Publication Search Results

ID: 159350
Type: article
Authors: Lazzarini, Margaret; Williams, Benjamin F.; Durbin, Meredith; Dalcanton, Julianne; Antoniou, Vallia; Binder, Breanna A.; Eracleous, Michael; Plucinsky, Paul P.; Sasaki, Manami; Vulic, Neven
Abstract: We present our analysis of high-quality high-mass X-ray binary (HMXB) candidates in M31 selected from point-source optical counterpart candidates from the Chandra-PHAT survey catalog. We fit the spectral energy distributions of optical counterpart candidates using the Bayesian Extinction and Stellar Tool. We used the best-fit luminosity, effective temperature, radius, and dust reddening for the companion stars in combination with the local star formation history, dust maps of M31, published X-ray spectral fits from XMM-Newton observations, IR colors, and Chandra X-ray hardness ratios to determine our best sample of HMXB candidates. The age distribution of the HMXB sample appears peaked between 10 and 50 Myr, consistent with findings in other nearby galaxies. Using the age distribution and mean star formation rate (SFR), we find that 80-136 HMXBs were produced per unit of SFR over the last 50 Myr and 89-163 HMXBs were produced per unit of SFR over the last 80 Myr, if we expand the assumed age limit beyond the lifetimes of single massive stars. We also calculate the HMXB production rate (HMXBs/M_⊙) over time, which ranges from 7 × 10^-7 to 4 × 10^-6 HMXBs/M_⊙ over the last 80 Myr, in agreement with both theoretical predictions and measured production rates in other galaxies.

ID: 154539
Type: article
Authors: Antoniou, Vallia; Zezas, Andreas; Drake, Jeremy J.; Badenes, Carles; Haberl, Frank; Wright, Nicholas J.; Hong, Jaesub; Di Stefano, Rosanne; Gaetz, Terrance J.; Long, Knox S.; Plucinsky, Paul P.; Sasaki, Manami; Williams, Benjamin F.; Winkler, P. Frank; SMC XVP collaboration
Abstract: We have compiled the most complete census of high-mass X-ray binaries (HMXBs) in the Small Magellanic Cloud with the aim to investigate the formation efficiency of young accreting binaries in its low-metallicity environment. In total, we use 123 X-ray sources with detections in our Chandra X-ray Visionary Program (XVP), supplemented by 14 additional (likely and confirmed) HMXBs identified by Haberl & Sturm that fall within the XVP area, but are neither detected in our survey (nine sources) nor matched with any of the 127 sources identified in the XVP data (five sources). Specifically, we examine the number ratio of the HMXBs [N(HMXBs)] to (a) the number of OB stars, (b) the local star formation rate (SFR), and (c) the stellar mass produced during the specific star formation burst, all as a function of the age of their parent stellar populations. Each of these indicators serves a different role, but in all cases we find that the HMXB formation efficiency increases as a function of time (following a burst of star formation) up to ∼40─60 Myr, and then gradually decreases. The formation efficiency peaks at ∼30─40 Myr with average rates of {\text{}}{{N}}({HMXB})/{SFR}={339}_-83⁺⁷⁸ {({M}_ȯ /{yr})}^-1, and N(HMXB)/M \star =({8.74}_-0.92^+1.0)× {10}^-6 {M}_ȯ ^-1, in good agreement with previous estimates of the average formation efficiency in the broad ∼20─60 Myr age range.

