Direction Of Galactic Anticenter Research Articles

Three new bricks in the wall: Berkeley 23, Berkeley 31 and King 8★

A comprehensive census of Galactic open cluster properties places unique constraints on the Galactic disc structure and evolution. In this framework we investigate the evolutionary status of three poorly-studied open clusters, Berkeley 31, Berkeley 23 and King 8, all located in the Galactic anti-centre direction. To this aim, we make use of deep LBT observations, reaching more than 6 mag below the main sequence Turn- Off. To determine the cluster parameters, namely age, metallicity, distance, reddening and binary fraction, we compare the observational colour-magnitude diagrams (CMDs) with a library of synthetic CMDs generated with different evolutionary sets (Padova, FRANEC and FST) and metallicities. We find that Berkeley 31 is relatively old, with an age between 2.3 and 2.9 Gyr, and rather high above the Galactic plane, at about 700 pc. Berkeley 23 and King 8 are younger, with best fitting ages in the range 1.1-1.3 Gyr and 0.8-1.3 Gyr, respectively. The position above the Galactic plane is about 500- 600 pc for the former, and 200 pc for the latter. Although a spectroscopic confirmation is needed, our analysis suggests a sub-solar metallicity for all three clusters.

The local Galactic magnetic field in the direction of Geminga

The Milagro hot spot A, close to the Galactic anticenter direction, has been tentatively attributed to cosmic rays from a local reservoir (at a distance ~100 pc), freely streaming along diverging and smooth magnetic field lines. This is at variance with the geometry of the ~kpc scale Galactic magnetic field, which is known to be aligned with the spiral arms. We investigate the information available on the geometry of the magnetic field on the scales (~100 pc) of relevance here. The magnetic field immediately upstream of the heliosphere has been investigated by previous authors by modeling the interaction of this field with the solar wind. At larger distances, we use the dispersion measure and the rotation measure of nearby pulsars (especially towards the third Galactic quadrant). Additional information about the local field towards the North Polar Spur is taken from previous studies of the diffuse radio emission and the polarization of starlight. The asymmetry of the heliosphere with respect to the incoming interstellar medium implies a magnetic field almost orthogonal to the local spiral arm, in the general direction of hot spot A, but more to the south. This is in good agreement with the nearby pulsar data on the one side, and the North Polar Spur data on the other. The local magnetic field on scales of ~100 parsecs around the Sun seems to be oriented so as to provide a direct connection between the Solar system and a possible site of the Geminga supernova; the residual angular difference and the shape and orientation of the Milagro hot spot can be attributed to the field trailing in the wake of the heliosphere.

Transiting exoplanets from the CoRoT space mission

Aims. The CoRoT space mission continues to photometrically monitor about 12 000 stars in its field-of-view for a series of target fields to search for transiting extrasolar planets ever since 2007. Deep transit signals can be detected quickly in the "alarm-mode" in parallel to the ongoing target field monitoring. CoRoT's first planets have been detected in this mode. Methods. The CoRoT raw lightcurves are filtered for orbital residuals, outliers, and low-frequency stellar signals. The phase folded lightcurve is used to fit the transit signal and derive the main planetary parameters. Radial velocity follow-up observations were initiated to secure the detection and to derive the planet mass. Results. We report the detection of CoRoT-5b, detected during observations of the LRa01 field, the first long-duration field in the galactic anticenter direction. CoRoT-5b is a "hot Jupiter-type" planet with a radius of 1.388(+0.046, -0.047) R_Jup, a mass of 0.467(+0.047, -0.024) M_Jup, and therefore, a mean density of 0.217(+0.031, -0.025) g cm-3. The planet orbits an F9V star of 14.0 mag in 4.0378962 +/- 0.0000019 days at an orbital distance of 0.04947(+0.00026, -0.00029) AU.

