arXiv subjects

A. Zezas

Publications and source records attributed to A. Zezas.

At least 37 records · Page 2Linked to original sources

arXiv2019

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 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 3$σ$ significance level (4$-$25 keV) and obtained upper limits on an additional 40 sources. We reached a 3$σ$ limiting luminosity in the 4$-$25 keV band of $\sim$ $10^{35}$ erg s$^{-1}$, 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 3$σ$ sample. We present the first detection of pulsations from a Be-XRB, SXP305 (CXO J005215.4$-$73191), with an X-ray pulse period of $305.69\pm0.16$ seconds and a likely orbital period of $\sim$1160-1180 days. Bright sources ($\gtrsim 5\times 10^{36}$ erg s$^{-1}$) in our sample have compact-object classifications consistent with their previously reported types in the literature. Lower luminosity sources ($\lesssim 5\times 10^{36}$ erg s$^{-1}$) 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.

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arXiv2019

Discrete star formation events in the central Bar of the Small Magellanic Cloud

We present the results of the photometric analysis of a large part of the main body of the Small Magellanic Cloud. Using the 6.5m Magellan Telescope at the Las Campanas Observatory in Chile, we have acquired deep B and I images in four fields (0.44 degree each in diameter), yielding accurate photometry for 1,068,893 stars down to 24th magnitude, with a spatial resolution of 0.20 arcsec per pixel. Colour-magnitude diagrams and (completeness corrected) luminosity functions have been constructed, yielding significant new results that indicate at least two discrete star formation events over a period from 2.7 to 4 Gyr ago. Also, we have derived star formation rates as a function of look back time and have found enhancements of SF between 4-6 Gyr and at younger ages.

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arXiv2019

A diagnostic tool for the identification of Supernova Remnants

We present new diagnostic tools for distinguishing supernova remnants (SNRs) from HII regions. Up to now, sources with flux ratio [S II]/H$\rmα$ higher than 0.4 have been considered as SNRs. Here, we present the combinations of three or two line ratios as more effective tools for the separation of these two kinds of nebulae, depicting them as 3D surfaces or 2D lines. The diagnostics are based on photoionization and shock excitation models (MAPPINGS III) analysed with Support Vector Machine (SVM) models for classification. The line-ratio combination that gives the most efficient diagnostic is: [O I]/H$\rmα$ - [O II]/H$\rmβ$ - [O III]/H$\rmβ$. This method gives $98.95\% $ completeness in the SNR selection and $1.20\%$ contamination. We also define the [O I]/H$\rmα$ SNR selection criterion and we measure its efficiency in comparison to other selection criteria.

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arXiv2019

Astro 2020 Science White Paper: Time Domain Studies of Neutron Star and Black Hole Populations: X-ray Identification of Compact Object Types

What are the most important conditions and processes governing the growth of stellar-origin compact objects? The identification of compact object type as either black hole (BH) or neutron star (NS) is fundamental to understanding their formation and evolution. To date, time-domain determination of compact object type remains a relatively untapped tool. Measurement of orbital periods, pulsations, and bursts will lead to a revolution in the study of the demographics of NS and BH populations, linking source phenomena to accretion and galaxy parameters (e.g., star formation, metallicity). To perform these measurements over sufficient parameter space, a combination of a wide-field (>5000 deg^2) transient X-ray monitor over a dynamic energy range (~1-100 keV) and an X-ray telescope for deep surveys with <5 arcsec PSF half-energy width (HEW) angular resolution are required. Synergy with multiwavelength data for characterizing the underlying stellar population will transform our understanding of the time domain properties of transient sources, helping to explain details of supernova explosions and gravitational wave event rates.

