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Added by
Hiromitsu Takahashi
Publication date
2001
fields
Physics
and research's language is
English
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Using ASCA, spatially integrated X-ray spectra of the central regions of M31 were studied. Data were accumulated over three different circular regions, with the radii of 3, 6 and 12, all centered on the nucleus. The spectra are relatively similar among the three regions. In the energy range above 1.5 keV, the spectra are reproduced by a combination of a disk black-body component and a black-body component, implying that the emission mainly comes from an assembly of low-mass X-ray binaries. In energies below 1.5 keV, the spectra involves two additional softer components, expressed with thin-thermal plasma emission models of temperatures ~ 0.9 keV and ~ 0.3 keV. Over the central 12 (2.4 kpc) region and in the 0.5-10 keV energy band, the binary component has a luminosity of 2.6 x 10^{39} erg/s, while the two softer components both exhibit luminosities of ~ 2 x 10^{38} erg/s. These results are compared with those from other missions, including Chandra and XMM-Newton in particular. Discussion is made on the nature of the two softer spectral components besides the binary one.
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Images obtained with NIRI on the Gemini North telescope are used to investigate the photometric properties of the central regions of M31 in the 3 - 5 micron wavelength range. The light distribution in the central arcsecond differs from what is seen in the near-infrared in the sense that the difference in peak brigh tness between P1 and P2 is larger in M than in K; no obvious signature of P3 is dete cted in M. These results can be explained if there is a source of emission that contributes ~ 20% of the peak M light of P1 and has an effective temperature of no more than a few hundred K that is located between P1 and P2. Based on the red K-M color of this source, it is suggested that the emission originates in a circumstellar dust shell surrounding a single bright AGB star. A similar bright source that is ~ 8 arcsec from the center of the galaxy is also detected in M. Finally, the (L, K-L) color-magnitude diagram of unblended stars shows a domin ant AGB population with photometric characteristics that are similar to those of the most luminous M giants in the Galactic bulge.
The archival XMM-Newton data of the central region of M31 were analyzed for diffuse X-ray emission. Point sources with the 0.5--10 keV luminosity exceeding $sim 4 times 10^{35}$ erg s$^{-1}$ were detected. Their summed spectra are well reproduced by a combination of a disk black-body component and a black-body component, implying that the emission mainly comes from an assembly of luminous low-mass X-ray binaries. After excluding these point sources, spectra were accumulated over a circular region of $6arcmin$ (1.2 kpc) centered on the nucleus. In the energy range above 2 keV, these residual spectra are understood mainly as contributions of unresolved faint sources and spill-over of photons from the excluded point sources. There is in addition a hint of a $sim 6.6$ keV line emission, which can be produced by a hot (temperature several keV) thin-thermal plasma. Below 2 keV, the spectra involve three additional softer components expressed by thin-thermal plasma emission models, of which the temperatures are $sim 0.6$, $sim 0.3$, and $sim 0.1$ keV. Their 0.5--10 keV luminosities within 6$arcmin$ are measured to be $sim 1.2 times 10^{38}$ erg s$^{-1}$, $sim 1.6 times 10^{38}$ erg s$^{-1}$, and $sim 4 times 10^{37}$ erg s$^{-1}$ in the order of decreasing temperature. The archival Chandra data of the central region of M31 yielded consistent results. By incorporating different annular regions, all the three softer thermal components were confirmed to be significantly extended. These results are compared with reports from previous studies. A discussion is presented on the origin of each thermal emission component.
We continue the analysis of the dataset of our spectroscopic observation campaign of M31, by deriving simple stellar population properties (age metallicity and alpha-elements overabundance) from the measurement of Lick/IDS absorption line indices. We describe their two-dimensional maps taking into account the dust distribution in M31. 80% of the values of our age measurements are larger than 10 Gyr. The central 100 arcsec of M31 are dominated by the stars of the classical bulge of M31. They are old (11-13 Gyr), metal-rich (as high as [Z/H]~0.35 dex) at the center with a negative gradient outwards and enhanced in alpha-elements ([alpha/Fe]~ 0.28+- 0.01 dex). The bar stands out in the metallicity map, where an almost solar value of [Z/H] (~0.02+-0.01 dex) with no gradient is observed along the bar position angle (55.7 deg) out to 600 arcsec from the center. In contrast, no signature of the bar is seen in the age and [alpha/Fe] maps, that are approximately axisymmetric, delivering a mean age and overabundance for the bar and the boxy-peanut bulge of 10-13 Gyr and 0.25-0.27 dex, respectively. The boxy/peanut-bulge has almost solar metallicity (-0.04+- 0.01 dex). The mass-to-light ratio of the three components is approximately constant at M/LV ~ 4.4-4.7 Msol/Lsol. The disk component at larger distances is made of a mixture of stars, as young as 3-4 Gyr, with solar metallicity and smaller M/LV (~3+-0.1 Msol/Lsol). We propose a two-phase formation scenario for the inner region of M31, where most of the stars of the classical bulge come into place together with a proto-disk, where a bar develops and quickly transforms it into a boxy-peanut bulge. Star formation continues in the bulge region, producing stars younger than 10 Gyr, in particular along the bar, enhancing its metallicity. The disk component appears to build up on longer time-scales.
We summarize the results of an X-ray spectroscopic survey of radio-loud AGNs observed with ASCA, using proprietary and archival data (public up to 1998 September). We briefly compare our results with those obtained for radio-quiet AGNs studied by other authors, and with the predictions of unified models.
ASCA observations of 4 high redshift radio--loud quasars with 1.44$<$z$<$3.21 are presented. The spectral analysis for three of them (PKS 0332-403, PKS 0537-286, PKS 2149-306) reveals that their X-ray continuum emission is well represented by a simple power--law model plus absorption with photon indices of $Gamma =1.92^{+0.30}_{-0.20}$ (PKS 0332-403), $Gamma =1.63^{+0.14}_{-0.12}$ (PKS 0537-286) and $Gamma =1.57pm 0.05$ (PKS 2149-306). The fourth and most distant object, PKS 1614+051 at z=3.21, was detected, but a detailed spectral analysis is impossible due to the small number of photons. We find evidence for excess absorption above the Galactic $N_H$--value in the ASCA data of PKS 2149-306, which is not confirmed by the ROSAT All-Sky Survey PSPC spectrum of this source. This could probably be due to variable absorption. The ROSAT spectrum of PKS 0537-286, deduced from a 10 ksec pointed PSPC observation, is consistent with the ASCA results. Thermal bremsstrahlung models also give acceptable fits to the ASCA data with best fit (rest frame) temperatures of 10.4, 33.5 and 45.8 keV for PKS 0332-403, PKS 0537-286 and PKS 2149-306, respectively. More complicated models for the X-ray continuum are not required, in particular, tight upper limits on the strength of the Fe-K emission line are given. The broad band spectral energy distributions from the radio to the $gamma$-rays are presented and discussed.
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