KONUS-WIND OBSERVATIONS OF THE NEW SOFT GAMMA-RAY REPEATER SGR 0501+4516

In 2008 August, the new soft gamma-ray repeater SGR 0501+4516 was discovered by Swift. The source was soon confirmed by several groups in space- and ground-based multi-wavelength observations. In this letter we report the analysis of five short bursts from the recently discovered SGR, detected with Konus-Wind gamma-ray burst spectrometer. Properties of the time histories of the observed events, as well as results of multi-channel spectral analysis, both in the 20--300 keV energy range, show, that the source exhibits itself as a typical SGR. The bursts durations are <0.75 s and their spectra above 20 keV can be fitted by optically-thin thermal bremsstrahlung (OTTB) model with kT of 20--40 keV. The spectral evolution is observed, which resembles the SGR 1627-41 bursts, where a strong hardness-intensity correlation was noticed in the earlier Konus-Wind observations. The peak energy fluxes of all five events are comparable to highest those for known SGRs, so a less distant source is implied, consistent with the determined Galactic anti-center direction. Supposing the young supernova remnant HB9 (at the distance of 1.5 kpc) as a natal environment of the source, the peak luminosities of the bursts are estimated to be (2--5)x10^{40} erg s-1. The values of the total energy release, given the same assumptions, amount to (0.6--6)x10^{39} erg. These estimations of both parameters are typical for short SGR bursts.

THE SOLAR WIND CHARGE-TRANSFER X-RAY EMISSION IN THE 1/4 keV ENERGY RANGE: INFERENCES ON LOCAL BUBBLE HOT GAS AT LOWZ

We present calculations of the heliospheric SWCX emission spectra and their contributions in the ROSAT 1/4 keV band. We compare our results with the soft X-ray diffuse background (SXRB) emission detected in front of 378 identified shadowing regions during the ROSAT All-Sky Survey (Snowden et al. 2000). This foreground component is principally attributed to the hot gas of the so-called Local Bubble (LB), an irregularly shaped cavity of ~50-150 pc around the Sun, which is supposed to contain ~10^6 K plasma. Our results suggest that the SWCX emission from the heliosphere is bright enough to account for most of the foreground emission towards the majority of low galactic latitude directions, where the LB is the least extended. In a large part of directions with galactic latitude above 30deg the heliospheric SWCX intensity is significantly smaller than the measured one. However, the SWCX R2/R1 band ratio differs slightly from the data in the galactic center direction, and more significantly in the galactic anti-centre direction where the observed ratio is the smallest. Assuming that both SWCX and hot gas emission are present and their relative contributions vary with direction, we tested a series of thermal plasma spectra for temperatures ranging from 10^5 to 10^6.5 K and searched for a combination of SWCX spectra and thermal emission matching the observed intensities and band ratios, while simultaneously being compatible with O VI emission measurements. In the frame of collisional equilibrium models and for solar abundances, the range we derive for hot gas temperature and emission measure cannot reproduce the Wisconsin C/B band ratio. We emphasize the need for additional atomic data, describing consistently EUV and X-ray photon spectra of the charge-exchange emission of heavier solar wind ions.

CHARACTERIZATION OF CoRoT TARGET FIELDS WITH BERLIN EXOPLANET SEARCH TELESCOPE. II. IDENTIFICATION OF PERIODIC VARIABLE STARS IN THE LRc2 FIELD

In this paper we report on observations of the CoRoT LRa1 field with the Berlin Exoplanet Search Telescope (BEST). The current paper is part of the series of papers describing the results of our stellar variability survey. BEST is a small aperture telescope with a wide field-of-view (FOV). It is dedicated to search for stellar variability within the target fields of the CoRoT space mission to aid in minimizing false-alarm rates and identify potential targets for additional science. The LRa1 field is CoRoT's third observed field and the second long run field located in the galactic anticenter direction. We observed the LRa1 stellar field on 23 nights between November and March 2005/2006. From 6099 stars marked as variable, 39 were classified as periodic variable stars and 27 of them are within the CoRoT FOV. We also confirmed the variability for 4 stars listed in GCVS catalog.

CHARACTERIZATION OFCoRoTTARGET FIELDS WITH THE BERLIN EXOPLANET SEARCH TELESCOPE: IDENTIFICATION OF PERIODIC VARIABLE STARS IN THE LRa1 FIELD

In this paper, we report on observations of the CoRoT LRa1 field with the Berlin Exoplanet Search Telescope (BEST). The current paper is part of a series of papers describing the results of our stellar variability survey. The BEST is a small aperture telescope with a wide field of view (FOV). It is dedicated to searching for stellar variability within the target fields of the CoRoT space mission to aid in minimizing false-alarm rates and identify potential targets for additional science. The LRa1 field is CoRoT's second long run field located in the galactic anticenter direction. We observed the LRa1 stellar field on 23 nights between November and March 2005/2006. From 6099 stars marked as variable, 39 were classified as periodic variable stars and 27 of them are within the CoRoT FOV. We also confirmed the variability for four stars listed in the General Catalogue of Variable Stars (GCVS).