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arXiv2019

Search for AGN counterparts of unidentified Fermi-LAT sources with optical polarimetry: Demonstration of the technique

The third Fermi-LAT catalog (3FGL) presented the data of the first four years of observations from the Fermi Gamma-ray Space Telescope mission. There are 3034 sources, 1010 of which still remain unidentified. Identifying and classifying gamma-ray emitters is of high significance with regard to studying high-energy astrophysics. We demonstrate that optical polarimetry can be an advantageous and practical tool in the hunt for counterparts of the unidentified gamma-ray sources (UGSs). Using data from the RoboPol project, we validated that a significant fraction of active galactic nuclei (AGN) associated with 3FGL sources can be identified due to their high optical polarization exceeding that of the field stars. We performed an optical polarimetric survey within $3σ$ uncertainties of four unidentified 3FGL sources. We discovered a previously unknown extragalactic object within the positional uncertainty of 3FGL J0221.2+2518. We obtained its spectrum and measured a redshift of $z=0.0609\pm0.0004$. Using these measurements and archival data we demonstrate that this source is a candidate counterpart for 3FGL J0221.2+2518 and most probably is a composite object: a star-forming galaxy accompanied by AGN. We conclude that polarimetry can be a powerful asset in the search for AGN candidate counterparts for unidentified Fermi sources. Future extensive polarimetric surveys at high galactic latitudes (e.g., PASIPHAE) will allow the association of a significant fraction of currently unidentified gamma-ray sources.

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arXiv2018

Extragalactic Gamma-ray Background from Star-forming Galaxies: Will Empirical Scalings Suffice?

Despite the influx of unprecedented-quality data from the Fermi Gamma-Ray Space Telescope that have been collected over nine years of operation, the contribution of normal star-forming galaxies to the extragalactic gamma-ray background (EGRB) remains poorly constrained. Different estimates are discrepant both their underlying physical assumptions and their results. With several detections and many upper limits for the gamma-ray fluxes of nearby starforming galaxies now available, estimates that rely on empirical scalings between gamma-ray and longer-wavelength luminosities have become possible and increasingly popular. In this paper we examine factors that can bias such estimates, including: a) possible sources of nontrivial redshift dependence; b) dependence on the choice of star-formation tracer; c) uncertainties in the slope and normalisation of empirical scalings. We find that such biases can be significant, pointing towards the need for more sophisticated models for the star-forming galaxy contribution to the gamma-ray background, implementing more, and more confident, physics in their buildup. Finally, we show that there are large regions of acceptable parameter space in observational inputs that significantly overproduce the gamma-ray background, implying that the observed level of the background can yield significant constraints on models of the average cosmic gamma-ray emissivity associated with star formation.

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arXiv2018

Do sub-galactic regions follow the galaxy-wide X-ray scaling relations? The example of NGC 3310 and NGC 2276

We present results from Chandra observations of the X-ray starburst galaxies NGC 3310 and NGC 2276. We detect 27 discrete sources in NGC 3310, and 19 discrete sources in NGC 2276 with luminosities above $\mathrm{1.0\times 10^{38}\ erg\ s^{-1}}$. The majority of the sources have photon indices of 1.7-2.0, typical for X-ray binaries. Both galaxies have large numbers of ultra-luminous X-ray sources (ULXs; sources with $\mathrm{L(0.3-10.0\ keV)>10^{39}\ erg\ s^{-1}}$), 14 for NGC 3310 concentrated on the circumnuclear star-forming ring and north spiral arm and 11 for NGC 2276 with the brighter ones on the west side of the galaxy which is compressed due to harassment by the intra-group medium it is moving into. We find for both galaxies that the ULX-hosting areas are located above the general Lx-SFR scaling relations while other areas either follow or fall below the scaling relations. This indicates that sub-galactic regions follow the galaxy-wide scaling relations but with much larger scatter resulting from the age (and possibly metallicity) of their local stellar populations in agreement with recent theoretical and observational results. Such differences in age could be the origin of the scatter we observe in the low SFR regime in the Lx-SFR scaling relations.

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arXiv2018

The Star Formation Reference Survey III: A Multi-wavelength View of Star Formation in Nearby Galaxies

We present multi-wavelength global star formation rate (SFR) estimates for 326 galaxies from the Star Formation Reference Survey (SFRS) in order to determine the mutual scatter and range of validity of different indicators. The widely used empirical SFR recipes based on 1.4 GHz continuum, 8.0 $μ$m polycyclic aromatic hydrocarbons (PAH), and a combination of far-infrared (FIR) plus ultraviolet (UV) emission are mutually consistent with scatter of $\raise{-0.8ex}\stackrel{\textstyle <}{\sim }$0.3 dex. The scatter is even smaller, $\raise{-0.8ex}\stackrel{\textstyle <}{\sim }$0.24 dex, in the intermediate luminosity range 9.3 }5\times10^9$ L$_{\odot}$ or with implied far-UV extinction <1 mag, the UV spectral slope gives extinction corrections with 0.22~dex uncertainty.

