Engineering topics

Zezas, A.

Publications and source records attributed to Zezas, A..

16 records

DOE OSTI2024

O Corona, where art thou? eROSITA’s view of UV-optical-IR variability-selected massive black holes in low-mass galaxies

Finding massive black holes (MBHs,M BH ≈ 10 4 –10 7 M ⊙ ) in the nuclei of low-mass galaxies $\left( {{M_*}\mathop {\mathop < \limits_ }\limits_ {{10}^{10}}{M_ \odot }} \right)$ is crucial to constrain seeding and growth of black holes over cosmic time, but it is particularly challenging due to their low accretion luminosities. Variability selection via long-term photometric ultraviolet, optical, or infrared (UVOIR) light curves has proved effective and identifies lower-Eddington ratios compared to broad and narrow optical spectral lines searches. In the inefficient accretion regime, X-ray and radio searches are effective, but they have been limited to small samples. Therefore, differences between selection techniques have remained uncertain. Here, we present the first large systematic investigation of the X-ray properties of a sample of known MBH candidates in dwarf galaxies. We extracted X-ray photometry and spectra of a sample of ~200 UVOIR variability-selected MBHs and significantly detected 17 of them in the deepest available SRG/eROSITA image, of which four are newly discovered X-ray sources and two are new secure MBHs. This implies that tens to hundreds of LSST MBHs will have SRG/eROSITA counterparts, depending on the seeding model adopted. Surprisingly, the stacked X-ray images of the many non-detected MBHs are incompatible with standard disk-corona relations, typical of active galactic nuclei, inferred from both the optical and radio fluxes. They are instead compatible with the X-ray emission predicted for normal galaxies. After careful consideration of potential biases, we identified that this X-ray weakness needs a physical origin. A possibility is that a canonical X-ray corona might be lacking in the majority of this population of UVOIR-variability selected low-mass galaxies or that unusual accretion modes and spectral energy distributions are in place for MBHs in dwarf galaxies. This result reveals the potential for severe biases in occupation fractions derived from data from only one waveband combined with SEDs and scaling relations of more massive black holes and galaxies.

Astronomy & Astrophysics↗

DOE OSTI2023

Star formation rate and stellar mass calibrations based on infrared photometry and their dependence on stellar population age and extinction

The stellar mass (M $\star$ ) and the star formation rate (SFR) are among the most important features that characterize galaxies. Measuring these fundamental properties accurately is critical for understanding the present state of galaxies, their history, and future evolution. Infrared (IR) photometry is widely used to measure the M $\star$ and SFR of galaxies because the near-IR traces the continuum emission of the majority of their stellar populations (SPs), and the mid/far-IR traces the dust emission powered by star-forming activity. This work explores the dependence of the IR emission of galaxies on their extinction, and the age of their SPs. It aims to provide accurate and precise IR-photometry SFR and M $\star$ calibrations that account for SP age and extinction while providing quantification of their scatter. We used the CIGALE spectral energy distribution (SED) fitting code to create model SEDs of galaxies with a wide range of star formation histories, dust content, and interstellar medium properties. We fit the relations between M $\star$ and SFR with IR and optical photometry of the model-galaxy SEDs with the Markov chain Monte Carlo (MCMC) method. As an independent confirmation of the MCMC fitting method, we performed a machine-learning random forest (RF) analysis on the same data set. The RF model yields similar results to the MCMC fits, thus validating the latter. This work provides calibrations for the SFR using a combination of the WISE bands 1 and 3, or the JWST NIR-F200W and MIRI-F2100W. It also provides mass-to-light ratio calibrations based on the WISE band-1, the JWST NIR-F200W, and the optical u - r or g - r colors. These calibrations account for the biases attributed to the SP age, while they are given in the form of extinction-dependent and extinction-independent relations. The proposed calibrations show robust estimations while minimizing the scatter and biases throughout a wide range of SFRs and stellar masses. The SFR calibration offers better results, especially in dust-free or passive galaxies where the contributions of old SPs or biases from the lack of dust are significant. Similarly, the M $\star$ calibration yields significantly better results for dusty and high-SFR galaxies where dust emission can otherwise bias the estimations.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2022

The eROSITA Final Equatorial-Depth Survey (eFEDS)