Revisiting the population of Galactic open clusters

We present results of a study of the galactic open cluster population based on the all-sky catalogue ASCC-2.5 (I/280A) compiled from Tycho-2, Hipparcos and other catalogues. The sample of optical clusters from ASCC-2.5 is complete up to about 850 pc from the Sun. The symmetry plane of the clusters' distribution is determined to be at Z 0 = -22 ± 4 pc, and the scale height of open clusters is only 56 ± 3 pc. The total surface density and volume density in the symmetry plane are Σ = 114 kpc -2 and D(Z 0 ) = 1015 kpc -3 , respectively. We find the total number of open clusters in the Galactic disk to be of order of 10 5 at present. Fluctuations in the spatial and velocity distributions are attributed to the existence of four open cluster complexes (OCCs) of different ages containing up to a few tens of clusters. Members in an OCC show the same kinematic behaviour, and a narrow age spread. We find, that the youngest cluster complex, OCC 1 (log t < 7.9), with 19 deg inclination to the Galactic plane, is apparently a signature of Gould's Belt. The most abundant OCC 2 complex has moderate age (log t 8.45). The clusters of the Perseus-Auriga group, having the same age as OCC 2, but different kinematics are seen in breaks between Perseus-Auriga clouds. The oldest (log t 8.85) and sparsest group was identified due to a large motion in the Galactic anticentre direction. Formation rate and lifetime of open clusters are found to be 0.23 ± 0.03 kpc -2 Myr -1 and 322 ± 31 Myr, respectively. This implies a total number of cluster generations in the history of the Galaxy between 30 to 40. We estimate that less than about 10% of the total Galactic stellar disk population has ever passed an open cluster membership.

Washington photometry of open cluster giants: two moderately metal-poor anticentre clusters

New photometric data in the Washington system are presented for red giant candidates in NGC 1817 and 2251, two open clusters located towards the Galactic anticentre direction. In the case of NGC 2251, the Washington data are supplemented with new UBV and David Dunlap Observatory (DDO) photoelectric photometry. Published radial velocities are used to separate field stars from cluster giants. The photometric data yield an effective temperature and metal abundance for each cluster member. Five independent Washington abundance indices yield mean metallicities of [Fe/H] = 0.25 ± 0.04 for NGC 1817 and 2251, respectively. From combined BV and DDO data, we also derive E(B − V ) = 0.21 ± 0.03 and [Fe/H]DDO = −0.14 ± 0.05 for NGC 2251. Both objects are then found to be on the metal-poor side of the distribution of open clusters, their metallicities being compatible with the existence of a radial abundance gradient in the disc. Using the WEBDA Open Cluster data base and the available literature, we re-examined the overall properties of a sample of 30 clusters located towards the Galactic anticentre with the distances, ages and metallicities available. This cluster sample presents no evidence of an abundance gradient perpendicular to the Galactic plane, nor is an age‐metallicity relation found. However, a radial abundance gradient of −0.093 dex kpc −1 is derived over a Galactocentric distance of 14 kpc, a gradient which is in keeping with most recent determinations. This value practically does not change when all clusters with basic parameters known up to this date are considered. Ke yw ords: methods: observational ‐ techniques: photometric ‐ open clusters and associations: individual: NGC 1817 ‐ open clusters and associations: individual: NGC 2251 ‐ Galaxy: abundances.

The relatively young, metal-poor and distant open cluster NGC 2324

We have obtained CCD photometry in the Johnson V, Kron-Cousins I and CT1 Washington systems for NGC 2324, a rich open cluster located ∼35 ◦ from the Galactic anticentre direction. We measured V magnitudes and V − I colours for 2865 stars and T1 magnitudes and C − T1 colours for 1815 stars in an area of 13. 6 × 13. 6. The comparison of the cluster colour-magnitude diagrams with isochrones of the Geneva group yield E(V − I) = 0.33 ± 0.07 and V − MV = 13.70 ± 0.15 for log t = 8.65 (t = 440 Myr) and Z = 0.008 ((Fe/H) = −0.40), and E(C − T1) = 0.40 ± 0.10 and T1 − MT 1 = 13.65 ± 0.15 for the same age and metallicity level. The resulting E(V − I) reddening value implies E(B − V) = 0.25 ± 0.05 and a distance from the Sun of (3.8 ± 0.5) kpc. Star counts carried out within and outside the cluster region allowed us to estimate the cluster angular radius as 5. 3 ± 0. 3 (5.9 pc). When using the E(B − V) reddening value here derived and the original Washington photometric data of Geisler et al. (1991) for the stars confirmed as red cluster giants from Coravel radial velocities, we found (Fe/H) = −0.31 ± 0.04, which is in good agreement with the best fits of isochrones. Therefore, NGC 2324 is found to be a relatively young, metal-poor and distant open cluster located beyond the Perseus spiral arm. A comparison of NGC 2324 with 10 well-known open clusters of nearly the same age shows that the cluster metal abundance and its position in the Galaxy are consistent with the existence of a radial abundance gradient of −0.07 dex kpc −1 in the Galactic disc.

