Abstract
We investigate the uncertainties of fitted X-ray model parameters and fluxes for relatively faint Chandra ACIS-I source spectra. Monte Carlo (MC) simulations are employed to construct a ...large set of 150,000 fake X-ray spectra in the low photon count statistics regime (from 10 to 350 net counts) using the XSPEC spectral model-fitting package. The simulations employed both absorbed thermal (APEC) and nonthermal (power-law) models, in concert with the Chandra ACIS-I instrument response and interstellar absorption. Simulated X-ray spectra were fit assuming a wide set of different input parameters and
C
-statistic minimization criteria to avoid numerical artifacts in the accepted solutions. Results provide an error estimate for each parameter (absorption,
N
H
, plasma temperature,
kT
, or power-law slope, Γ, and flux) and for different background contamination levels. The distributions of these errors are studied as a function of the 1
σ
quantiles, and we show how these correlate with different model parameters, net counts in the spectra, and relative background level. Maps of uncertainty in terms of the 1
σ
quantiles for parameters and flux are computed as a function of spectrum net counts. We find very good agreement between our estimated X-ray spectral parameter and flux uncertainties and those recovered from spectral fitting for a subset of the X-ray sources detected in the Chandra Cygnus OB2 Legacy Survey diagnosed to be association members and that have between 20 and 350 net counts. Our method can provide uncertainties for spectral parameters whenever formal X-ray spectral fits cannot be well constrained, or are unavailable, and predictions useful for computing Chandra ACIS-I exposure times for observation planning.
Abstract
We analyze the X-ray spectra of the ∼8000 sources detected in the Cygnus OB2 Chandra Legacy Survey (this focus issue), with the goals of characterizing the coronal plasma of the young ...low-mass stars in the region and estimating their intrinsic X-ray luminosities. We adopt two different strategies for X-ray sources for which more or less than 20 photons were detected. For the brighter sample we fit the spectra with absorbed isothermal models. In order to limit uncertainties, for most of the fainter Cygnus OB2 members in this sample we constrain the spectral parameters to characteristic ranges defined from the brightest stars. For X-ray sources with <20 net photons we adopt a conversion factor from detected photon flux to intrinsic flux. This was defined, building on the results for the previous sample, as a function of the 20% quantile of the detected photon energy distributions, which we prove to also correlate well with extinction. We then use the X-ray extinction from the spectral fits to constrain the ratio between optical and X-ray extinction toward Cyg OB2, finding it consistent with standard “Galactic” values, when properly accounting for systematics. Finally, we exploit the large number of sources to constrain the average coronal abundances of several elements, through two different ensemble analyses of the X-ray spectra of low-mass Cyg OB2 members. We find the pattern of abundances to be largely consistent with that derived for the young stellar coronae in the Orion Nebula Cluster.
Abstract
In our Galaxy, star formation occurs in a variety of environments, with a large fraction of stars formed in clusters hosting massive stars. OB stars have an important feedback on the ...evolution of protoplanetary disks orbiting around nearby young stars and likely on the process of planet formation occurring in them. The nearby massive association Cyg OB2 is an outstanding laboratory to study this feedback. It is the closest massive association to our Sun and hosts hundreds of massive stars and thousands of low-mass members, both with and without disks. In this paper, we analyze the spatial variation of the disk fraction (i.e., the fraction of cluster members bearing a disk) in Cyg OB2 and study its correlation with the local values of far-ultraviolet (FUV) and extreme-ultraviolet (EUV) radiation fields and the local stellar surface density. We present definitive evidence that disks are more rapidly dissipated in the regions of the association characterized by intense local UV fields and large stellar density. In particular, the FUV radiation dominates disk dissipation timescales in the proximity (i.e., within 0.5 pc) of the O stars. In the rest of the association, EUV photons potentially induce a significant mass loss from the irradiated disks across the entire association, but the efficiency of this process is reduced at increasing distances from the massive stars owing to absorption by the intervening intracluster material. We find that disk dissipation due to close stellar encounters is negligible in Cyg OB2 and likely to have affected 1% or fewer of the stellar population. Disk dissipation is instead dominated by photoevaporation. We also compare our results to what has been found in other young clusters with different massive populations, concluding that massive associations like Cyg OB2 are potentially hostile to protoplanetary disks but that the environments where disks can safely evolve in planetary systems are likely quite common in our Galaxy.
Context
. With a mass exceeding several 10
4
M
⊙
and a rich and dense population of massive stars, supermassive young star clusters represent the most massive star-forming environment that is ...dominated by the feedback from massive stars and gravitational interactions among stars.
