arXiv subjects

F. Yusef-Zadeh

Publications and source records attributed to F. Yusef-Zadeh.

At least 109 records · Page 6Linked to original sources

arXiv2005

Discovery of X-ray eclipses from the transient source CXOGC J174540.0-290031 with XMM-Newton

We present the XMM-Newton observations obtained during four revolutions in Spring and Summer 2004 of CXOGC J174540.0-290031, a moderately bright transient X-ray source, located at only 2.9" from SgrA*. We report the discovery of sharp and deep X-ray eclipses, with a period of 27,961+/-5 s and a duration of about 1,100+/-100 s, observed during the two consecutive XMM revolutions from August 31 to September 2. No deep eclipses were present during the two consecutive XMM revolutions from March 28 to April 1, 2004. The spectra during all four observations are well described with an absorbed power law continuum. While our fits on the power law index over the four observations yield values that are consistent with Gamma=1.6-2.0, there appears to be a significant increase in the column density during the Summer 2004 observations, i.e. the period during which the eclipses are detected. The intrinsic luminosity in the 2-10 keV energy range is almost constant with 1.8-2.3 x 10^34 (d_8kpc)^2 erg/s over the four observations. In the framework of eclipsing semidetached binary systems, we show that the eclipse period constrains the mass of the assumed main-sequence secondary star to less than 1.0 M_odot. Therefore, we deduce that CXOGC J174540.0-290031 is a low-mass X-ray binary (LMXB). Moreover the eclipse duration constrains the mass of the compact object to less than about 60 M_odot, which is consistent with a stellar mass black hole or a neutron star. The absence of deep X-ray eclipses during the Spring 2004 observations could be explained if the centroid of the X-ray emitting region moves from a position on the orbital plane to a point above the compact object, possibly coincident with the base of the jet which was detected in radio at this epoch. [Abstract truncated].

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arXiv2005

HST and UKIRT Observations of the Center of the Trifid Nebula: Evidence for the Photoevaporation of a Proplyd and a Protostellar Condensation

The Trifid nebula (M20) is a well-known prominent optical HII region trisected by bands of obscuring dust lanes and excited by an O7.5 star HD 164492A. Previous near-IR, mid-IR and radio continuum observations of the cluster of stars at the center of the Trifid nebula indicated circumstellar disks associated with hot stars with envelopes that are photoionized externally by the UV radiation from the hot central star, HD 164492A. Using WFPC2 of the HST, we present evidence of a resolved proplyd in H$α$ and [SII] line emission from a stellar source emitting cool dust emission. Using UKIRT, an infrared observation of the stellar source with a proplyd indicates a late F -- mid G spectral type. We also note a remarkable complex of filamentary and sheet-like structures which appear to arise from the edge of a protostellar condensation. These observations are consistent with a picture in which the bright massive star HD 164492A is responsible for the photoevaporation of protoplanetary disks of other less-massive members of the cluster as well as the closest protostellar condensation facing the central cluster. Using the evidence for a proplyd, we argue that the massive and intermediate mass members of the cluster, HD 164492C (B6 star) and HD 164492 (Herbig Be star) have disks associated with them.

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arXiv2005

A Radio Study of the Mouse, G359.23--0.82

The recent detection of a young pulsar powering ``the Mouse'', G359.23--0.82, as well as detailed imaging of surrounding nebular X-ray emission, have motivated us to investigate the structural details and polarization characteristics of the radio emission from this axisymmetric source with a supersonic bow shock. Using polarization data at 3.6 and 6cm, we find that the magnetic field wraps around the bow shock structure near the apex of the system, but downnstream runs parallel to the inferred direction of the pulsar's motion. The rotation measure (RM) distribution of the Mouse also suggests that the low degree of polarization combined with a high RM ahead of the pulsar result from internal plasma within the bowshock region. In addition, using sub-arcsecond radio image of the Mouse, we identify modulations in the brightness distribution of the Mouse that may be associated with the unshocked pulsar wind behind the pulsar. Lastly, we discuss the relationship between the Mouse and its neighboring shell-type supernova remnant G359.1--0.5 and argue that these two sources could potentially have the same origin.