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arXiv2018

The integrated properties of the CALIFA galaxies: Model-derived galaxy parameters and quenching of star formation

We present a study of the integrated properties of the 835 galaxies in the CALIFA survey. To derive the main physical parameters of the galaxies we have fitted their UV-to-IR spectral energy distributions (SED) with sets of theoretical models using CIGALE. We perform a comparison of the integrated galaxy parameters derived from multi-band SED fitting with those obtained from modelling the Integral Field Unit (IFU) spectra and show the clear advantage of using the SED-derived star formation rates (SFR). A detailed analysis of galaxies in the SFR/Mstar plane as a function of their properties reveals that quenching of star formation is caused by a combination of gas deficiency and the inefficiency of the existing gas to form new stars. Exploring the plausible mechanisms that could produce this effect, we find a strong correlation with galaxy morphology and the build-up of central bulge. On the other hand, the presence of AGN and/or a stellar bar, as well as the local environment have only temporal effects on the current star formation, a result also consistent with their model-derived star formation histories.

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arXiv2018

A long hard-X-ray look at the dual active galactic nuclei of M51 with NuSTAR

We present a broadband X-ray spectral analysis of the M51 system, including the dual active galactic nuclei (AGN) and several off-nuclear point sources. Using a deep observation by NuSTAR, new high-resolution coverage of M51b by Chandra, and the latest X-ray torus models, we measure the intrinsic X-ray luminosities of the AGN in these galaxies. The AGN of M51a is found to be Compton thick, and both AGN have very low accretion rates ($λ_{\rm Edd} <10^{-4}$). The latter is surprising considering that the galaxies of M51 are in the process of merging, which is generally predicted to enhance nuclear activity. We find that the covering factor of the obscuring material in M51a is $0.26 \pm 0.03$, consistent with the local AGN obscured fraction at $L_{\rm X}\sim 10^{40}$ erg s$^{-1}$. The substantial obscuring column does not support theories that the torus, presumed responsible for the obscuration, disappears at these low accretion luminosities. However, the obscuration may have resulted from the gas infall driven by the merger rather than the accretion process. We report on several extra-nuclear sources with $L_{\rm X}>10^{39}$ erg s$^{-1}$ and find that a spectral turnover is present below 10 keV in most such sources, in line with recent results on ultraluminous X-ray sources.

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arXiv2018

Young Accreting Compact Objects in M31: The Combined Power of NuSTAR, Chandra, and Hubble

We present 15 high mass X-ray binary (HMXB) candidates in the disk of M31 for which we are able to infer compact object type, spectral type of the donor star, and age using multiwavelength observations from NuSTAR, Chandra, and the Hubble Space Telescope (HST). The hard X-ray colors and luminosities from NuSTAR permit the tentative classification of accreting X-ray binary systems by compact object type, distinguishing black hole from neutron star systems. We find hard state black holes, pulsars, and non-magnetized neutron stars associated with optical point source counterparts with similar frequency. We also find nine non-magnetized neutron stars coincident with globular clusters and an equal number of pulsars with and without point source optical counterparts. We perform spectral energy distribution (SED) fitting for the most likely optical counterparts to the HMXB candidates, finding 7 likely high mass stars and 1 possible red Helium burning star. The remaining 7 HMXB optical counterparts have poor SED fits, so their companion stars remain unclassified. Using published star formation histories, we find that the majority of HMXB candidates --- X-ray sources with UV-bright point source optical counterpart candidates --- are found in regions with star formation bursts less than 50 Myr ago, with 3 associated with young stellar ages (<10 Myr). This is consistent with similar studies of HMXB populations in the Magellanic Clouds, M33, NGC 300, and NGC 2403.