The relatively young and metal-poor Galactic open cluster NGC 2194

We present CCD VIKC photometry down to V≈ 21 mag in the field of the rich open cluster NGC 2194, which is projected towards the Galactic anticentre direction. We measured V magnitudes and V−I colours for a total of 2515 stars in a field of 13.6 × 13.6 arcmin2. These data are supplemented with CCD photometry in the C, M and T1 filters of the Washington system and photoelectric CMT1T2 photometry of 20 red giant candidates. Based on the best fits of isochrones computed by the Geneva and Padova groups to the (V, V−I) colour — magnitude diagram, we derive a colour excess E(V−I) = 0.75, a cluster distance of 3.2 kpc and an age of 400 Myr. Five independent Washington abundance indices yield a mean cluster metallicity of [Fe/H]=−0.27 ± 0.06, which is compatible with the existence of both a radial and Z gradient in the Galactic disc. NGC 2194 is a relatively young open cluster, whose deficiency in metal content can be accounted for by the fact that it was born not only far from the Galactic centre in the outer disc, but also at a relatively high Z value.

One ring to encompass them all: a giant stellar structure that surrounds the Galaxy

We present evidence that the curious stellar population found by the Sloan Digital Sky Survey in the Galactic anticentre direction extends to other distant fields that skirt the plane of the Milky Way. New data, taken with the Isaac Newton Telescope Wide Field Camera, show a similar population, narrowly aligned along the line of sight, but with a galactocentric distance that changes from similar to15 to similar to20 kpc (over similar to100degrees on the sky). Despite being narrowly concentrated along the line of sight, the structure is fairly extended vertically out of the plane of the disc, with a vertical scaleheight of 0.75 +/- 0.04 kpc. This finding suggests that the outer rim of the Galaxy ends in a low surface brightness stellar ring. Presently available data do not allow us to ascertain the origin of the structure. One possibility is that it is the wraith of a satellite galaxy devoured long ago by the Milky Way, although our favoured interpretation is that it is a perturbation of the disc, possibly the result of ancient warps. Assuming that the ring is smooth and axisymmetric, the total stellar mass in the structure may amount to similar to2 x 10(8) M. up to similar to10(9) M(.).

Old open clusters: the interesting case of Berkeley 21

ABSTRACT We present CCD BVI photometry of the old open cluster Berkeley 21, one of themost distant clusters in the Galactic anticentre direction, and possibly the lowestmetallicity object in the open clusters sample. Its position and metal abundance makeit very important for the study of the Galactic disc.Using the synthetic Colour - Magnitude Diagram method, we estimate values fordistance modulus (m-M) 0 = 13.4–13.6, reddening E(B−V)= 0.74–0.78 (with possibledifferential absorption), and age = 2.2–2.5 Gyr.Key words: Hertzsprung-Russell (HR) diagram – open clusters and associations:general – open clusters and associations: individual: Berkeley 21 1 INTRODUCTIONOld open clusters cover a large range of distances, metal-licities, and ages (Friel 1995), and that warrants their usein investigations of the chemical and dynamical evolution ofour Galaxy. To study the metallicity and age distribution ofopen clusters with Galactocentric distance, and avoid unnec-essary and dangerous biases, a key requisite is homogenousanalysis of very accurate observational data, as discussed by,e.g, Janes & Phelps (1994, JP94) Carraro & Chiosi (1994,CC94), Friel (1995), Twarog et al. (1997,TAAT97).This is the fifth paper of a series dedicated to the exami-nation of old open clusters of different ages and metallicities,and located at different Galactic radii: for them we measurein a homogenous way distance, age, reddening and metal-licity. These quantities are derived from comparison of theobserved colour-magnitude diagrams (CMDs) to syntheticones generated by a numerical code based on stellar evolu-tion tracks and taking into account theoretical and obser-vational uncertainties (Tosi et al. 1991). These simulationsare much more powerful than the classical isochrone fittingmethod to study the evolutionary status of the analysedregion and have been successfully applied both to nearby ir-regular galaxies (Greggio et al. 1998 and references therein)and to galactic open clusters (NGC2243: Bonifazi et al. 1990;Cr261: Gozzoli et al. 1996; NGC6253: Bragaglia et al. 1997;NGC2506: Marconi et al. 1997).Berkeley 21 (Be21) is located toward the Galactic an-ticentre, at coordinates RA(1950) = 5:48:42, DEC(1950)= 21:46, and l