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arXiv2005

A powerful bursting radio source towards the Galactic Centre

Transient astronomical sources are typically powered by compact objects and usually signify highly explosive or dynamic events. While radio astronomy has an impressive record of obtaining high time resolution observations, usually it is achieved in quite narrow fields-of-view. Consequently, the dynamic radio sky is poorly sampled, in contrast to the situation in the X- and gamma-ray bands in which wide-field instruments routinely detect transient sources. Here we report a new transient source, GCRT J1745-3009, detected in 2002 during a moderately wide-field radio transient monitoring program of the Galactic center (GC) region at 0.33 GHz. The characteristics of its bursts are unlike those known for any other class of radio transient. If located in or near the GC, its brightness temperature (~10^16 K) and the implied energy density within GCRT J1745-3009 vastly exceeds that observed in most other classes of radio astronomical sources, and is consistent with coherent emission processes rarely observed. We conclude that GCRT J1745-3009 is the first member of a new class of radio transient sources, the first of possibly many new classes to be identified through current and upcoming radio surveys.

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arXiv2005

The Nature of Nonthermal X-ray Filaments Near the Galactic Center

Recent Chandra and XMM-{\it Newton} observations reported evidence of two X-ray filaments G359.88-0.08 (SgrA-E) and G359.54+0.18 (the ripple filament) near the Galactic center. The X-ray emission from these filaments has a nonthermal spectrum and coincides with synchrotron emitting radio sources. Here, we report the detection of a new X-ray feature coincident with a radio filament G359.90-0.06 (SgrA-F) and show more detailed VLA, Chandra and BIMA observations of the radio and X-ray filaments. In particular, we show that radio emission from the nonthermal filaments G359.90-0.06 (SgrA-F) and G359.54+0.18 (the ripple) has a steep spectrum whereas G359.88-0.08 (SgrA-E) has a flat spectrum. The X-ray emission from both these sources could be due to synchrotron radiation. However, given that the 20 \kms molecular cloud, with its intense 1.2mm dust emission, lies in the vicinity of SgrA-F, it is possible that the X-rays could be produced by inverse Compton scattering of far-infrared photons from dust by the relativistic electrons responsible for the radio synchrotron emission. The production of X-ray emission from ICS allows an estimate of the magnetic field strength of ~0.08 mG within the nonthermal filament. This should be an important parameter for any models of the Galactic center nonthermal filaments.

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arXiv2005

Starburst Driven Thermal and Non-thermal Structures in the Galactic Center Region

We briefly review the prominent thermal and nonthermal sources near the Galactic center. These sources include the young stellar clusters, the Sgr B complex as well as the large-scale nonthermal filaments and lobes. Some of the recent radio images of this region based on VLA and Green Bank Telescope observations are also presented. We then argue that the origin of the large-scale features within the inner two degrees of the Galactic center is tied to a past starburst activity in the nucleus of the Galaxy.

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arXiv2005

Proper Motion of the Irradiated Jet HH 399 in the Trifid Nebula

HH 399 is one of the first Herbig Haro flows recognized to be irradiated by the UV radiation of the massive O7.5 star in the Trifid nebula. We present the proper motion of the first irradiated jet based on two epochs of HST observations of HH 399 separated nearly by five years using H$α$ and [SII] line filters. High proper motion with continuous velocities between 200$\pm$55 and 528$\pm24$ \kms are detected in both lines along the 18$''$ extent of the jet axis. The irradiated fully-ionized jet consists of numerous knots along the jet but also shows the evidence for a number of isolated blob-like structures running immediately outside the jet with lower transverse velocities. The transverse velocities combined with radial velocity measurements indicate that the jet axis lies away from the plane of the sky by only few degrees. We argue that the jet is fully ionized based on [SII]/H$α$ line ratio as well as radio continuum emission detected from the full extent of the jet at 3.6cm wavelength. The stellar mass-loss rate producing HH 399 is estimated to be $\approx 2\times10^{-6} \msol $yr$^{-1}$.