ID: 150497
Type: article
Authors: Guarcello, M. G.; Micela, G.; Sciortino, S.; López-Santiago, J.; Argiroffi, C.; Reale, F.; Flaccomio, E.; Alvarado-Gómez, Julian D.; Antoniou, Vallia; Drake, Jeremy J.; Pillitteri, Ignazio; Rebull, L. M.; Stauffer, J.
Abstract: Context. Flares are powerful events ignited by a sudden release of magnetic energy which triggers a cascade of interconnected phenomena, each resulting in emission in different electromagnetic bands. In fact, in the Sun flares are observed across the whole electromagnetic spectrum. Multi-band observations of stellar flares are instead rare. This limits our ability to extend what we learn from solar flares to the case of flares occurring in stars with different properties. Aims: With the aim of studying flares in the 125-Myr-old stars in the Pleiades observed simultaneously in optical and X-ray light, we obtained new XMM-Newton observations of this cluster during the observations of Kepler K2 Campaign 4. The objective of this paper is to characterize the most powerful flares observed in both bands and to constrain the energy released in the optical and X-ray, the geometry of the loops, and their time evolution. We also aim to compare our results to existing studies of flares occurring in the Sun and stars at different ages. Methods: We selected bright X-ray/optical flares that occurred in 12 known members of the Pleiades from their K2 and XMM-Newton light curves. The sample includes ten K-M stars, one F9 star, and one G8 star. Flare average properties were obtained from integrated analysis of the light curves during the flares. The time evolution of the plasma in the magnetic loops is constrained with time-resolved X-ray spectral analysis. Results: Most of the flares studied in this work emitted more energy in optical than in X-rays, as in most solar flares, even if the Pleiades flares output a larger fraction of their total energy in X-rays than typical solar flares do. Additionally, the energy budget in the two bands is weakly correlated. We also found comparable flare duration in optical and X-rays and observed that rapidly rotating stars (e.g., with rotation period shorter than 0.5 days) preferentially host short flares. We estimated the slope of the cooling path of the flares in the log(EM)-vs.-log(T) plane. The values we obtained are affected by large uncertainties, but their nominal values suggest that the flares analyzed in this paper are mainly due to single loops with no sustained heating occurring during the cooling phase. We also observed and analyzed oscillations with a period of 500 s during one of the flares. Conclusions: The flares observed in the Pleiades can be classified as "superflares" based on their energy budget in the optical, and share some of the properties of the flares observed in the Sun, despite being more energetic. For instance, as in most solar flares, more energy is typically released in the optical than in X-rays and the duration of the flares in the two bands is correlated. We have attempted a comparison between the X-ray flares observed in the Pleiades and those observed in clusters with different ages, but to firmly address any evolutionary pattern of flare characteristics, similar and uniform multi-wavelength analyses on more complete samples are necessary.

ID: 154709
Type: article
Authors: Lazzarini, M.; Williams, B. F.; Hornschemeier, A. E.; Antoniou, Vallia; Vasilopoulos, G.; Haberl, F.; Vulic, N.; Yukita, M.; Zezas, Andreas; Bodaghee, A.; Lehmer, B. D.; Maccarone, T. J.; Ptak, A.; Wik, D.; Fornasini, Francesca M.; Hong, Jaesub; Kennea, J. A.; Tomsick, J. A.; Venters, T.; Udalski, A.; Cassity, A.
Abstract: 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 ̃10³⁵ 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 3σ sample. We present the first detection of pulsations from a Be-XRB, 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 × 10³⁶ erg s^-1) in our sample have compact-object classifications consistent with their previously reported types in the literature. Lower-luminosity sources (≲5 × 10³⁶ 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.

ID: 154599
Type: article
Authors: Strantzalis, A.; Hatzidimitriou, D.; Zezas, A.; Antoniou, Vallia; Lianou, S.; Tsilia, S.
Abstract: 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 deg each in diameter), yielding accurate photometry for 1068 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 and 6 Gyr and at younger ages.

ID: 150223
Type: article
Authors: West, Lacey A.; Lehmer, Bret D.; Wik, Daniel; Yang, Jun; Walton, Dominic J.; Antoniou, Vallia; Haberl, Frank; Hornschemeier, Ann; Maccarone, Thomas J.; Plucinsky, Paul P.; Ptak, Andrew; Williams, Benjamin F.; Vulic, Neven; Yukita, Mihoko; Zezas, Andreas
Abstract: We present nearly simultaneous NuSTAR and XMM-Newton observations of the nearby (832 kpc) ultraluminous X-ray source (ULX) M33 X-8. M33 X-8 has a 0.3–10 keV luminosity of L _X ≈ 1.4 × 10³⁹ erg s^‑1, near the boundary of the “ultraluminous” classification, making it an important source for understanding the link between typical Galactic X-ray binaries and ULXs. Past studies have shown that the 0.3–10 keV spectrum of X-8 can be characterized using an advection-dominated accretion disk model. We find that when fitting to our NuSTAR and XMM-Newton observations, an additional high-energy (≳10 keV) Comptonization component is required, which allows us to rule out single advection-dominated disk and classical sub-Eddington models. With our new constraints, we analyze XMM-Newton data taken over the last 17 yr to show that small (≈30%) variations in the 0.3–10 keV flux of M33 X-8 result in spectral changes similar to those observed for other ULXs. The two most likely phenomenological scenarios suggested by the data are degenerate in terms of constraining the nature of the accreting compact object (i.e., black hole versus neutron star). We further present a search for pulsations using our suite of data; however, no clear pulsations are detected. Future observations designed to observe M33 X-8 at different flux levels across the full 0.3–30 keV range would significantly improve our constraints on the nature of this important source.