The eROSITA Final Equatorial Depth Survey (eFEDS), completed in survey mode during the calibration and performance verification phase of the eROSITA instrument on Spectrum Roentgen Gamma, delivers data at and beyond the final depth of the 4-yr eROSITA all-sky survey (eRASS:8), f 0.5–2 keV = 1.1 × 10 –14 erg s –1 cm –2 , over 140 deg 2 . It provides the first view of normal galaxy X-ray emission from X-ray binaries (XRBs) and the hot interstellar medium at the full depth of eRASS:8. We used the Heraklion Extragalactic Catalogue (HECATE) of galaxies to correlate with eFEDS X-ray sources and identify X-ray detected normal galaxies. This flux-limited X-ray survey is relatively free from selection effects and enables the study of integrated normal galaxy X-ray emission and its relation to galaxy parameters such as the stellar mass, star formation rate (SFR), and metallicity. We cross-correlated 32 646 eFEDS X-ray sources to 1181 HECATE normal galaxies and obtained 94 matches. We classified galaxies as star-forming, early-type, composite, and active galactic nuclei (AGN) using Sloan Digital Sky Survey (SDSS) and Six-degree Field (6dF) optical spectroscopy. The eFEDS field harbours 37 normal galaxies: 36 late-type (star-forming) galaxies and one early-type galaxy. There are 1.9 times as many normal galaxies as predicted by scaling relations via simulations, with an overabundance of late-type galaxies and a dearth of early-type galaxies. When compared with empirical relations, eFEDS dwarf galaxies with a high specific SFR have elevated L X /SFR at a fixed specific SFR and metallicity, indicating an increase in XRB emission due to low metallicity. We expect that eRASS:8 will detect 12 500 normal galaxies, the majority of which will be star-forming, with the caveat that there are unclassified sources in eFEDS and galaxy catalogue incompleteness issues that could increase the actual number of detected galaxies over these current estimates. eFEDS observations detected a rare population of galaxies – the metal-poor dwarf starbursts – that do not follow known scaling relations. eRASS is expected to discover significant numbers of these high-redshift analogues, which are important for studying the heating of the intergalactic medium at high redshift. Further investigation of the hot gas emission from normal galaxies and stochastic effects in the dwarf galaxy population are required to constrain their X-ray output.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2019

Neutron Stars and Black Holes in the Small Magellanic Cloud: The SMC NuSTAR Legacy Survey

We present a source catalog from the first deep hard X-ray (E > 10 keV) survey of the Small Magellanic Cloud (SMC), the Nuclear Spectroscopic Telescope Array (NuSTAR) Legacy Survey of the SMC. We observed three fields, for a total exposure time of 1 Ms, along the bar of this nearby star-forming galaxy. Fields were chosen for their young stellar and accreting binary populations. We detected 10 sources above a 3σ significance level (4–25 keV) and obtained upper limits on an additional 40 sources. We reached a 3σ limiting luminosity in the 4–25 keV band of ∼10(exp 35) erg s(exp −1), allowing us to probe fainter X-ray binary (XRB) populations than has been possible with other extragalactic NuSTAR surveys. We used hard X-ray colors and luminosities to constrain the compact-object type, exploiting the spectral differences between accreting black holes and neutron stars at E > 10 keV. Several of our sources demonstrate variability consistent with previously observed behavior. We confirmed pulsations for seven pulsars in our 3σ sample. We present the first detection of pulsations from a BeXRB, SXP 305 (CXO J005215.4–73191), with an X-ray pulse period of 305.69 ± 0.16 s and a likely orbital period of ∼1160–1180 days. Bright sources (≳5 × 10(exp 36) erg s(exp −1) ) in our sample have compact-object classifications consistent with their previously reported types in the literature. Lower-luminosity sources (≲5 × 10(exp 36) erg s(exp −1) ) have X-ray colors and luminosities consistent with multiple classifications. We raise questions about possible spectral differences at low luminosity between SMC pulsars and the Galactic pulsars used to create the diagnostic diagrams

