Galactic Center Survey Research Articles

JASMINE: Near-infrared astrometry and time-series photometry science

Abstract The Japan Astrometry Satellite Mission for INfrared Exploration (JASMINE) is a planned M-class science space mission by the Institute of Space and Astronautical Science, the Japan Aerospace Exploration Agency. JASMINE has two main science goals. One is Galactic archaeology with a Galactic Center survey, which aims to reveal the Milky Way’s central core structure and formation history from Gaia-level (∼25 ${\mu} $as) astrometry in the near-infrared (NIR) Hw band (1.0–1.6 ${\mu} $m). The other is an exoplanet survey, which aims to discover transiting Earth-like exoplanets in the habitable zone from NIR time-series photometry of M dwarfs when the Galactic Center is not accessible. We introduce the mission, review many science objectives, and present the instrument concept. JASMINE will be the first dedicated NIR astrometry space mission and provide precise astrometric information on the stars in the Galactic Center, taking advantage of the significantly lower extinction in the NIR. The precise astrometry is obtained by taking many short-exposure images. Hence, the JASMINE Galactic Center survey data will be valuable for studies of exoplanet transits, asteroseismology, variable stars, and microlensing studies, including discovery of (intermediate-mass) black holes. We highlight a swath of such potential science, and also describe synergies with other missions.

Extraterrestrial Axion Search with the Breakthrough Listen Galactic Center Survey.

Axion dark matter (DM) may efficiently convert to photons in the magnetospheres of neutron stars (NSs), producing nearly monochromatic radio emission. This process is resonantly triggered when the plasma frequency induced by the underlying charge distribution approximately matches the axion mass. We search for evidence of this process using archival Green Bank Telescope data collected in a survey of the Galactic Center in the C band by the Breakthrough Listen project. While Breakthrough Listen aims to find signatures of extraterrestrial life in the radio band, we show that their high-frequency resolution spectral data of the Galactic Center region is ideal for searching for axion-photon transitions generated by the population of NSs in the inner pc of the Galaxy. We use data-driven models to capture the distributions and properties of NSs in the inner Galaxy and compute the expected radio flux from each NS using state-of-the-art ray tracing simulations. We find no evidence for axion DM and set leading constraints on the axion-photon coupling, excluding values down to the level g_{aγγ}∼10^{-11} GeV^{-1} for DM axions for masses between 15 and 35 μeV.

Constraints on very high energy gamma-ray emission from the Fermi bubbles with future ground-based experiments

The origin of sub-TeV gamma rays detected by the \textit{Fermi}-Large Area Telescope (LAT) from the \textit{Fermi} Bubbles (FBs) at the Galactic center is still uncertain. In a hadronic model, acceleration of protons and/or nuclei and their subsequent interactions with gas in the bubble volume can produce the observed gamma rays. Recently the High Altitude Water Cherenkov (HAWC) observatory reported an absence of gamma-ray excess from the Northern FB at $b\gtrsim 6^\circ$ Galactic latitude, which resulted in flux upper limits in the energy range of $1.2-126$ TeV. These upper limits are consistent with the gamma-ray spectrum measured by \textit{Fermi}-LAT at $|b|\ge 10^\circ$, where an exponential cutoff at energies $\gtrsim 100$ GeV is evident. However, the FB gamma-ray spectrum at $|b|\le 10^\circ$, without showing any sign of cutoff up to around 1 TeV in the latest results, remains unconstrained. The upcoming Cherenkov Telescope Array (CTA) will perform a Galactic center survey with unprecedented sensitivity in the energy between 20 GeV and 300 TeV. In this work, we perform both morphological and classic on/off analyses with planned CTA deep central and extended survey and estimate the sensitivity of CTA to the FB hadronic gamma-ray flux models that best fit the spectrum at $|b|\le 10^\circ$ and whose counterpart neutrino flux model best fits the optimistic neutrino spectrum from IceCube Neutrino Observatory. We also perform sensitivity analysis with a future ground-based Cherenkov detector the Large High Altitude Air Shower Observatory (LHAASO). We find that CTA will be able to discover or constrain the FB gamma-ray flux at $|b|\le 10^\circ$ in the $\approx$ 200 GeV -- 100 TeV range with planned observation strategy, while LHAASO may constrain emission in the $\approx 100$~GeV -- 100~TeV range if $\lesssim 10\%$ systematic uncertainties can be achieved.