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arXiv2018

Discovery of X-ray pulsations in the Be/X-ray binary IGR J06074+2205

IGR J06074+2205 is a poorly studied X-ray source with a Be star companion. It has been proposed to belong to the group of Be/X-ray binaries. In Be/X-ray binaries, accretion onto the neutron star occurs via the transfer of material from the Be star's circumstellar disk. Thus, in the absence of the disk, no X-ray should be detected. The main goal of this work is to study the quiescent X-ray emission of IGR J06074+2205 during a disk-loss episode. We show that at the time of the XMM-Newton observation the decretion disk around the Be star had vanished. Still, accretion appears as the source of energy that powers the high-energy radiation in IGR J06074+2205. We report the discovery of X-ray pulsations with a pulse period of 373.2 s and a pulse fraction of ~50%. The $0.4-12$ keV spectrum is well described by an absorbed power law and blackbody components with the best fitting parameters: $N_{\rm H}=(6.2\pm0.5) \times 10^{21}$ cm$^{-2}$, $kT_{\rm bb}=1.16\pm0.03$ keV, and $Γ=1.5\pm0.1$ The absorbed X-ray luminosity is $L_{\rm X}=1.4 \times 10^{34}$ erg s$^{-1}$ assuming a distance of 4.5 kpc. The detection of X-ray pulsations confirms the nature of IGR J06074+2205 as a Be/X-ray binary. We discuss various scenarios to explain the quiescent X-ray emission of this pulsar. We rule out cooling of the neutron star surface and magnetospheric emission and conclude that accretion is the most likely scenario. The origin of the accreted material remains an open question.

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arXiv2017

The Star Formation Reference Survey. II. Activity demographics and host-galaxy properties for Infrared-selected galaxies

We present activity demographics and host-galaxy properties of infrared-selected galaxies in the local Universe, using the representative Star Formation Reference Survey (SFRS). Our classification scheme is based on a combination of optical emission-line diagrams (BPT) and IR-color diagnostics. Using the weights assigned to the SFRS galaxies based on its parent sample, a far-infrared-selected sample comprises 71\% H\,\textsc{ii} galaxies, 13\% Seyferts, 3\% Transition Objects (TOs), and 13\% Low-Ionization Nuclear Emission-Line Regions (LINERs). For the SFRS H\,\textsc{ii} galaxies we derive nuclear star-formation rates and gas-phase metallicities. We measure host-galaxy metallicities for all galaxies with available long-slit spectroscopy and abundance gradients for a subset of 12 face-on galaxies. The majority of H\,\textsc{ii} galaxies show a narrow range of metallicities, close to solar, and flat metallicity profiles. Based on their host-galaxy and nuclear properties, the dominant ionizing source in the far-infrared selected TOs is star-forming activity. LINERs are found mostly in massive hosts (median of $10^{10.5}$ M$_{\odot} $), median $L(60μm) = 10^{9}$ L$_{\odot}$, median dust temperatures of $ F60/F100 = 0.36 $, and median $L_{\textrm{H}α}$ surface density of $ 10^{40.2} $ erg s$ ^{-1} $kpc$ ^{-2} $, indicating older stellar populations as their main ionizing source rather than AGN activity.

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arXiv2017

On the Spatially Resolved Star Formation History in M51 I: Hybrid UV+IR Star Formation Laws and IR Emission from Dust Heated by Old Stars

We present Lightning, a new spectral energy distribution (SED) fitting procedure, capable of quickly and reliably recovering star formation history (SFH) and extinction parameters. The SFH is modeled as discrete steps in time. In this work, we assumed lookback times of 0-10 Myr, 10-100 Myr, 0.1-1 Gyr, 1-5 Gyr, and 5-13.6 Gyr. Lightning consists of a fully vectorized inversion algorithm to determine SFH step intensities and combines this with a grid-based approach to determine three extinction parameters. We apply our procedure to the extensive FUV-to-FIR photometric data of M51, convolved to a common spatial resolution and pixel scale, and make the resulting maps publicly available. We recover, for M51a, a peak star formation rate (SFR) between 0.1 and 5 Gyr ago, with much lower star formation activity over the last 100 Myr. For M51b, we find a declining SFR toward the present day. In the outskirt regions of M51a, which includes regions between M51a and M51b, we recover a SFR peak between 0.1 and 1 Gyr ago, which corresponds to the effects of the interaction between M51a and M51b. We utilize our results to (1) illustrate how UV+IR hybrid SFR laws vary across M51, and (2) provide first-order estimates for how the IR luminosity per unit stellar mass varies as a function of the stellar age. From the latter result, we find that IR emission from dust heated by stars is not always associated with young stars, and that the IR emission from M51b is primarily powered by stars older than 5 Gyr.