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arXiv2004

Young Stellar Clusters, WR-type Phenomenon and the Origin of the Galactic Center Nonthermal Radio Filaments

Recent observations of the Arches cluster located within a projected distance of 30 pc from the dynamical center of the Galaxy have shown the presence of diffuse and discrete X-ray continuum emission, diffuse 6.4keV line emission as well as thermal and nonthermal radio continuum emission. This young and dense stellar cluster is also recognized to be within the 95% error circle of an identified steady source of $γ$-ray emission associated with the EGRET source 3EG J1746--2851. Much of the thermal and nonthermal emission can be explained by shocked gas resulting from colliding winds originating from massive binaries within the cluster. In particular, we argue that nonthermal particles could upscatter the radiation field of the cluster by ICS and account for the $γ$-ray emission. We also consider that the fluorescent 6.4 keV line emission may be the result of the impact of low-energy relativistic particles on neutral gas distributed in the vicinity of the cluster. Lastly, we sketch an interpretation in which young stellar clusters and massive young binary systems are responsible for the origin of nonthermal radio filaments found throughout the inner 300pc of the Galaxy. The collimation of the nonthermal filaments may be done in the colliding wind region by the ionized surface of individual mass-losing stars of massive binary systems. In this picture, a WR-type phenomenon is expected to power a central star burst in the Galactic center in order to account for all the observed filaments.

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arXiv2004

Discovery of an OH(1720 MHz) Maser in the LMC

We report the first study of OH(1720 MHz) masers in the LMC in order to probe regions where supernova remnants interact with adjacent molecular clouds. Using the ATCA, we observed four sources in the LMC and detected a single OH(1720 MHz) maser with a flux density of 377 mJy toward 30 Doradus. No main line emission at 1665 or 1667 MHz was detected. The observed OH(1720 MHz) maser emission from 30 Dor shows characteristics similar to the well-known collisionally-pumped supernova remnant masers found in the Galaxy, though 30 Dor is known as a star forming region. It is possible that shocks driven by a nearby supernova remnant or by strong stellar winds from young stars are responsible for production of OH(1720 MHz) maser in 30 Dor. Future studies are required to distinguish between collisional and radiative pumping mechanisms for the 30 Dor OH (1720 MHz) maser.

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arXiv2004

A $λ$20cm Survey of the Galactic Center Region I: Detection of Numerous Linear Filaments

We present a sensitive $λ$20cm VLA continuum survey of the Galactic center region using new and archival data based on multi-configuration observations taken with relatively uniform {\it uv} coverage. The high dynamic range images cover the regions within $-2^\circ < l < 5^\circ$ and $-40' < b < 40'$ with a spatial resolution of $\approx30''$ and 10$''$. The wide field imaging technique is used to construct a low-resolution mosaic of 40 overlapping pointings. The mosaic image includes the Effelsburg observations filling the low spatial frequency {\it uv} data. We also present high resolution images of twenty three overlapping fields using DnC and CnB array configurations. The survey has resulted in a catalog of 345 discrete sources as well as 140 images revealing structural details of HII regions, SNRs, pulsar wind nebulae and more than 80 linear filaments distributed toward the complex region of the Galactic center. These observations show the evidence for an order of magnitude increase in the number of faint linear filaments with typical lengths of few arcminutes. Many of the filaments show morphological characteristics similar to the Galactic center nonthermal radio filaments (NRFs). The linear filaments are not isolated but are generally clustered in star forming regions where prominent NRFs had been detected previously. The extensions of many of these linear filaments appear to terminate at either a compact source or a resolved shell-like thermal source. A relationship between the filaments, the compact and extended thermal sources as well as a lack of preferred orientation for many RFs should constrain models that are proposed to explain the origin of nonthermal radio filaments in the Galactic center.

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arXiv2004

Shocked molecular hydrogen towards the Tornado nebula

We present near-infrared and millimetre-line observations of the Tornado nebula (G357.7-0.1). We detected 2.12 micron_m H2 1-0 S(1) line emission towards the suspected site of interaction with a molecular cloud revealed by the presence of an OH(1720 MHz) maser. The distribution of the H2 emission is well correlated with the nonthermal radio continuum emission from the Tornado, and the velocity of the H2 emission spans over 100 km/s, which both imply that the H2 emission is shock excited. We also detected millimetre-lines from 12CO and 13CO transitions at the velocity of the maser, and mapped the distribution of the molecular cloud in a 2 x 2 arcmin^2 region around the maser. The peak of the molecular cloud aligns well with an indentation in the nebula's radio continuum distribution, suggesting that the nebula's shock is being decelerated at this location, which is consistent with the presence of the OH(1720 MHz) maser and shocked H2 emission at that location.

