We present spectroscopic evidence for a low-luminosity, low-excitation active galactic nucleus (AGN) in NGC 1042, powered by an intermediate-mass black hole. These findings are significant in that the AGN is coincident with a compact star cluster known to reside in the nucleus, thus providing an example where the two types of central mass concentration coexist. The existence of a central black hole is additionally remarkable in that NGC 1042 lacks a stellar bulge. Objects such as NGC 1042 may have an important role in testing theories for the genesis of massive black holes in galaxy nuclei, and the extent to which they are in symbiosis with the larger stellar host.
NGC 4395 is one of the least luminous and nearest known type 1 Seyfert galaxies, and it also lacks a bulge. We present an HST I-band image of its nuclear region and Keck high-resolution (8 km/s) echelle spectra containing the Ca II near-infrared triplet. In addition to the unresolved point source, there is a nuclear star cluster of size r ~ 3.9 pc; the upper limit on its velocity dispersion is only 30 km/s. We thus derive an upper limit of 6.2x10^6 solar masses for the mass of the compact nucleus. Based on the amount of spatially resolved light in the HST image, a sizable fraction of this is l…
We study the extensively monitored Seyfert galaxy NGC 5548 and compare its nuclear emission with models of accretion disk with accreting corona. To obtain the intrinsic nuclear spectra from the observed spectra we had to estimate and subtract the contribution from circumnuclear components such as stars, the Balmer continuum and blended FeII lines, and the FC2 extended, featureless continuum. The true nuclear spectra were compared with a two parameter model of the accreting disk with an accreting corona, described by the mass of the central black hole and viscosity. The model that best fitted t…
The spectra of 34 galaxies within 20 arcmins (\sim 100 kpc) of the lenticular galaxy NGC 5084 have been obtained using the FOCAP system on the Anglo-Australian 3.9m telescope. Nine objects are found with projected separations \lesssim 80 kpc and with radial velocities within \pm 630 km/s of the parent galaxy redshift. Using various techniques, their velocity differences and projected separations are used to estimate the mass of this S0 galaxy, which ranges from 6 \times 10^{12} M_{\odot} to 1 \times 10^{13} M_{\odot}. With such a mass, NGC 5084 is one of the most massive disk galaxy known, wit…
Close to its center, the bulgeless galaxy NGC 2139 hosts a star cluster that is younger and less massive than any actual nuclear star cluster (NC) studied so far. We have measured the H-alpha velocity field around the photometric center of this galaxy using the VLT ARGUS integral field unit and GIRAFFE spectrograph in order to constrain different proposed theories of NC formation. We observe that the best-fit kinematic center and the candidate NC appear to be separated by 2.8 arcsec (320 pc). Indeed, the kinematic center also is offset from the galaxy's photometric center and a possible bar or…
We have derived simplified equations for a freely expanding, pressure driven jet model as a function of jet power and applied it successfully to the radio cores in the black hole candidates GRS 1915+105, NGC 4258, and M81 which are observationally well defined systems, and to Sgr A*. By using equipartition assumptions, the model has virtually no free parameters and can explain all sources by just scaling the jet power. In GRS 1915+105 it also naturally explains the jet velocity and the radio time delay. The jet powers we derive for the radio cores of the first three sources are comparable to t…
We have studied an ultraluminous X-ray source (ULX) in NGC 4559 with XMM-Newton, and its peculiar star-forming environment with HST/WFPC2. The X-ray source is one of the brightest in its class (L_x ~ 2 x 10^{40} erg/s). Luminosity and timing arguments suggest a mass >~ 50 M_sun for the accreting black hole. The ULX is located near the rim of a young (age < 30 Myr), large (diameter ~ 700 pc) ring-like star forming complex possibly triggered by the impact of a dwarf satellite galaxy through the gas-rich outer disk of NGC 4559. We speculate that galaxy interactions (including the infall of high-v…