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arXiv2017

On the Spatially Resolved Star-Formation History in M51 II: X-ray Binary Population Evolution

We present a new technique for empirically calibrating how the X-ray luminosity function (XLF) of X-ray binary (XRB) populations evolves following a star-formation event. We first utilize detailed stellar population synthesis modeling of far-UV to far-IR photometry of the nearby face-on spiral galaxy M51 to construct maps of the star-formation histories (SFHs) on subgalactic (~400 pc) scales. Next, we use the ~850 ks cumulative Chandra exposure of M51 to identify and isolate 2-7 keV detected point sources within the galaxy, and we use our SFH maps to recover the local properties of the stellar populations in which each X-ray source is located. We then divide the galaxy into various subregions based on their SFH properties (e.g., star-formation rate [SFR] per stellar mass [M*] and mass-weighted stellar age) and group the X-ray point sources according to the characteristics of the regions in which they are found. Finally, we construct and fit a parameterized XLF model that quantifies how the XLF shape and normalization evolves as a function of the XRB population age. Our best-fit model indicates the XRB XLF per unit stellar mass declines in normalization, by ~3-3.5 dex, and steepens in slope from ~10 Myr to ~10 Gyr. We find that our technique recovers results from past studies of how XRB XLFs and XRB luminosity scaling relations vary with age and provides a self-consistent picture for how the XRB XLF evolves with age.

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arXiv2017

Connecting traces of galaxy evolution: the missing core mass -- morphological fine structure relation

Deep exposure imaging of early-type galaxies (ETGs) are revealing the second-order complexity of these objects, which have been long considered uniform, dispersion-supported spheroidals. "Fine structure" features (e.g. ripples, plumes, tidal tails, rings) as well as depleted stellar cores (i.e. central light deficits) characterize a number of massive ETG galaxies, and can be interpreted as the result of galaxy-galaxy interactions. We discuss how the timescale for the evolution of cores and fine structures are comparable, and hence it is expected that they develop in parallel after the major interaction event which shaped the ETG. Using archival data, we compare the "depleted stellar mass" (i.e. the mass missing from the depleted stellar core) against the prominence of the fine structure features, and observe that they correlate inversely. This result confirms our expectation that, while the Super Massive Black Hole (SMBH) binary (constituted by the SMBHs of the merger progenitors) excavates the core via three-body interactions, the gravitational potential of the newborn galaxy relaxes, and the fine structures fade below detection levels. We expect the inverse correlation to hold at least within the first Gyr from the merger which created the SMBH binary; after then, the fine structure evolves independently.

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arXiv2017

Reconciling inverse-Compton Doppler factors with variability Doppler factors in blazar jets

Blazar population models have shown that the inverse-Compton and variability Doppler factor estimates yield consistent results at the population level for flat spectrum radio quasars (FSRQs). The two methods, however, are inconsistent when compared on a source-by-source basis. In this work, we attempt to understand the source of the discrepancy by tracing the potential sources of systematic and statistical error for the inverse-Compton Doppler factors. By eliminating these sources of error, we provide stronger constrains on the value of the Doppler factor in blazar jets. We re-estimate the inverse-Compton Doppler factor for 11 sources that meet certain criteria for their synchrotron peak frequency and the availability of Doppler factor estimates in the literature. We compare these estimates with the average of two different estimates of the variability Doppler factor obtained using various datasets and methodologies to identify any discrepancies and, in each case, trace their sources in the methodology or assumptions adopted. We identify three significant sources of error for the inverse-Compton Doppler factors: a) contamination of the X-ray flux by non-synchrotron self-Compton emission; b) radio observations at frequencies other than the synchrotron turnover frequency; c) non-simultaneity between radio and X-ray observations. We discuss key aspects in the correct application of the inverse-Compton method in light of these potential errors. We are able to constrain the Doppler factor of 3C273, 3C345, 3C454.3, PKS1510-089, and PKS1633+382 effectively, since all available estimates from both methods converge to the same values for these five sources.

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arXiv2017

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 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 Gamma ~ 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 Msun) 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 Msun) companion or a symbiotic X-ray binary. We discuss other possibilities in more detail.

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