Aims
. In this paper we present the Extended Westerlund 1 and 2 Open Clusters Survey (EWOCS) project, which aims to investigate the influence of the starburst environment on the formation of stars and planets, and on the evolution of both low and high mass stars. The primary targets of this project are Westerlund 1 and 2, the closest supermassive star clusters to the Sun.
Methods
. The project is based primarily on recent observations conducted with the
Chandra
and JWST observatories. Specifically, the
Chandra
survey of Westerlund 1 consists of 36 new ACIS-I observations, nearly co-pointed, for a total exposure time of 1 Msec. Additionally, we included 8 archival
Chandra/
ACIS-S observations. This paper presents the resulting catalog of X-ray sources within and around Westerlund 1. Sources were detected by combining various existing methods, and photon extraction and source validation were carried out using the ACIS-Extract software.
Results
. The EWOCS X-ray catalog comprises 5963 validated sources out of the 9420 initially provided to ACIS-Extract, reaching a photon flux threshold of approximately 2 × 10
−8
photons cm
−2
s
−1
. The X-ray sources exhibit a highly concentrated spatial distribution, with 1075 sources located within the central 1 arcmin. We have successfully detected X-ray emissions from 126 out of the 166 known massive stars of the cluster, and we have collected over 71 000 photons from the magnetar CXO J164710.20-455217.
Abstract
We present a large-scale study of diffuse X-ray emission in the nearby massive stellar association Cygnus OB2 as part of the Chandra Cygnus OB2 Legacy Program. We used 40 Chandra X-ray ...ACIS-I observations covering ∼1.0 deg
2
. After removing 7924 point sources detected in our survey and applying adaptive smoothing to the background-corrected X-ray emission, the adaptive smoothing reveals large-scale diffuse X-ray emission. Diffuse emission was detected in the subbands soft (0.5−1.2 keV) and medium (1.2−2.5 keV) and marginally in the hard (2.5−7.0 keV) band. From X-ray spectral analysis of stacked spectra we compute a total (0.5–7.0 keV) diffuse X-ray luminosity of
L
X
diff
≈
4.2 × 10
34
erg s
−1
, characterized by plasma temperature components at
kT
≈ 0.11, 0.40, and 1.18 keV, respectively. The H
i
absorption column density corresponding to these temperatures has a distribution consistent with
N
H
= (0.43, 0.80, 1.39) × 10
22
cm
−2
. The extended medium-band energy emission likely arises from O-type stellar winds thermalized by wind−wind collisions in the most populated regions of the association, while the soft-band emission probably arises from less energetic termination shocks against the surrounding interstellar medium. Supersoft and soft diffuse emission appears more widely dispersed and intense than the medium-band emission. The diffuse X-ray emission is generally spatially coincident with low-extinction regions that we attribute to the ubiquitous influence of powerful stellar winds from massive stars and their interaction with the local interstellar medium. Diffuse X-ray emission is volume filling, rather than edge brightened, oppositely to other star-forming regions. We reveal the first observational evidence of X-ray halos around some evolved massive stars.
Context.
Nearly a dozen star-forming galaxies have been detected in
γ
-rays by the
Fermi
observatory in the last decade. A remarkable property of this sample is the quasi-linear relation between the
...γ
-ray luminosity and the star formation rate, which was obtained assuming that the latter is well traced by the infrared luminosity of the galaxies. The non-linearity of this relation has not been fully explained yet.
Aims.
We aim to determine the biases derived from the use of the infrared luminosity as a proxy for the star formation rate and to shed light on the more fundamental relation between the latter and the
γ
-ray luminosity. We expect to quantify and explain some trends observed in this relation.
Methods.
We compiled a near-homogeneous set of distances, ultraviolet, optical, infrared, and
γ
-ray fluxes from the literature for all known
γ
-ray emitting, star-forming galaxies. From these data, we computed the infrared and
γ
-ray luminosities, and star formation rates. We determined the best-fitting relation between the latter two, and we describe the trend using simple, population-orientated models for cosmic-ray transport and cooling.
Results.
We find that the
γ
-ray luminosity–star formation rate relation obtained from infrared luminosities is biased to shallower slopes. The actual relation is steeper than previous estimates, having a power-law index of 1.35 ± 0.05, in contrast to 1.23 ± 0.06.
Conclusions.
The unbiased
γ
-ray luminosity–star formation rate relation can be explained at high star formation rates by assuming that the cosmic-ray cooling region is kiloparsec-sized and pervaded by mild to fast winds. Combined with previous results about the scaling of wind velocity with star formation rate, our work provides support to advection as the dominant cosmic-ray escape mechanism in galaxies with low star formation rates.