HST UV STIS imaging and spectroscopy of the low luminosity AGN (LLAGN) NGC 4303 have identified the previously detected UV-bright nucleus of this galaxy, as a compact, massive and luminous stellar cluster. The cluster with a size (FWHM) of 3.1 pc, and an ultraviolet luminosity log L (1500 A}(erg/s/A)= 38.33 is identified as a nuclear super star cluster (SSC) like those detected in the circumnuclear regions of spirals and starburst galaxies. The UV spectrum showing the characteristic broad P Cygni lines produced by the winds of massive young stars, is best fitted by the spectral energy distribu…
The nuclear gas kinematics probed by water vapor maser emission has shown that two nearby active galaxies, NGC 1068 and NGC 4258, have a supermassive object in their nuclei and their masses are nearly comparable; a few 10^7 solar masses. Nevertheless, the activity of the central engine of NGC 1068 is more powerful by two orders of magnitude than that of NGC 4258. Since it is generally considered that the huge luminosities of active galactic nuclei are attributed to the mass accretion onto a supermassive black hole, the above observational results suggest that the accretion rate in NGC 1068 is …
Spectral studies of a luminous X-ray source, NGC 253 X21, are presented based on archival Chandra and XMM-Newton data. The Chandra observation on 1999 December 16 detected the source at a bolometric luminosity of 0.3 x 10^39 erg/s (assuming isotropic emission), while an XMM-Newton observation on 2000 June 3 revealed a short-term source variation in the range of (0.6--1.3) x 10^39 erg/s. All spectra from these observations were successfully modeled by emission from an optically thick accretion disk. The average inner disk radius was kept constant at 63 x (cos 60/cos i)^(1/2) km, where i is the …
We present Keck LRIS spectroscopy of the black hole-hosting globular cluster RZ2109 in the Virgo elliptical galaxy NGC 4472. We find that this object has extraordinarily broad [OIII]5007 and [OIII]4959 emission lines, with velocity widths of approximately 2,000 k/ms. This result has significant implications for the nature of this accreting black-hole system and the mass of the globular cluster black hole. We show that the broad [OIII]5007 emission must arise from material driven at high velocity from the black hole system. This is because the volume available near the black hole is too small b…
X-ray bursts from bright sources in globular clusters of our galaxy show that at least 11 (of a total 13) of these sources are neutron stars. One of the low-luminosity X-ray sources in omega Cen is a neutron star accreting at a low rate. Together with the discoveries that M~15 contains two bright X-ray sources, and that 47 Tuc and NGC 6440 contain 2 and 5 low-luminosity X-ray sources with a neutron star, this indicates that the total number of binaries with neutron stars in globular clusters is higher than previously suspected. The discovery of very bright X-ray sources in globular clusters ne…
It was found that the advection-dominated accretion flow (ADAF)+thin disk model calculations can reproduce the observed spectral energy distributions (SED) of the two low luminosity AGN, provided they are accreting at ~0.01-0.03 Eddington rates and the thin disks are truncated to ADAFs at ~100 R_s (Schwarzschild radii) for M81 and NGC4579 (Quataert et al., 1999). However, the black hole masses adopted in their work are about one order of magnitude lower than recent measurements on these two sources. Adopting the well estimated black hole masses, our ADAF+thin disk model calculations can reprod…
Using the Giant Metrewave Radio Telescope (GMRT), we have discovered PSR J0514-4002A, a binary millisecond pulsar in the globular cluster NGC 1851. This pulsar has a rotational period of 4.99 ms and the most eccentric pulsar orbit yet found: e = 0.89. The orbital period is 18.8 days, and companion has a minimum mass of 0.9 M_sun; its nature is presently unclear. After accreting matter from a low-mass stellar companion, this pulsar exchanged it for its more massive present companion. This system presents the strongest evidence to date of such a process.