Schmidt survey in the Galactic anticentre direction. 1. Investigation of open clusters

AbstractA study of four open clusters in the direction of the Galactic anticentre (l = 186°, b = +2°) is presented. In a field of 8.32 square degrees proper motions and B magnitudes for about 79 000 stars down to 19.5 were determined on Tautenburg Schmidt plates. For more than 15 500 of them U magnitudes down to 17.3 could be obtained. Additionally, OCA Schmidt plates were used to determine V, R magnitudes in a larger field of 24.45 square degrees for 271 000 stars down to V = 18.2. For stars brighter than V = 15.5 an accuracy of about 1.5 mas/yr has been estimated for proper motions. The rms errors of stellar magnitudes and colour indices are 0.09 – 0.12 mag.Several open clusters have been already known in this direction of the sky, e.g. NGC 2168 or M 35 (C 0605+243), NGC 2158 (C 0604+241) and IC 2157 (C 0601+240). Inspecting the plates and analysing the colour‐magnitude diagrams and published data, we could identify an additional anonymous cluster C 0605+242 with a projection on the sky near the centre of M 35 but at a larger distance from the Sun.The cluster membership determination was carried out using information on spatial and proper motion distributions of stars in the field. The colour‐magnitude diagrams were derived down to the limiting stellar magnitude. For each cluster the interstellar extinction Av, the diameters of the core and corona, the ages and spatial velocity components (V,W) relative to the LSR in the Y,Z – Galactic directions were determined.The distances to the clusters of 960 pc, 2 600 pc, 2 520 pc and 3 700 pc were obtained for M 35, IC 2157, C 0605+242 and NGC 2158. They show the loci of the clusters in the Local and Perseus spiral arms and at external border of Perseus arm, respectively.

SiO Maser Sources in the Outer Disk of the Galaxy

view Abstract Citations (42) References (59) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS SiO Maser Sources in the Outer Disk of the Galaxy Jiang, B. W. ; Deguchi, S. ; Yamamura, I. ; Nakada, Y. ; Cho, S. H. ; Yamagata, T. Abstract The observations in the 28SiO j = 1-0 V = 1, J = 1-0 V = 2 lines and the 29SiO j = 1-0, V = 0 lines were simultaneously made toward 181 color-selected IRAS sources in the outer disk of the Galaxy (at longitudes between 90° and 250°). The 28SiO lines were detected in 63 sources (56 new) and the 29SiO line in 11 sources (9 new). Most of the detections are optically variable stars, mainly Mira-type variables and a few semiregular variables. The detection rate (35%) is much lower than that of the bulge survey performed by the same observational system the 45 m telescope at Nobeyama Radio Observatory. The lower detection rate of SiO maser emission is partly attributed to the increase of contaminations by stars with C-rich circumstellar envelopes and young stellar objects in the outer disk sample. The kinematical implications of the observational results are also discussed. The luminosity distances to the SiO maser sources are estimated from their IRAS fluxes at 12 μm and colors. By combining the SiO radial velocity and the luminosity distance, the rotation curve of the outer Galactic disk is found to be slightly falling within the Galactocentric distance between 8.5 and 12 kpc. The sources located within 20° from the Galactic anticenter direction are used to check for a peculiar motion of the local standard of rest. Publication: The Astrophysical Journal Supplement Series Pub Date: October 1996 DOI: 10.1086/192345 Bibcode: 1996ApJS..106..463J Keywords: GALAXY: KINEMATICS AND DYNAMICS; ISM: MOLECULES; MASERS; STARS: VARIABLES: OTHER LONG-PERIOD VARIABLES full text sources ADS | data products SIMBAD (182) CDS (2) Related Materials (1) Catalog: 1997yCat..21060463J

How do the Stellar Disc and Thick Disc Stop?

