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Effective temperature scale and bolometric corrections from 2MASS photometry
We present a method to determine effective temperatures, angularsemi-diameters and bolometric corrections for population I and II FGKtype stars based on V and 2MASS IR photometry. Accurate calibration isaccomplished by using a sample of solar analogues, whose averagetemperature is assumed to be equal to the solar effective temperature of5777 K. By taking into account all possible sources of error we estimateassociated uncertainties to better than 1% in effective temperature andin the range 1.0-2.5% in angular semi-diameter for unreddened stars.Comparison of our new temperatures with other determinations extractedfrom the literature indicates, in general, remarkably good agreement.These results suggest that the effective temperaure scale of FGK starsis currently established with an accuracy better than 0.5%-1%. Theapplication of the method to a sample of 10 999 dwarfs in the Hipparcoscatalogue allows us to define temperature and bolometric correction (Kband) calibrations as a function of (V-K), [m/H] and log g. Bolometriccorrections in the V and K bands as a function of T_eff, [m/H] and log gare also given. We provide effective temperatures, angularsemi-diameters, radii and bolometric corrections in the V and K bandsfor the 10 999 FGK stars in our sample with the correspondinguncertainties.
| The XMM-Newton/Chandra monitoring campaign of the Galactic center region. Description of the program and preliminary results
We present the first results of our X-ray monitoring campaign on a 1.7square degree region centered on Sgr A* using the satellites XMM-Newtonand Chandra. The purpose of this campaign is to monitor the behavior(below 10 keV) of X-ray sources (both persistent and transient) whichare too faint to be detected by monitoring instruments aboard othersatellites currently in orbit (e.g., Rossi X-ray Timing Explorer;INTEGRAL). Our first monitoring observations (using the HRC-I aboardChandra) were obtained on June 5, 2005. Most of the sources detectedcould be identified with foreground sources, such as X-ray active stars.In addition we detected two persistent X-ray binaries (1E 1743.1-2843;1A 1742-294), two faint X-ray transients (GRS 1741.9-2853; XMMJ174457-2850.3), as well as a possible new transient source at aluminosity of a few times 1034 erg s-1. We reporton the X-ray results on these systems and on the non-detection of thetransients in follow-up radio data using the Very Large Array. Wediscuss how our monitoring campaign can help to improve ourunderstanding of the different types of X-ray transients (i.e., the veryfaint ones).
| Repeated X-Ray Flaring Activity in Sagittarius A*
Investigating the spectral and temporal characteristics of the X-rayscoming from Sagittarius A* (Sgr A*) is essential to our development of amore complete understanding of the emission mechanisms in thissupermassive black hole located at the center of our Galaxy. SeveralX-ray flares with varying durations and spectral features have alreadybeen observed from this object. Here we present the results of two longXMM-Newton observations of the Galactic nucleus carried out in 2004, fora total exposure time of nearly 500 ks. During these observations wedetected two flares from Sgr A* with peak 2-10 keV luminosities about 40times (LX~9×1034 ergs s-1) abovethe quiescent luminosity: one on 2004 March 31 and another on 2004August 31. The total duration of the first flare was about 4 ks. Thesecond, simultaneously detected in the near-IR, lasted about 10 ks. Thecombined fit to the EPIC spectra yields photon indices of about 1.5 and1.9 for the first and second flares, respectively. This hard photonindex strongly suggests the presence of an important population ofnonthermal electrons during the event and supports the view that themajority of flaring events tend to be hard and not very luminous.
| 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 revolutionsin Spring and Summer 2004, of CXOGC J174540.0-290031, a moderatelybright transient X-ray source, located at only 2.9'' from Sgr A*. Wereport the discovery of sharp and deep X-ray eclipses, with a period of27 961±5 s and a duration of about 1100±100 s, observedduring the two consecutive XMM-Newton revolutions from August 31 toSeptember 2. No deep eclipses were present during the two consecutiveXMM-Newton revolutions from March 28 to April 1, 2004. The spectraduring all four observations are described well with an absorbed powerlaw continuum. While our fits on the power law index over the fourobservations yielded values that are consistent with Γ=1.6-2.0,there appeared to be a significant increase in the column density duringthe Summer 2004 observations, i.e. the period during which the eclipsesare detected. The intrinsic luminosity in the 2-10 keV energy range wasalmost constant with 1.8{-}2.3 × 1034 ({d8kpc})2 erg s-1 over the four observations.In the framework of eclipsing semidetached binary systems, we show thatthe eclipse period constrains the mass of the assumed main-sequencesecondary star to less than 1.0 M_ȯ. Therefore, we deduce thatCXOGC J174540.0-290031 is a low-mass X-ray binary (LMXB). Moreover theeclipse duration constrains the mass of the compact object to less thanabout 60 M_ȯ, which is consistent with a stellar mass black hole ora neutron star. The absence of deep X-ray eclipses during the Spring2004 observations could be explained if the centroid of the X-rayemitting region moves from a position on the orbital plane to a pointabove the compact object, possibly coincident with the base of the jetwhich was detected in radio at this epoch. According to our study, CXOGCJ174540.0-290031 is a LMXB, and is more likely to have a black hole thana neutron star as its primary, which would entail an inclination anglegreater than 75°; i.e. the binary system and the accretion disk areseen close to edge-on.