ID: 144706
Type: article
Authors: Hong, Jaesub; Antoniou, Vallia; Zezas, Andreas; Haberl, Frank; Sasaki, Manami; Drake, Jeremy J.; Plucinsky, Paul P.; Laycock, Silas
Abstract: We report the timing analysis results of X-ray pulsars from a recent deep Chandra survey of the Small Magellanic Cloud (SMC). We analyzed a total exposure of 1.4 Ms from 31 observations over a 1.2 deg² region in the SMC under a Chandra X-ray Visionary Program. Using the Lomb-Scargle and epoch-folding techniques, we detected periodic modulations from 20 pulsars and a new candidate pulsar. The survey also covered 11 other pulsars with no clear sign of periodic modulation. The 0.5-8 keV X-ray luminosity (L _X ) of the pulsars ranges from 10³⁴ to 10³⁷ erg s^-1 at 60 kpc. All of the Chandra sources with L _X ? 4 × 10³⁵ erg s^-1 exhibit X-ray pulsations. The X-ray spectra of the SMC pulsars (and high-mass X-ray binaries) are in general harder than those of the SMC field population. All but SXP 8.02 can be fitted by an absorbed power-law model with a photon index of ? ? 1.5. The X-ray spectrum of the known magnetar SXP 8.02 is better fitted with a two-temperature blackbody model. Newly measured pulsation periods of SXP 51.0, SXP 214, and SXP 701, are significantly different from the previous XMM-Newton and RXTE measurements. This survey provides a rich data set for energy-dependent pulse profile modeling. Six pulsars show an almost eclipse-like dip in the pulse profile. Phase-resolved spectral analysis reveals diverse spectral variations during pulsation cycles: e.g., for an absorbed power-law model, some exhibit an (anti)-correlation between absorption and X-ray flux, while others show more intrinsic spectral variation (I.e., changes in photon indices).

ID: 140229
Type: article
Authors: Hong, Jaesub; Antoniou, Vallia; Zezas, Andreas; Haberl, Frank; Drake, Jeremy J.; Plucinsky, Paul P.; Gaetz, Terrance; Sasaki, Manami; Williams, Benjamin; Long, Knox S.; Blair, William P.; Winkler, P. Frank; Wright, Nicholas J.; Laycock, Silas; Udalski, Andrzej
Abstract: Located in the Small Magellanic Cloud (SMC), SXP 214 is an X-ray pulsar in a high mass X-ray binary system with a Be-star companion. A recent survey of the SMC under a Chandra X-ray Visionary program found that the source was in a transition when the X-ray flux was on a steady rise. The Lomb-Scargle periodogram revealed a pulse period of 211.49 ± 0.42 s, which is significantly (>5?) shorter than the previous measurements made with XMM-Newton and RXTE. This implies that the system has gone through sudden spin-up episodes recently. The pulse profile shows a sharp eclipse-like feature with a modulation amplitude of >95%. The linear rise of the observed X-ray luminosity from ?2× to 7× {10}³⁵ erg s^-1 is correlated with a steady softening of the X-ray spectrum, which can be described by the changes in the local absorption from N _H ~ 10²⁴ to ?10²⁰ cm^-2 for an absorbed power-law model. The soft X-ray emission below 2 keV was absent in the early part of the observation when only the pulsating hard X-ray component was observed, whereas at later times, both soft and hard X-ray components were observed to be pulsating. A likely explanation is that the neutron star was initially hidden in the circumstellar disk of the companion, and later came out of the disk with the accreted material that continued fueling the observed pulsation.

ID: 120964
Type: article
Authors: Maccarone, Thomas J.; Lehmer, Bret D.; Leyder, J. C.; Antoniou, Vallia; Hornschemeier, Ann; Ptak, Andrew; Wik, Daniel; Zezas, Andreas
Abstract: We have discovered a persistent, but highly variable X-ray source in the nearby starburst galaxy NGC 253. The source varies at the level of a factor of about 5 in count rate on time-scales of a few hours. Two long observations of the source with Chandra and XMM-Newton show suggestive evidence for the source having a period of about 14-15 hours, but the time sampling in existing data is insufficient to allow a firm determination that the source is periodic. Given the amplitude of variation and the location in a nuclear starburst, the source is likely to be a Wolf-Rayet X-ray binary, with the tentative period being the orbital period of the system. In light of the fact that we have demonstrated that careful examination of the variability of moderately bright X-ray sources in nearby galaxies can turn up candidate Wolf-Rayet X-ray binaries, we discuss the implications of Wolf-Rayet X-ray binaries for predictions of the gravitational wave source event rate, and, potentially, interpretations of the events.