Lazzarini, M.↗

NASA NTRS2018

Black Holes and Neutron Stars in Nearby Galaxies: Insights from NuSTAR

Nearby galaxy surveys have long classified X-ray binaries (XRBs) by the mass category of their donor stars (high-mass and low-mass). The NuSTAR (Nuclear Spectroscopic Telescope Array) observatory, which provides imaging data at E greater than 10 keV (kiloelectronvolts), has enabled the classification of extragalactic XRBs by their compact object type: neutron star (NS) or black hole (BH). We analyzed NuSTAR/Chandra/XMM-Newton observations from a NuSTAR-selected sample of 12 galaxies within 5 Mpc (megaparsecs) having stellar masses (M (sub asterisk)) 10 (sup 7-11) solar masses and star formation rates (SFRs) approximately 0.01-15 solar masses per year. We detected 128 NuSTAR sources to a sensitivity of 10 (sup 38) ergs per second. Using NuSTAR color-intensity and color-color diagrams we classified 43 of these sources as candidate NSs and 47 as candidate BHs. We further subdivide BHs by accretion states (soft, intermediate, and hard) and NSs by weak (Z/Atoll) and strong (accreting pulsar) magnetic field. Using eight normal (Milky Way-type) galaxies in the sample, we confirm the relation between the SFR and galaxy X-ray point source luminosity in the 4-25 and 12-25 keV energy bands. We also constrained galaxy X-ray point source luminosity using the relation L (sub X) equals M (sub asterisk) plus Beta SFR, finding agreement with previous work. The X-ray luminosity function (XLF) of all sources in the 4-25 and 12-25 keV energy bands matches the alpha = 1.6 slope for high-mass XRBs. We find that NS XLFs suggest a decline beginning at the Eddington limit for a 1.4 solar mass NS, whereas the BH fraction shows an approximate monotonic increase in the 4-25 and 12-25 keV energy bands. We calculate the overall ratio of BH to NS to be approximately 1 for 4-25 keV and approximately 2 for 12-25 keV.

Vulic, N.↗

NASA NTRS2017

Identification of the Hard X-Ray Source Dominating the E > 25 keV Emission of the Nearby Galaxy M31

We report the identification of a bright hard X-ray source dominating the M31 bulge above 25 kiloelectronvolts from a simultaneous NuSTAR-Swift observation. We find that this source is the counterpart to Swift J0042.6+4112, which was previously detected in the Swift BAT All-Sky Hard X-Ray Survey. This Swift BAT source had been suggested to be the combined emission from a number of point sources; our new observations have identified a single X-ray source from 0.5 to 50 kiloelectronvolts as the counterpart for the first time. In the 0.5-10 kiloelectronvolt band, the source had been classified as an X-ray Binary candidate in various Chandra and XMM-Newton studies; however, since it was not clearly associated with Swift J0042.6+4112, the previous E is less than 10 kiloelectronvolts observations did not generate much attention. This source has a spectrum with a soft X-ray excess (kT approximately equal to 0.2 kiloelectronvolts) plus a hard spectrum with a power law of gamma approximately equal to 1 and a cutoff around 15-20 kiloelectronvolts, typical of the spectral characteristics of accreting pulsars. Unfortunately, any potential pulsation was undetected in the NuSTAR data, possibly due to insufficient photon statistics. The existing deep HST (Hubble Space Telescope) images exclude high-mass (greater than 3 times the radius of the moon) donors at the location of this source. The best interpretation for the nature of this source is an X-ray pulsar with an intermediate-mass (less than 3 times the radius of the moon M) companion or a symbiotic X-ray binary. We discuss other possibilities in more detail.

galaxies: bulges â€“ galaxies: individual (M31)↗

NASA NTRS2017

Identification of the Hard X-Ray Source Dominating the E>25 keV Emission of the Nearby Galaxy M31

We report the identification of a bright hard X-ray source dominating the M31 bulge above 25 keV from a simultaneous NuSTAR-Swift observation. We find that this source is the counterpart to Swift J0042.6+4112, which was previously detected in the Swift BAT All-sky Hard X-ray Survey. This Swift BAT source had been suggested to be the combined emission from a number of point sources; our new observations have identified a single X-ray source from 0.5 to 50 keV as the counterpart for the first time. In the 0.5-10 keV band, the source had been classified as an X-ray Binary candidate in various Chandra and XMM-Newton studies; however, since it was not clearly associated with Swift J0042.6+4112, the previous E<10 keV observations did not generate much attention. This source has a spectrum with a soft X-ray excess (kT ∼ 0.2 keV) plus a hard spectrum with a power law of G ~ 1 and a cutoff around 15-20 keV, typical of the spectral characteristics of accreting pulsars. Unfortunately, any potential pulsation was undetected in the NuSTAR data, possibly due to insufficient photon statistics. The existing deep HST images exclude high-mass (>3 solar mass) donors at the location of this source. The best interpretation for the nature of this source is an X-ray pulsar with an intermediate-mass (<3 solar mass) companion or a symbiotic X-ray binary. We discuss other possibilities in more detail.