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arXiv2004

The Mouse That Soared: High Resolution X-ray Imaging of the Pulsar-Powered Bow Shock G359.23-0.82

We present an observation with the Chandra X-ray Observatory of the unusual radio source G359.23-0.82 ("the Mouse"), along with updated radio timing data from the Parkes radio telescope on the coincident young pulsar J1747-2958. We find that G359.23-0.82 is a very luminous X-ray source (L_X [0.5-8.0 keV] = 5e34 ergs/s for a distance of 5 kpc), whose morphology consists of a bright head coincident with PSR J1747-2958, plus a 45"-long narrow tail whose power-law spectrum steepens with distance from the pulsar. We thus confirm that G359.23-0.82 is a bow-shock pulsar wind nebula powered by PSR J1747-2958; the nebular stand-off distance implies that the pulsar is moving with a Mach number of ~60, suggesting a space velocity ~600 km/s through gas of density ~0.3 cm^-3. We combine the theory of ion-dominated pulsar winds with hydrodynamic simulations of pulsar bow shocks to show that a bright elongated X-ray and radio feature extending 10'' behind the pulsar represents the surface of the wind termination shock. The X-ray and radio "trails" seen in other pulsar bow shocks may similarly represent the surface of the termination shock, rather than particles in the postshock flow as is usually argued. The tail of the Mouse contains two components: a relatively broad region seen only at radio wavelengths, and a narrow region seen in both radio and X-rays. We propose that the former represents material flowing from the wind shock ahead of the pulsar's motion, while the latter corresponds to more weakly magnetized material streaming from the backward termination shock. This study represents the first consistent attempt to apply our understanding of "Crab-like" nebulae to the growing group of bow shocks around high-velocity pulsars.

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arXiv2004

The INTEGRAL View of the Galactic Nucleus

We present the preliminary results of the observational campaign performed in 2003 to study the Galactic Nucleus with INTEGRAL. The mosaicked images obtained with the IBIS/ISGRI coded aperture instrument in the energy range above 20 keV, give a yet unseen view of the high-energy sources of this region in hard X and gamma-rays, with an angular resolution of 12'. We report on the discovery of a source, IGR J17456-2901, compatible with the instrument's point spread function and coincident with the Galactic Nucleus Sgr A* to within 0.9'. The source is visible up to 60-80 keV with a 20-100 keV luminosity at 8 kpc of 3 x 10E35 erg/s. Although we cannot unequivocally associate the new INTEGRAL source to the Galactic Nucleus, this is the first report of significant hard X-ray emission from within the inner 10' of the Galaxy and a contribution from the galactic center supermassive black hole itself cannot be excluded. Here we discuss the results obtained and the perspectives for future observations of the Galactic Nucleus with INTEGRAL and other observatories

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arXiv2004

X-ray Observations of Stellar Clusters Near the Galactic Center

We report the first detection of X-ray emission from the Quintuplet star cluster and compare its X-ray emission to that of the Arches star cluster. Four point sources are significantly detected near the core of the Quintuplet cluster with a total, absorption-corrected luminosity of ~1\times10^{33} ergs s^{-1}. Diffuse, thermal emission is also detected near the core of the Quintuplet cluster with an absorption-corrected luminosity of ~1\times10^{34} ergs s^{-1}. We analyze the diffuse and point-like emission from the Arches and Quintuplet and discuss the possibility that they are host to cluster wind outflows. We also present the results of our search for X-ray emission from candidate star clusters in the Galactic center (GC) region. We use extinction estimated by near-IR colors and X-ray spectral fits, as well as other IR properties, to determine if the candidate clusters are new, GC star clusters. We find that three of the six candidate clusters found toward the GC are likely foreground clusters, two of the candidate clusters are not detected in the X-ray data, but have near-IR extinctions consistent with a GC location, and one of the candidate clusters has X-ray and near-IR extinctions consistent with being in the GC. The X-ray and IR emission from the candidate clusters is compared to the known, massive, GC star clusters.