As part of a stellar population sampling program, a series of photometric probes at various field sizes and depths have been obtained in a low extinction window in the galactic anticentre direction. Very deep CCD frames probe the most external parts of the disc, providing strong evidence that the galactic density scale length for the old disc population is rather short (2.5 kpc) and drops abruptly beyond 5.5-6 kpc.Deeper frames in the I band allow to estimate photometric distances and confirm the position of the disk edge. A few stars are found at larger distances. Their number is exactly what we expect if the thick disk does not have any cutoff. We discuss the implications for the formation and evolution of the disc, for the star formation threshold, and for the origin of the thick disc population.

UBV (RI)C-H photometry and GRISM spectroscopy of the young cluster Bochum 2 in the anticentre direction

UBV(RI)C–Hα CCD photometry and GRISM spectroscopy are presented for Bochum 2, a distant cluster in the galactic anticentre direction. The cluster is young, with late O-type members still on the main sequence. No red giant/supergiant is associated with the cluster. 28 possible cluster members are identified. The distance and mean reddening are ~ 6 kpc and E(B – V) = 0.80, respectively. The reddening law for Bochum 2 matches the standard one. The upper main sequence of the cluster is well defined over a range ≥ 6.5 mag, with some spread due to differential reddening. No strong emission line or Wolf-Rayet star has been detected. There is no bright H II region associated with the cluster.

The Stellar Structure of the Orion Spiral ARM in Selected Directions. I. The Galactic Anticenter

To investigate the spatial distribution of various stellar groups in the Hertzsprung-Russell diagram, it is necessary to have homogeneous observational data of MK spectral classification and photometry. This enables us to perform some statistical analyses with sufficient accuracy.This paper reports BV photographic photometry of about 700 stars complete to mV = 12, in the direction of galactic anticenter with (l,b) = (169°, −12.4°), (178°, −2.4°), (184°, 8.9°). For the MK spectral data we used the catalogue by Bartaya and Kharadze (1992), which contains spectral and luminosity classes of 6042 stars in Kapteyn Areas Nos. 44–67. The observational data for this catalogue were obtained using the Abastumani Astrophysical Observatory 70 cm meniscus telescope and an 8°objective prism with dispersion 166 Åmm−1 near Hγ. The errors in classification are equal to ± 0.6 spectral subclasses and ± 0.5 luminosity classes, with a limiting magnitude of mV = 12.5.

The edge of the Galactic disk

view Abstract Citations (84) References (13) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Edge of the Galactic Disk Robin, Annie C. ; Creze, Michel ; Mohan, Vijay Abstract As part of a stellar population sampling program, a series of photometric probes at various field sizes and depths have been obtained in a low extinction window in the galactic anticentre direction. Such data set strong constraints on the radial structure of the disc. At the forefront of this "drilling" program, very deep CCD frames probe the most external parts of the disc. Over the whole effective magnitude range (18 to 25), all contributions in the statistics which should be expected from old disc stars beyond 6 kpc vanish, although such stars dominate by far at distances less than 5 kpc. This is the signature of a sharp cut-off in the star density: the edge of the galactic disc between 5.5 and 6 kpc. As a consequence, the galactic radius does not exceed 14 kpc (assuming $R/-/(//sun/)$=8.5). Colours of elliptical galaxies measured in the field rule out the risk of being misled by undetected extinction. Publication: The Astrophysical Journal Pub Date: November 1992 DOI: 10.1086/186640 arXiv: arXiv:astro-ph/9210001 Bibcode: 1992ApJ...400L..25R Keywords: Galactic Structure; Milky Way Galaxy; Star Distribution; Astronomical Photometry; Charge Coupled Devices; Star Formation; Stellar Magnitude; Astrophysics; GALAXY: STRUCTURE; GALAXY: STELLAR CONTENT; Astrophysics E-Print: 8 pages, Besancon Observatory preprint n. 2 full text sources arXiv | ADS |

A color gradient in the soft X-ray diffuse background

It is shown that the deviations of the soft X-ray diffuse background B band to C band intensity ratio from a constant value can be described as a simple dipole-like variation across the sky. In terms of the observed Wisconsin B/C band intensity ratio, the mean value is 0.355, the dipole magnitude is 0.106, and the positive dipole axis points toward l = 168.7 deg, b = 11.2 deg, almost in the galactic anticenter direction. This gradient in the spectral hardness can be due to several causes; the simplest is a temperature gradient in the X-ray emitting plasma of the local cavity from about 10 exp 6.2 K toward the galactic center to about 10 exp 5.9 K in the anticenter direction. While the physical origin of such a temperature gradient is uncertain, the alignment of the dipole with the higher temperature (and absorbed) Loop I region may be significant.