Yukita, M.↗

NASA NTRS2013

Energy Feedback from X-ray Binaries in the Early Universe

X-ray photons, because of their long mean-free paths, can easily escape the galactic environments where they are produced, and interact at long distances with the intergalactic medium, potentially having a significant contribution to the heating and reionization of the early universe. The two most important sources of X-ray photons in the universe are active galactic nuclei (AGNs) and X-ray binaries (XRBs). In this Letter we use results from detailed, large scale population synthesis simulations to study the energy feedback of XRBs, from the first galaxies (z (redshift) approximately equal to 20) until today.We estimate that X-ray emission from XRBs dominates over AGN at z (redshift) greater than or approximately equal to 6-8. The shape of the spectral energy distribution of the emission from XRBs shows little change with redshift, in contrast to its normalization which evolves by approximately 4 orders of magnitude, primarily due to the evolution of the cosmic star-formation rate. However, the metallicity and the mean stellar age of a given XRB population affect significantly its X-ray output. Specifically, the X-ray luminosity from high-mass XRBs per unit of star-formation rate varies an order of magnitude going from solar metallicity to less than 10% solar, and the X-ray luminosity from low-mass XRBs per unit of stellar mass peaks at an age of approximately 300 Myr (million years) and then decreases gradually at later times, showing little variation for mean stellar ages 3 Gyr (Giga years, or billion years). Finally, we provide analytical and tabulated prescriptions for the energy output of XRBs, that can be directly incorporated in cosmological simulations.

galaxies↗

NASA NTRS2013

Nustar and Chandra Insight into the Nature of the 3-40 Kev Nuclear Emission in Ngc 253

We present results from three nearly simultaneous Nuclear Spectroscopic Telescope Array (NuSTAR) and Chandra monitoring observations between 2012 September 2 and 2012 November 16 of the local star-forming galaxy NGC 253. The 3-40 kiloelectron volt intensity of the inner approximately 20 arcsec (approximately 400 parsec) nuclear region, as measured by NuSTAR, varied by a factor of approximately 2 across the three monitoring observations. The Chandra data reveal that the nuclear region contains three bright X-ray sources, including a luminous (L (sub 2-10 kiloelectron volt) approximately few × 10 (exp 39) erg per s) point source located approximately 1 arcsec from the dynamical center of the galaxy (within the sigma 3 positional uncertainty of the dynamical center); this source drives the overall variability of the nuclear region at energies greater than or approximately equal to 3 kiloelectron volts. We make use of the variability to measure the spectra of this single hard X-ray source when it was in bright states. The spectra are well described by an absorbed (power-law model spectral fit value, N(sub H), approximately equal to 1.6 x 10 (exp 23) per square centimeter) broken power-law model with spectral slopes and break energies that are typical of ultraluminous X-ray sources (ULXs), but not active galactic nuclei (AGNs). A previous Chandra observation in 2003 showed a hard X-ray point source of similar luminosity to the 2012 source that was also near the dynamical center (Phi is approximately equal to 0.4 arcsec); however, this source was offset from the 2012 source position by approximately 1 arcsec. We show that the probability of the 2003 and 2012 hard X-ray sources being unrelated is much greater than 99.99% based on the Chandra spatial localizations. Interestingly, the Chandra spectrum of the 2003 source (3-8 kiloelectron volts) is shallower in slope than that of the 2012 hard X-ray source. Its proximity to the dynamical center and harder Chandra spectrum indicate that the 2003 source is a better AGN candidate than any of the sources detected in our 2012 campaign; however, we were unable to rule out a ULX nature for this source. Future NuSTAR and Chandra monitoring would be well equipped to break the degeneracy between the AGN and ULX nature of the 2003 source, if again caught in a high state.

Chandra↗

NASA NTRS2013

Modeling X-Ray Binary Evolution in Normal Galaxies: Insights from SINGS

We present the largest-scale comparison to date between observed extragalactic X-ray binary (XRB) populations and theoretical models of their production. We construct observational X-ray luminosity functions (oXLFs) using Chandra observations of 12 late-type galaxies from the Spitzer Infrared Nearby Galaxy Survey (SINGS). For each galaxy, we obtain theoretical XLFs (tXLFs) by combining XRB synthetic models, constructed with the population synthesis code StarTrack, with observational star formation histories (SFHs). We identify highest-likelihood models both for individual galaxies and globally, averaged over the full galaxy sample. Individual tXLFs successfully reproduce about half of oXLFs, but for some galaxies we are unable to find underlying source populations, indicating that galaxy SFHs and metallicities are not well matched and/or XRB modeling requires calibration on larger observational samples. Given these limitations, we find that best models are consistent with a product of common envelope ejection efficiency and central donor concentration approx.. = 0.1, and a 50% uniform - 50% "twins" initial mass-ratio distribution. We present and discuss constituent subpopulations of tXLFs according to donor, accretor and stellar population characteristics. The galaxy-wide X-ray luminosity due to low-mass and high-mass XRBs, estimated via our best global model tXLF, follows the general trend expected from the L(sub X) - star formation rate and L(sub X) - stellar mass relations of Lehmer et al. Our best models are also in agreement with modeling of the evolution both of XRBs over cosmic time and of the galaxy X-ray luminosity with redshift.