| The Spectra and Variability of X-Ray Sources in a Deep Chandra Observation of the Galactic Center
We examine the X-ray spectra and variability of the sample of X-raysources with LX~1031-1033 ergss-1 identified within the inner 9' of the Galaxy by Muno etal. Very few of the sources exhibit intraday or intermonth variations.We find that the spectra of the point sources near the Galactic centerare very hard between 2-8 keV, even after accounting for absorption.When modeled as power laws, the median photon index is Γ=0.7,while when modeled as thermal plasma we can only obtain lower limits tothe temperature of kT>8 keV. The combined spectra of the pointsources is similarly hard, with a photon index of Γ=0.8. Strongline emission is observed from low-ionization, He-like, and H-like Fe,both in the average spectra and in the brightest individual sources. Theline ratios of the highly ionized Fe in the average spectra areconsistent with emission from a plasma in thermal equilibrium. This lineemission is observed whether average spectra are examined as a functionof the count rate from the source or as a function of the hardnessratios of individual sources. This suggests that the hardness of thespectra may in fact be to due local absorption that partially covers theX-ray-emitting regions in the Galactic center systems. We suggest thatmost of these sources are intermediate polars, which (1) often exhibithard spectra with prominent Fe lines, (2) rarely exhibit either flareson short timescales or changes in their mean X-ray flux on longtimescales, and (3) are the most numerous hard X-ray sources withcomparable luminosities in the Galaxy.
| Chandra X-Ray Spectroscopic Imaging of Sagittarius A* and the Central Parsec of the Galaxy
We report the results of the first-epoch observation with the ACIS-Iinstrument on the Chandra X-Ray Observatory of Sagittarius A* (Sgr A*),the compact radio source associated with the supermassive black hole(SMBH) at the dynamical center of the Milky Way. This observationproduced the first X-ray (0.5-7 keV) spectroscopic image with arcsecondresolution of the central 17'×17'(40pc×40pc) of the Galaxy. We report the discovery of an X-raysource, CXOGC J174540.0-290027, coincident with Sgr A* within0.27"+/-0.18". The probability of a false match is estimated to be<~0.5%. The spectrum is well fitted either by an absorbed power lawwith photon index Γ~2.7 or by an absorbed optically thin thermalplasma with kT~1.9 keV and column densityNH~1×1023 cm-2. The observed fluxin the 2-10 keV band is ~1.3×10-13 ergs cm-2s-1, and the absorption-corrected luminosity is~2.4×1033 ergs s-1. The X-ray emission atthe position of Sgr A* is extended, with an intrinsic size of ~1.4"(FWHM), consistent with the Bondi accretion radius for a2.6×106 Msolar black hole. A compactcomponent within the source flared by up to a factor of 3 over a periodof ~1 hr at the start of the observation. The search for Kα lineemission from iron was inconclusive, yielding an upper limit on theequivalent width of 2.2 keV. Several potential stellar origins for theX-ray emission at Sgr A* are considered, but we conclude that thevarious properties of the source favor accretion onto the SMBH as theorigin for the bulk of the emission. These data are inconsistent with``standard'' advection-dominated accretion flow (ADAF) models or Bondimodels, unless the accretion rate from stellar winds is much lower thananticipated. The central parsec of the Galaxy contains an ~1.3 keVplasma with electron densityne~26η-1/2fcm-3, whereηf is the filling factor. This plasma should supply~10-6 Msolar yr-1 of material to theaccretion flow at the Bondi radius, whereas measurements of linearpolarization at 150 GHz and above limit the accretion rate near theevent horizon to <~10-8 Msolar yr-1,assuming an equipartition magnetic field. Taken together, the X-ray andradio results imply that outflows or convection are playing a role inADAF models and subequipartition magnetic fields in Bondi models, orelse the X-ray emission must be generated predominantly via thesynchrotron self-Compton (SSC) process. The measured extent of thesource and the detection of short timescale variability are evidencethat the emission from Sgr A* contains both thermal and nonthermalemission components at comparable levels. We also discuss the complexstructure of the X-ray emission from the Sgr A radio complex and alongthe Galactic plane. Morphological evidence is presented that Sgr A* andthe H II region Sgr A West lie within the hot plasma in the centralcavity of Sgr A East, which we interpret as a supernova remnant that mayhave passed through the position of the SMBH, leading to a period ofincreased activity that ended within the past ~300 yr. Similarly, wehave discovered bright clumps of X-ray emission located on oppositesides of the Galactic plane, along a line passing through the centralparsec of the Galaxy. The arrangement of these lobes suggests that SgrA* may have experienced an earlier period of increased activity lastingseveral thousand years during which it expelled hot gas in a bipolaroutflow oriented roughly perpendicular to the Galactic plane.Additionally, we present an analysis of stellar emission within thecentral parsec of the Galaxy.