We present a Chandra X-ray observation of the X-ray bright E2 elliptical galaxy NGC4649. In addition to bright diffuse emission, we resolve 165 discrete sources, most of which are presumably low-mass X-ray binaries (LMXBs). As found in previous studies, the luminosity function of the resolved sources is well-fit by a broken power-law. In NGC4697 and NGC1553, the break luminosity was comparable to the Eddington luminosity of a $1.4 M_{\odot}$ neutron star. One possible interpretation of this result is that those sources with luminosities above the break are accreting black holes and those below…
The radio emission from supermassive black holes in nearby early-type galaxies can be used to test possible explanations for their low luminosities. We calculate the expected contribution from thermal synchrotron emission from hot accretion flows to the high radio frequency observations of NGC 2300, NGC 1399, NGC 4278 and NGC 4594. We find that, in all cases, and in accordance with our previous findings, hot flows accreting close to their Bondi rates overestimate significantly the observed fluxes. This implies that simply assuming a low radiative efficiency for the accreting gas is not enough …
Using the Chandra/ACIS-I detector, we have identified an X-ray source (CXOU 132619.7-472910.8) in the globular cluster NGC 5139 with a thermal spectrum identical to that observed from transiently accreting neutron stars in quiescence. The absence of intensity variability on timescales as short as 4 seconds (< 25% rms variability) and as long as 5 years (<50% variability) supports the identification of this source as a neutron star, most likely maintained at a high effective temperature (approximately 1e6 K) by transient accretion from a binary companion. The ability to spectrally identify quie…
We report on intermittent X-ray pulsations with a frequency of 442.36 Hz from the neutron-star X-ray binary SAX J1748.9--2021 in the globular cluster NGC 6440. The pulsations were seen during both 2001 and 2005 outbursts of the source, but only intermittently, appearing and disappearing on timescales of hundreds of seconds. We find a suggestive relation between the occurrence of type-I X-ray bursts and the appearance of the pulsations but the relation is not strict. This behavior is very similar to that of the intermittent accreting millisecond X-ray pulsar HETE J1900.1--2455. The reason for t…
Chandra observations of the core of the nearby starburst galaxy NGC 253 reveal a heavily absorbed source of hard X-rays embedded within the nuclear starburst region. The source has an unabsorbed, 2 to 10 keV luminosity of >=10^39 erg/s and photoionizes the surrounding gas. We observe this source through a dusty torus with a neutral absorbing column density of NH ~ 2x10^23 cm^-2. The torus is hundreds of pc across and collimates the starburst-driven nuclear outflow. We suggest that the ionizing source is an intermediate-mass black hole or a weakly accreting supermassive black hole, which may si…
We present multi-frequency simultaneous VLBA observations at 15, 22 and 43 GHz towards the nucleus of the nearby radio galaxy NGC 1052. These three continuum images reveal a double-sided jet structure, whose relative intensity ratios imply that the jet axis is oriented close to the sky plane. The steeply rising spectra at 15-43 GHz at the inner edges of the jets strongly suggest that synchrotron emission is absorbed by foreground thermal plasma. We detected H2O maser emission in the velocity range of 1550-1850 km/s, which is redshifted by 50-350 km/s with respect to the systemic velocity of NG…
We use a combination of new AAOmega multi-object wide-field spectroscopic observations and literature data to define 111 spectroscopically confirmed members of the massive NGC 5044 group with M_B <= -13.5 mag, providing a three-fold increase in group members over previous analyses of this group. We find the group to have a dynamical mass of 9.2 x 10^13 solar masses, placing it on the border between rich groups and poor clusters. However, comparison to the L_x-sigma and L_x-mass relations shows it more closely follows cluster scaling relations. Using a combination of crossing time, X-ray contou…