Normal Galaxies↗

NASA NTRS2013

Modeling the Redshift Evolution of the Normal Galaxy X-Ray Luminosity Function

Emission from X-ray binaries (XRBs) is a major component of the total X-ray luminosity of normal galaxies, so X-ray studies of high-redshift galaxies allow us to probe the formation and evolution of XRBs on very long timescales (approximately 10 Gyr). In this paper, we present results from large-scale population synthesis models of binary populations in galaxies from z = 0 to approximately 20. We use as input into our modeling the Millennium II Cosmological Simulation and the updated semi-analytic galaxy catalog by Guo et al. to self-consistently account for the star formation history (SFH) and metallicity evolution of each galaxy. We run a grid of 192 models, varying all the parameters known from previous studies to affect the evolution of XRBs. We use our models and observationally derived prescriptions for hot gas emission to create theoretical galaxy X-ray luminosity functions (XLFs) for several redshift bins. Models with low common envelope efficiencies, a 50% twins mass ratio distribution, a steeper initial mass function exponent, and high stellar wind mass-loss rates best match observational results from Tzanavaris & Georgantopoulos, though they significantly underproduce bright early-type and very bright (L(sub x) greater than 10(exp 41)) late-type galaxies. These discrepancies are likely caused by uncertainties in hot gas emission and SFHs, active galactic nucleus contamination, and a lack of dynamically formed low-mass XRBs. In our highest likelihood models, we find that hot gas emission dominates the emission for most bright galaxies. We also find that the evolution of the normal galaxy X-ray luminosity density out to z = 4 is driven largely by XRBs in galaxies with X-ray luminosities between 10(exp 40) and 10(exp 41) erg s(exp −1).

Tremmel, M.↗

NASA NTRS2012

Probing the X-Ray Binary Populations of the Ring Galaxy NGC 1291

We present Chandra studies of the X-ray binary (XRB) populations in the bulge and ring regions of the ring galaxy NGC 1291. We detect 169 X-ray point sources in the galaxy, 75 in the bulge and 71 in the ring, utilizing the four available Chandra observations totaling an effective exposure of 179 ks. We report photometric properties of these sources in a point-source catalog. There are approx. 40% of the bulge sources and approx. 25% of the ring sources showing > 3(sigma) long-term variability in their X-ray count rate. The X-ray colors suggest that a significant fraction of the bulge (approx. 75%) and ring (approx. 65%) sources are likely low-mass X-ray binaries (LMXBs). The spectra of the nuclear source indicate that it is a low-luminosity AGN with moderate obscuration; spectral variability is observed between individual observations. We construct 0.3-8.0 keV X-ray luminosity functions (XLFs) for the bulge and ring XRB populations, taking into account the detection incompleteness and background AGN contamination. We reach 90% completeness limits of approx.1.5 x 10(exp 37) and approx. 2.2 x 10(exp 37) erg/s for the bulge and ring populations, respectively. Both XLFs can be fit with a broken power-law model, and the shapes are consistent with those expected for populations dominated by LMXBs. We perform detailed population synthesis modeling of the XRB populations in NGC 1291 , which suggests that the observed combined XLF is dominated by aD old LMXB population. We compare the bulge and ring XRB populations, and argue that the ring XRBs are associated with a younger stellar population than the bulge sources, based on the relative over-density of X-ray sources in the ring, the generally harder X-ray color of the ring sources, the overabundance of luminous sources in the combined XLF, and the flatter shape of the ring XLF.