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arXiv2004

A Jet Model of the Galactic Center Nonthermal Radio Filaments

Protostellar sources in star forming regions are responsible for driving jets with flow velocities ranging between 300 and 400 km s$^{-1}$. This class of jets consists of highly collimated outflows which include thermal knots with number densities estimated to be greater than that of their ambient medium. On the other hand, extragalactic FR I jets consist of light fluid with low Mach number burrowing through a denser medium as the magnetized jets radiate nonthermal emission. Both protostellar and extragalactic jets are believed to be launched by accretion disks. Here we consider a jet model in which the characteristics common to both protostellar and extragalactic jets are used to explain the origin of nonthermal filaments in the Galactic center region. We argue that these filaments are analogous to FR I extragalactic sources but are launched by embedded young stars or clusters of stars in star-forming regions.

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arXiv2003

The Eye of the Tornado - an isolated, high mass young stellar object near the Galactic centre

We present infrared (AAT, UKIRT) and radio (VLA, SEST) observations of the Eye of the Tornado, a compact source apparently near the head of the Tornado Nebula. The near-infrared Br-gamma and He I lines are broad (FWHM 40 and 30 km/s, respectively) and have a line centre at Vlsr = -205 km/s. This corresponds to a feature at the same velocity in the 12CO J=1-0 line profile. The kinematic velocity derived from Galactic rotation places the Eye at the distance of the Galactic Centre (i.e. 8.5 kpc) and separated (probably foreground) from the Tornado Nebula. Four knots of emission are seen in the Br-gamma line and at 6 and 20 cm. Together with the flat radio spectral index, we confirm that the Eye contains ionized gas, but that this is embedded within a dense molecular core. The spectral energy distribution can be modelled as a two-component blackbody + greybody, peaking at far-IR wavelengths. The knots are UC HII regions, and the core contains a luminous (2 x 10^4 Lsun), embedded, massive young stellar source. We also propose a geometrical model for the Eye to account for both its spectral energy distribution and its morphology.

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arXiv2003

The Origin of the Galactic Center Nonthermal Radio Filaments: Young Stellar Clusters

The unusual class of magnetized nonthermal radio filaments (NTF), threads and streaks with their unique physical characteristics are found only within the inner couple of degrees of the Galactic center. Also, a number of young, mass-losing and rare stellar clusters are recognized to lie in the Galactic center region. The latter characteristic of the Galactic center region is used to explain the origin of the nonthermal radio filaments. We consider a mechanism in which the collective winds of massive WR and OB stars within a dense stellar environment produce shock waves that can accelerate particles to relativistic energies. This mechanism is an extension of a model originally proposed by Rosner and Bodo (1996), who suggested that energetic nonthermal particles are produced in a terminal shock of mass-losing stars. The large-scale distribution of the magnetic field in the context of this model is argued to have neither poloidal geometry nor pervasive throughout the Galactic center region.

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arXiv2003

HST Paschen alpha and 1.9 micron imaging of Sgr A West

We present HST/NICMOS images at 0.2" resolution of the HI Paschen Alpha (PaA) emission line in a 70" x 90" region of the Galactic center centered on the non-thermal radio source Sgr A*. The majority of the emission arises from ionized gas in the mini-spiral in the central parsec. PaA emission is also seen from 26 stellar sources, presumably early-type stars with mass-loss winds. The new data reveal significant small-scale structure (<1"~0.04pc) in the ionized gas of the mini-spiral; low surface brightness emission features are also seen for the first time. Extinction, estimated from the ratio of observed PaA emission to 6-cm continuum emission, varies from 20 to 50 mag with a median Av=31.1 mag, in excellent agreement with earlier estimates for the stellar sources and indepedent measurements derived using H92alpha recombination line data. Large increases in extinction are seen along the periphery of the ionized gas, suggesting that the ionized gas is partially extincted by dust in the molecular clouds at the outside of the ionized regions. The small-scale, filamentary structures in the ionized gas have a free thermal expansion time of only ~ 3000 yrs; either magnetic fields or mass-loss winds from the hot emission line stars may contain the ionized filaments. For both the ionized gas and the stellar continuum, the centroids of the emission remain within ~+/- 1" from a radius of 2" out to 40", providing further evidence that Sgr A* is indeed at or extremely close to the dynamical center of the Galactic nucleus stellar distribution. The 1.9 micron surface brightness increases inwards to 0.9" and then decreases or levels off closer to Sgr A*, possibly indicating the core radius of the central stellar distribution or depletion of the late-type stars by stellar collisions near the central black hole.

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