Presented here are the highlights from the deep Chandra observation of the elliptical galaxy NGC 3379. From the multi-epoch observation of this galaxy, 132 discrete X-ray sources have been detected within the region overlapped by all observations, 98 of which lie within the D25 ellipse of the galaxy. Of these 132 sources, 71 exhibit long-term variability, indicating that they are accreting compact objects. 11 of these sources have been identified as transient candidates, with a further 7 possible transients. In addition to this, from the joint Hubble/Chandra field of view, nine globular cluste…
We studied the colliding galaxy NGC 7714 with two XMM-Newton observations, six months apart. The galaxy contains two bright X-ray sources: we show that they have different physical nature. The off-nuclear source is an accreting compact object, one of the brightest ultraluminous X-ray sources (ULXs) found to date. It showed spectral and luminosity changes between the two observations, from a low/soft to a high/hard state; in the high state, it reached L_x ~ 6 x 10^{40} erg/s. Its lightcurve in the high state suggests variability on a ~ 2-hr timescale. Its peculiar location, where the tidal brid…
We present sensitive, high angular resolution 1.3 cm and 7 mm observations of the massive core NGC 6334 I(N), a region known to be undergoing massive star formation. At 1.3 cm we detect three sources, of which two had been previously detected at centimeter or millimeter wavelengths. At 7 mm we detect four sources. We suggest that three of these sources are subcomponents of the millimeter source SMA 1, that at these wavelengths is the dominant source in the region. The fourth 7 mm source appears to be associated with the relatively isolated source SMA 6. In all four 7 mm sources, the continuum …
We present 26 point-sources discovered with Chandra within 200" (~20kpc) of the center of the barred supergiant galaxy NGC 1365. The majority of these sources are high-mass X-ray binaries, containing a neutron star or a black hole accreting from a luminous companion at a sub-Eddington rate. Using repeat Chandra and XMM-Newton as well as optical observations, we discuss in detail the natures of two highly-variable ultraluminous X-ray sources (ULXs): NGC 1365 X1, one of the most luminous ULXs known since the ROSAT era, which is X-ray variable by a factor of 30, and NGC 1365 X2, a newly discovere…
The temporal and spectral properties of ultraluminous X-ray sources (ULXs, L>2x10^39 ergs/s) and bright X-ray sources (L>3x10^38 ergs/s) are examined and compared in two extremely different host environments: the old elliptical galaxy NGC 1399 and the young, starforming Antennae galaxies (NGC 4038/4039). ULXs in NGC 1399 show little variability on either long or short time scales. Only 1 of 8 ULXs and 10 of 63 bright sources in NGC 1399 are variable at a confidence level of 90%. On long timescales, the NGC 1399 sources are steadier than most Galactic black hole X-ray binaries, but similar to G…
We present results from HST/STIS long-slit spectroscopy of the gas motions in the nuclear region of the Seyfert 2 galaxy NGC 5252. The observed velocity field is consistent with gas in regular rotation with superposed localized patches of disturbed gas. The dynamics of the circumnuclear gas can be accurately reproduced by adding to the stellar mass component a compact dark mass of MBH = 0.95 (-0.45;+1.45) 10E9 M(sun), very likely a supermassive black hole. Contrarily to results obtained in similar studies rotational broadening is sufficient to reproduce also the behaviour of line widths. The M…
We report the detection of an absorbed central X-ray source and its strong, rapid variability in NGC4395, the least luminous Seyfert nucleus known. The X-ray source exhibits a number of flares with factors of 3-4 flux changes during a half day ASCA observation. Such X-ray variability is in constrast to the behaviour of other low luminosity active galaxies. It provides further support for an accreting black hole model rather than an extreme stellar process in accounting for the nuclear activity of NGC4395. The soft X-ray emission below 3 keV is strongly attenuated by absorption. The energy spec…
We present the discovery of a triplet of emission-line nuclei in the disturbed disk galaxy NGC 3341, based on archival data from the Sloan Digital Sky Survey and new observations from the Keck Observatory. This galaxy contains two offset nuclei within or projected against its disk, at projected distances of 5.1 and 8.4 kpc from its primary nucleus and at radial velocity separation of less than 200 km/s from the primary. These appear to be either dwarf ellipticals or the bulges of low-mass spirals whose disks have already been stripped off while merging into the primary galaxy. The inner offset…