Luo, B.↗

NASA NTRS2011

Investigating the Nuclear Activity of Barred Spiral Galaxies: The Case of NGC 1672

We have performed an X-ray study of the nearby barred spiral galaxy NGC 1672, primarily to ascertain the effect of the bar on its nuclear activity. We use both Chandra and XMM-Newton observations to investigate its X-ray properties, together with supporting high-resolution optical imaging data from the Hubble Space Telescope (HST) infrared imaging from the Spitzer Space Telescope, and Australia Telescope Compact Array ground-based radio data. We detect 28 X-ray sources within the D25 area of the galaxy; many are spatially correlated with star formation in the bar and spiral arms, and two are identified as background galaxies in the HST images. Nine of the X-ray sources are ultraluminous X-ray sources, with the three brightest (LX 5 * 10(exp 39) erg s(exp -1)) located at the ends of the bar. With the spatial resolution of Chandra, we are able to show for the first time that NGC 1672 possesses a hard (1.5) nuclear X-ray source with a 2-10 keV luminosity of 4 * 10(exp 38) erg s(exp -1). This is surrounded by an X-ray-bright circumnuclear star-forming ring, comprised of point sources and hot gas, which dominates the 2-10 keV emission in the central region of the galaxy. The spatially resolved multiwavelength photometry indicates that the nuclear source is a low-luminosity active galactic nucleus (LLAGN), but with star formation activity close to the central black hole. A high-resolution multiwavelength survey is required to fully assess the impact of both large-scale bars and smaller-scale phenomena such as nuclear bars, rings, and nuclear spirals on the fueling of LLAGN.

COLOR FIGURES↗

NASA NTRS2006

Rapid Compton-thick/Compton-thin Transitions in the Seyfert 2 Galaxy NGC 1365

We present multiple Chandra and XMM-Newton observations of the type 1.8 Seyfert galaxy NGC 1365, which shows the most dramatic X-ray spectral changes observed so far in an active galactic nucleus: the source switched from reflection-dominated to transmission-dominated and back in just 6 weeks. During this time the soft thermal component, arising from a approx. 1 kpc region around the center, remained constant. The reflection component is constant at all timescales, and its high flux relative to the primary component implies the presence of thick gas covering a large fraction of the solid angle. The presence of this gas, and the fast variability timescale, suggest that the Compton-thick to Compton-thin change is due to variation in the line-of-sight absorber rather than to extreme intrinsic emission variability. We discuss a structure of the circumuclear absorber/reflector that can explain the observed X-ray spectral and temporal properties.

Risaliti, G.↗

NASA NTRS2006

Highly Ionized Iron Absorption Lines from Outflowing Gases in the X-ray Spectrum of NGC 1365

We present the discovery of four absorption lines in the X-ray spectrum of the Seyfert galaxy NGC 1365, at energies between 6.7 and 8.3 keV. The lines are detected with high statistical confidence (from >20 sigma for the strongest to -4 sigma for the weakest) in two XMM-Newton observations 60 ks long. We also detect the same lines, with a lower signal-to-noise ratio (but still >2 sigma for each line), in two previous shorter (-10 ks) XMM-Newton observations. The spectral analysis identifies these features as Fe XXV and Fe XXVI Kalpha and Kbeta lines, outflowing with velocities varying between -1000 and -5000 km/s among the observations. These are the highest quality detections of such lines so far. The high equivalent widths [EW (Kalpha) approximately 100 eV] and the Kalpha/Kbeta ratios imply that the lines are due to absorption of the AGN continuum by a highly ionized gas with column density NH-5?1023 cm(exp -2) at a distance of -(50-100)RS from the continuum source.

Risaliti, G.↗

NASA NTRS2003

Chandra Observations of NGC 4698: A Seyfert 2 Galaxy with No Absorption

We present Chandra ACIS-S observations of the enigmatic Seyfert 2 galaxy NGC 4698. This object, together with several other bona fide Seyfert 2 galaxies, shows no absorption in the low spatial resolution ASCA data, in contrast to the standard unification models. Our Chandra observations of NGC 4698 probe directly the nucleus, allowing us to check whether nearby sources contaminate the ASCA spectrum. Indeed, the Chandra observations show that the ASCA spectrum is dominated by two nearby AGNs. The X-ray flux of NGC 4698 is dominated by a nuclear source with luminosity L(sub 0.3-8 keV) approximately 10(exp 39) ergs per second, coincident with the radio nucleus. Its spectrum is well represented by a power law, GAMMA approximately equal to 2.2, obscured by a small column density of 5 x 10(exp 20) per square centimeter, suggesting that NGC 4698 is an atypical Seyfert galaxy. On the basis of its low luminosity, we then interpret NGC 4698 as a Seyfert galaxy that lacks a broad-line region.

Georgantopoulos, I.↗