X-RAY EMISSION FROM MASSIVE STARS IN CYG OB2 Rauw, G.; Nazé, Y.; Wright, N. J. ...
The Astrophysical journal. Supplement series,
11/2015, Volume:
221, Issue:
1
Journal Article, Web Resource
Peer reviewed
Open access
ABSTRACT We report on the analysis of the Chandra-ACIS data of O, B, and WR stars in the young association Cyg OB2. X-ray spectra of 49 O-stars, 54 B-stars, and 3 WR-stars are analyzed and for the ...brighter sources, the epoch dependence of the X-ray fluxes is investigated. The O-stars in Cyg OB2 follow a well-defined scaling relation between their X-ray and bolometric luminosities: . This relation is in excellent agreement with the one previously derived for the Carina OB1 association. Except for the brightest O-star binaries, there is no general X-ray overluminosity due to colliding winds in O-star binaries. Roughly half of the known B-stars in the surveyed field are detected, but they fail to display a clear relationship between LX and Lbol. Out of the three WR stars in Cyg OB2, probably only WR 144 is itself responsible for the observed level of X-ray emission, at a very low . The X-ray emission of the other two WR-stars (WR 145 and 146) is most probably due to their O-type companion along with a moderate contribution from a wind-wind interaction zone.
ABSTRACT
NGC 2071 is a star-forming region that overlaps with three γ-ray sources detected by the Fermi Space Telescope. We propose that strong flare activity in T Tauri stars could produce γ-ray ...emission in a way that makes them a counterpart to some unidentified sources detected by the Large Area Telescope aboard the Fermi satellite. We have performed a spectral and temporal analysis for two Fermi data sets: the first 2 yr and the entire 14 yr of observations. We have found that the γ-ray source is detectable at 3.2σ above the background at energies above 100 GeV during the first 2 yr of observation. The analysis of the expected frequency of the highest energy flares occurring in T Tauri stars is consistent with our estimate. In addition, we have determined the minimum energy of the flare that would produce γ-ray emission, which is ∼5 × 1037 erg. This agreement becomes a hard observational constraint supporting previous hypotheses about rare flares as the origin of unidentified γ-ray sources in star-forming regions.
ABSTRACT
We present a detailed spatially resolved X-ray study of Chandra observations towards the central region of the supernova remnant (SNR) G359.1-0.5. We removed 168 point-like sources and ...performed an adaptive smoothing on the entire field-of-view of the Chandra ACIS-I observation. The images obtained show an extended X-ray emission below $4.0\, {\rm keV}$ that partially fills the interior of G359.1-0.5. In order to characterize the X-ray emission, we divide the emitting area into 10 different regions inside the remnant. We identify the presence of at least four emission lines that confirm the thin thermal plasma origin. All spectral regions are well fitted by a recombining plasma model (VRNEI) with an absorbing column density range from 1.5 to $2.1\times 10^{22}\, {\rm cm}^{-2}$, and electron temperatures from 0.14 to $0.19\, {\rm keV}$. We also calculated an average electron density of $0.7\, {\rm cm}^{-3}$, estimated an age of 2.8 × 104 yr for the remnant, which can be lower if the filling factor is considered, and an X-ray luminosity of $1.6 \times 10^{35} \, {\rm erg}\, {\rm s}^{-1}$. Finally, we show that the X-ray emission is morphologically anti-correlated with adjacent CO clouds interacting with the SNR. Our results favour that the recombining plasma inside G359.1-0.5 could be explained via adiabatic cooling.
ABSTRACT
We report new CO observations and a detailed molecular-line study of the mixed morphology supernova remnant G359.1−0.5, which contains six OH (1720 MHz) masers along the radio shell, ...indicative of shock-cloud interaction. Observations of 12CO and 13CO J:1–0 lines were performed in a ∼38 × 38 arcmin area with the on-the-fly technique using the Kit Peak 12 Meter telescope. The molecular study has revealed the existence of a few clumps with densities ∼103 cm−3 compatible in velocity and position with the OH (1720 MHz) masers. These clumps, in turn, appear to be part of a larger, elongated molecular structure ∼34 arcmin long extending between −12.48 and +1.83 km s−1, adjacent to the western edge of the radio shell. According to the densities and relative position with respect to the masers, we conclude that the CO clouds depict unshocked gas, as observed in other remnants with OH (1720 MHz) masers. In addition, we investigated the distribution of the molecular gas towards the adjacent γ-ray source HESS J1745-303 (Aharonian et al. 2006) but could not find any morphological correlation between the γ-rays and the CO emission at any velocity in this region.
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