed when a supermassive black hole tidally disrupts a star joins an accretion flow. Combining a relativistic hydrodynamic simulation of the stellar disruption with a relativistic hydrodynamics simulation of the tidal debris motion, we track such a system until ~80% of the stellar mass bound to the black hole has settled into an accretion flow. Shocks near the stellar pericenter and also near the apocenter of the most tightly-bound debris dissipate orbital energy, but only enough to make the characteristic radius comparable to the semi-major axis of the most-bound material, not the tidal radius as previously thought. The outer shocks are caused by post-Newtonian effects, both on the stellar orbit during its disruption and on the tidal forces. Accumulation of mass into the accretion flow is non-monotonic and slow, requiring ~3--10x the orbital period of the most tightly-bound tidal streams, while the inflow time for most of the mass may be comparable to or longer than the mass accumulation time. Deflection by shocks does, however, remove enough angular momentum and energy from some mass for it to move inward even before most of the mass is accumulated into the accretion flow. Although the accretion rate rises sharply and then decays roughly as a power-law, its maximum is ~0.1x the previous expectation, and the duration of the peak is ~5x longer than previously predicted. The geometric mean of the black hole mass and stellar mass inferred from a measured event timescale is therefore ~0.2x the value given by classical theory.

HUBBLE SPACE TELESCOPEPROPER MOTIONS ALONG THE SAGITTARIUS STREAM. I. OBSERVATIONS AND RESULTS FOR STARS IN FOUR FIELDS

We present a multi-epoch Hubble Space Telescope (HST) study of stellar proper motions (PMs) for four fields spanning 200 degrees along the Sagittarius (Sgr) stream: one trailing arm field, one field near the Sgr dwarf spheroidal tidal radius, and two leading arm fields. We determine absolute PMs of dozens of individual stars per field, using established techniques that use distant background galaxies as stationary reference frame. Stream stars are identified based on combined color-magnitude diagram and PM information. The results are broadly consistent with the few existing PM measurements for the Sgr galaxy and the trailing arm. However, our new results provide the highest PM accuracy for the stream to date, the first PM measurements for the leading arm, and the first PM measurements for individual stream stars; we also serendipitously determine the PM of the globular cluster NGC~6652. In the trailing-arm field, the individual PMs allow us to kinematically separate trailing-arm stars from leading-arm stars that are 360 degrees further ahead in their orbit. Also, in three of our fields we find indications that two distinct kinematical components may exist within the same arm and wrap of the stream. Qualitative comparison of the HST data to the predictions of the Law & Majewski and Penarrubia et al. N-body models show that the PM measurements closely follow the predicted trend with Sgr longitude. This provides a successful consistency check on the PM measurements, as well as on these N-body approaches (which were not tailored to fit any PM data).

Evidence of tidal distortions and mass-loss from the old open cluster NGC 6791

We present the first evidence of clear signatures of tidal distortions in the density distribution of the fascinating open cluster NGC 6791. We used deep and wide-field data obtained with the Canada-France-Hawaii-Telescope covering a 2x2 square degrees area around the cluster. The two-dimensional density map obtained with the optimal matched filter technique shows a clear elongation and an irregular distribution starting from ~300" from the cluster center. At larger distances, two tails extending in opposite directions beyond the tidal radius are also visible. These features are aligned to both the absolute proper motion and to the Galactic center directions. Moreover, other overdensities appear to be stretched in a direction perpendicular to the Galactic plane. Accordingly to the behaviour observed in the density map, we find that both the surface brightness and the star count density profiles reveal a departure from a King model starting from ~600" from the center. These observational evidence suggest that NGC 6791 is currently experiencing mass loss likely due to gravitational shocking and interactions with the tidal field. We use this evidence to argue that NGC 6791 should have lost a significant fraction of its original mass. A larger initial mass would in fact explain why the cluster survived so long. Using available recipes based on analytic studies and N-body simulations, we derived the expected mass loss due to stellar evolution and tidal interactions and estimated the initial cluster mass to be M_ini=(1.5-4) x 10^5 M_sun.

Gamma-ray emission from globular clusters 2MS-GC01, IC 1257, FSR 1735, NGC 5904 and 6656

Globular clusters (GCs) are the oldest stellar system in the Galaxy, in which the millisecond pulsars are widely believed to be the only steady {\gamma}-ray emitters. So far 9 {\gamma}-ray GCs have been identified and a few candidates such as 2MS-GC01 and IC 1257 have been suggested. In this work, after analyzing the publicly-available Fermi-LAT data we confirm the significant {\gamma}-ray emission from 2MS-GC01 and IC 1257 and report the discovery for {\gamma}-ray emission from NGC 5904 and NGC 6656 within their tidal radii. Also a strong evidence of significant {\gamma}-ray emission is found from FSR 1735. From the observed {\gamma}-ray luminosities, the numbers of MSPs that are expected to be present in these GCs are estimated.

SERENDIPITOUS DISCOVERY OF A DWARF NOVA IN THEKEPLERFIELD NEAR THE G DWARF KIC 5438845

The Kepler satellite provides a unique window into stellar temporal variability by observing a wide variety of stars with multi-year, near-continuous, high precision, optical photometric time series. While most Kepler targets are faint stars with poorly known physical properties, many unexpected discoveries should result from a long photometric survey of such a large area of sky. During our Kepler Guest Observer programs that monitored late-type stars for starspot and flaring variability, we discovered a previously unknown dwarf nova that lies within a few arcseconds of the mid-G dwarf star KIC 5438845. This dwarf nova underwent nine outbursts over a 4 year time span. The two largest outbursts lasted ~17–18 days and show strong modulations with a 110.8 minute period and a declining amplitude during the outburst decay phase. These properties are characteristic of an SU UMa-type cataclysmic variable. By analogy with other dwarf nova light curves, we associate the 110.8 minute (1.847 hr) period with the superhump period, close to but slightly longer than the orbital period of the binary. No precursor outbursts are seen before the super-outbursts and the overall super-outburst morphology corresponds to Osaki & Meyer Case B outbursts, which are initiated when the outer edge of the disk reaches the tidal truncation radius. Case B outbursts are rare within the Kepler light curves of dwarf novae. The dwarf nova is undergoing relatively slow mass transfer, as evidenced by the long intervals between outbursts, but the mass transfer rate appears to be steady, because the smaller normal outbursts show a strong correlation between the integrated outburst energy and the elapsed time since the previous outburst. At super-outburst maximum the system was at V ~ 18, but in quiescence it is fainter than V ~ 22, which will make any detailed quiescent follow-up of this system difficult.