The fluorescent iron K$α$ emission line profile provides an excellent probe of the innermost regions of active galactic nuclei. Fe XXV and Fe XXVI in diffuse plasma above the accretion disc can affect the X-ray spectrum by iron K$α$ resonant absorption. This in turn can influence the interpretation of the data and the estimation of the accretion disc and black hole parameters. We embark on a fully relativistic computation of this effect and calculate the iron line profile in the framework of a specific model in which rotating, highly ionized and resonantly-absorbing plasma occurs close to the …
We present a 60 ks Chandra ACIS-S observation of the isolated edge-on spiral NGC 3556, together with a multiwavelength analysis of various discrete X-ray sources and diffuse X-ray features. Among 33 discrete X-ray sources detected within the I_B = 25 mag per square arcsec isophote ellipse of the galaxy, we identify a candidate for the galactic nucleus, an ultraluminous X-ray source that might be an accreting intermediate-mass black hole, a possible X-ray binary with a radio counterpart, and two radio-bright giant HII regions. We detect large amounts of extraplanar diffuse X-ray emission, which…
An analysis of the X-ray variability of the low luminosity Seyfert nucleus NGC 4395, based on a long XMM-Newton observation, is presented. The power spectrum shows a clear break from a flat spectrum (alpha ~ 1) to a steeper spectrum (alpha ~ 2) at a frequency f_br = 0.5 - 3.0 * 10^-3 Hz, comparable to the highest characteristic frequency found previously in a Seyfert galaxy. This extends the measured M_BH-f_br values to lower M_BH than previous studies of Seyfert galaxies, and is consistent with an inverse scaling of variability frequency with black hole mass. The variations observed are among…
We describe a series of observations of NGC 1313 using the ROSAT HRI with a combined exposure time of 183.5 ksec. The observations span an interval between 1992 and 1998; the purpose of observations since 1994 was to monitor the X-ray flux of SN1978K, one of several luminous sources in the galaxy. No diffuse emission is detected in the galaxy to a level of ~1-2x10^37 ergs/s/arcmin^-2. A total of eight sources are detected in the summed image within the D_25 diameter of the galaxy. The luminosities of five of the eight range from \~6x10^37 to ~6x10^38 erg/s; these sources are most likely accret…
We report on the {\it Chandra} observations of the elliptical galaxy NGC 1399, concentrating on the X-ray sources identified with globular clusters (GCs). A large fraction of the 2-10 keV X-ray emission in the $8' \times 8'$ {\it Chandra} image is resolved into point sources with luminosities $\ge 5 \times 10^{37}$ \ergsec. These sources are most likely Low Mass X-ray Binaries (LMXBs). In a region imaged by {\it HST} about 70% of the X-ray sources are located within GCs. This association suggests that in giant elliptical galaxies luminous X-ray binaries preferentially form in GCs. Many of the …
We report on the XMM-Newton observation of the Be/neutron star X-ray binary system RX J0146.9+6121, a long period (~ 23 m) pulsar in the NGC 663 open cluster. The X-ray luminosity decreased by a factor two compared to the last observation carried out in 1998, reaching a level of ~ 10^34 erg/s, the lowest ever observed in this source. The spectral analysis reveals the presence of a significant excess at low energies over the main power-law spectral component. The soft excess can be described by a black-body spectrum with a temperature of about 1 keV and an emitting region with a radius of ~ 140…
After 3 years of quiescence, the globular cluster NGC 6440 exhibited a bright transient X-ray source turning on in August 2001, as noted with the RXTE All-Sky Monitor. We carried out a short target of opportunity observation with the Chandra X-ray Observatory and are able to associate the transient with the brightest of 24 X-ray sources detected during quiescence in July 2000 with Chandra. Furthermore, we securely identify the optical counterpart and determine that the 1998 X-ray outburst in NGC 6440 was from the same object. This is the first time that an optical counterpart to a transient in…