| ROSAT-HRI observations of SIGMA hard X-ray sources of the Galactic Center region.
We report on the ROSAT HRI observations of three Galactic Center hardX-ray sources discovered by the ART-P/SIGMA experiments aboard theGRANAT satellite, namely GRS1734-292, GRS1747-341, and GRS1743-290. Wefound a possible counterpart for GRS1734-292 located at a positionα=17h37m28.2s and δ= -29deg08'03.1" (equinox 2000) with anassociated error radius of 3arcsec (90% confidence level). The HRI X-rayflux is consistent with previous measurements by ART-P. This variablesource was also clearly detected during two PSPC pointed observations ofthe Galactic Center region. Although the significance of our detectionis low (~3σ), we may have also an X-ray counterpart for thepersistent SIGMA source GRS1747-341; its position would beα=17h50m55.0s and δ=-34d11'37.0" (equinox 2000) with anassociated error radius of 5.5arcsec (90% confidence level). On theother hand, we failed to detect GRS1743-290; another source whichdisplays persistent emission in the SIGMA hard X-ray band. This resultis consistent with the source being highly absorbed as suggested byprevious observations.
| The Henry Draper Extension Charts: A catalogue of accurate positions, proper motions, magnitudes and spectral types of 86933 stars
The Henry Draper Extension Charts (HDEC), published in the form offinding charts, provide spectral classification for some 87000 starsmostly between 10th and 11th magnitude. This data, being highlyvaluable, as yet was practically unusable for modern computer-basedastronomy. An earlier pilot project (Roeser et al. 1991) demonstrated apossibility to convert this into a star catalogue, using measurements ofcartesian coordinates of stars on the charts and positions of theAstrographic Catalogue (AC) for subsequent identification. We presenthere a final HDEC catalogue comprising accurate positions, propermotions, magnitudes and spectral classes for 86933 stars of the HenryDraper Extension Charts.
| ROSAT observation of the SGR A region
The Galactic Center has been observed with the ROSAT-PSPC for 50000sec.15 sources were found within the central 30'x30', field. 1E1742.5-2859,the source commonly associated with Sgr A*, could be resolved into threeindividual objects, out of which at least one is a foreground object.One component of the 1E1742.5-2859 complex is coincident with Sgr A*within less than 10". This makes it to be likely the nucleus itself or asource in its immediate vicinity. The flux derived from the ROSATobservation is much less than one would expect on the basis of previousmeasurements at higher energies. In order to explain this discrepancy,we assume an X-ray absorption much excess of N_H_=5x10^22^cm^-2^, theexpected value for the interstellar absorption towards the GalacticCenter. There is an indication that at least a part of the apparentlydiffuse emission can be resolved into individual sources.
| UVBY observations of A, F, G and K field stars
Photoelectric data in the uvby system have been obtained for about 800southern stars of the Hipparcos Input Catalog (Grenon, 1982, 1985). Mostof the stars are F and G main sequence and fall in the magnitude range V= 8-11.
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Observation and Astrometry data
| Constellation: | Sagittaire |
| Right ascension: | 17h45m30.00s |
| Declination: | -29°07'04.2" |
| Apparent magnitude: | 10.165 |
| Proper motion RA: | -0.8 |
| Proper motion Dec: | -12.2 |
| B-T magnitude: | 10.677 |
| V-T magnitude: | 10.208 |
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