We present an analysis of the compact X-ray source population in the Seyfert~2 galaxy NGC 1068, imaged with Chandra. We find a total of 84 compact sources, of which 66 are projected onto the galactic disk of NGC 1068. Spectra of the brightest sources have been modeled with both multi-color disk blackbody and power-law models. The power-law model provides the better description of the spectrum for most of these sources. Five sources have 0.4-8 keV intrinsic luminosities greater than 10^{39} erg/s, assuming that their emission is isotropic and that they are associated with NGC 1068. We refer to …
[ABRIDGED] IC10 X-1 has recently been confirmed as a black hole (BH) + Wolf-Rayet (WR) X-ray binary, and NGC300 X-1 is thought to be. IC10 X-1 and NGC300 X-1 have similar X-ray properties, with luminosities ~10^38 erg/s, and orbital periods ~30 hr. We investigate similarities between these two, as well as differences between them and the known Galactic BH binary systems. We have examined XMM-Newton observations of NGC300 X-1 and IC10 X-1. We extracted lightcurves and spectra; power density spectra (PDS) were constructed from the lightcurves, and the X-ray emission spectra were modeled. Each so…
Chandra images of the elliptical galaxy NGC4261 have revealed an anisotropy in the spatial distribution of the off-nuclear X-ray sources, interpreted by Zezas et al. as evidence of an association with a young stellar population. Our independent analysis of archive X-ray (Chandra) and optical (INT and HST) observations confirms the anisotropy of the X-ray sources but conducts to a different interpretation for their origin. We find that nearly half of the X-ray sources are associated to a globular cluster (optical counterpart) suggesting that they are accreting low-mass X-ray binaries. Where col…
The edge-on spiral galaxy NGC 4565 was observed for $\sim$ 35 ks with ASCA in the 0.5-10 keV energy band. The X-ray emission was dominated by two bright sources, which can be identified with two point-like X-ray sources seen in the ROSAT HRI image. The observed 0.5-10 keV fluxes of these sources, $1.7 \times 10^{-12} erg/s cm^{-2}$ and $0.7 \times 10^{-12}erg/s cm^{-2}$, %$1.66 \times 10^{-12} erg/s cm^{-2}$ %$0.66 \times 10^{-12} erg/s cm^{-2}$ imply bolometric luminosities of $1.0\times 10^{40} erg/s$ and $4 \times 10^{39} erg/s$, respectively. They exhibit similar spectra, which can be expl…
We present the results from the Fourier Resolved Spectroscopy of archival XMM-Newton data of five AGN, namely, Mrk 766, NGC 3516, NGC 3783, NGC 4051 and Ark 564. This work supplements the earlier study of MCG-6-30-15 as well as those of several Galactic Black Hole Candidate sources. Our results exhibit much larger diversity than those of Galactic sources, a fact we attribute to the diversity of their masses. When we take into account this effect and combine our results with those from Cyg X-1, it seems reasonable to conclude that, at high frequencies, the slope of the Fourier-resolved spectra …
A Chandra observation of the intermediate luminosity (M_B=-20) elliptical galaxy NGC 3379 resolves 75% of the X-ray emission within the central 5kpc into point sources. Spectral analysis of the remaining unresolved emission within the central 770pc indicates that 90% of the emission probably arises from undetected point sources, while 10% arises from thermal emission from kT=0.6keV gas. Assuming a uniform density distribution in the central region of the galaxy gives a gas mass of 5.0e5 Mo.Such a small amount of gas can be supplied by stellar mass loss in only 1.0e7 years. Thus, the gas must b…
We have examined X-ray and optical observations of two ultra-luminous X-ray sources, X7 and X10 in NGC4559, using XMM-Newton, Chandra and HST. The UV/X-ray luminosity of X7 exceeds 2.1e40 erg/s in the XMM-Newton observation, and that of X10 is >1.3e40 erg/s. X7 has both thermal and power-law spectral components. The characteristic temperature of the thermal component is 0.12 keV. The power-law components in the two sources both have slopes with photon index \~2.1. A timing analysis of X7 indicates a break frequency at 28 mHz in the power spectrum, while that for X10 is consistent with an unbro…
Uncorrelated variability between direct X-ray continuum and reflection components (including the Fe line, if present) has been reported in many accreting black hole systems, challenging theoretical models of disc reflection. We explore a model based on gravitational light bending in the near vicinity of the central black hole in which the X-ray variability is controlled by the height of the primary source of X-rays above the accretion disc. The relationship between the direct power law component (PLC) and the reflection-dominated component (RDC) is not trivial and allows to identify three diff…