Cloud–cloud collision in the Galactic center 50 km s−1 molecular cloud

Abstract We performed a search of star-forming sites influenced by external factors, such as SNRs, H ii regions, and cloud–cloud collisions (CCCs), to understand the star-forming activity in the Galactic center region using the NRO Galactic Center Survey in SiO v = 0, J = 2–1, H13CO+J = 1–0, and CS J = 1–0 emission lines obtained with the Nobeyama 45 m telescope. We found a half-shell-like feature (HSF) with a high integrated line intensity ratio of ∫TB(SiO v = 0, J = 2–1)dv/∫TB(H13CO+J = 1–0)dv ∼ 6–8 in the 50 km s−1 molecular cloud; the HSF is a most conspicuous molecular cloud in the region and harbors an active star-forming site where several compact H ii regions can be seen. The high ratio in the HSF indicates that the cloud contains huge shocked molecular gas. The HSF can be also seen as a half-shell feature in the position–velocity diagram. A hypothesis explaining the chemical and kinetic properties of the HSF is that the feature originates from a CCC. We analyzed the CS J = 1–0 emission line data obtained with the Nobeyama Millimeter Array to reveal the relation between the HSF and the molecular cloud cores in the cloud. We made a cumulative core mass function (CMF) of the molecular cloud cores within the HSF. The CMF in the CCC region is not truncated at least up to ∼2500 M⊙, although the CMF of the non-CCC region reaches the upper limit of ∼1500 M⊙. Most massive molecular cores with Mgas > 750 M⊙ are located only around the ridge of the HSF and adjoin the compact H ii region. These may be a sign of massive star formation induced by CCCs in the Galactic center region.

Comparison of 3.6–8.0 μmSpitzer/IRAC Galactic Center Survey Point Sources withChandraX‐Ray Point Sources in the Central 40 × 40 Parsecs

We have studied the correlation between 2357 Chandra X-ray point sources in a 40 × 40 pc field and ~20,000 infrared sources we observed in the corresponding subset of our 2° × 1.4° Spitzer/IRAC Galactic Center Survey at 3.6-8.0 μm, using various spatial and X-ray hardness thresholds. The correlation was determined for source separations of less than 0.5'', 1'', or 2''. Only the soft X-ray sources show any correlation with infrared point sources on these scales, and that correlation is very weak. The upper limit on hard X-ray sources that have infrared counterparts is <1.7% (3 σ). However, because of the confusion limit of the IR catalog, we only detect IR sources with absolute magnitudes ≲1. As a result, a stronger correlation with fainter sources cannot be ruled out. Only one compact IR source, IRS 13, coincides with any of the dozen prominent X-ray emission features in the 3 × 3 pc region centered on Sgr A*, and the diffuse X-ray and IR emission around Sgr A* seems to be anticorrelated on a few-arcsecond scale. We compare our results with previous identifications of near-infrared companions to Chandra X-ray sources.

A Statistical Analysis of Point-like Sources in the Chandra Galactic Center Survey

{\it Chandra} Galactic Center Survey detected $\sim 800$ X-ray point-like sources in the $2^{\circ} \times 0.8^{\circ}$ sky region around the Galactic Center. In this paper, we study the spatial and luminosity distributions of these sources according to their spectral properties. Fourteen bright sources detected are used to fit jointly an absorbed power-law model, from which the power-law photon index is determined to be $\sim$2.5. Assuming that all other sources have the same power-law form, the relation between hardness ratio and HI column density $N_H$ is used to estimate the $N_H$ values for all sources. Monte Carlo simulations show that these sources are more likely concentrated in the Galactic center region, rather than distributed throughout the Galactic disk. We also find that the luminosities of the sources are positively correlated with their HI column densities, i.e. a more luminous source has a higher HI column density. From this relation, we suggest that the X-ray luminosity comes from the interaction between an isolated old neutron star and interstellar medium (mainly dense molecular clouds). Using the standard Bondi accretion theory and the statistical information of molecular clouds in the Galactic center, we confirm this positive correlation and calculate the luminosity range in this scenario, which is consistent with the observation ($10^{32}\sim 10^{35}$ ergs s$^{-1}$).

Comparison of Spitzer/IRAC Galactic Center 3.6-8.0 μum survey results with X-ray emission in the central 40 × 40 parsecs

We have studied the correlation between 2357 Chandra X-ray point sources identified by Muno et al. (2003) in a 40 × 40 parsec field and ∼20,000 infrared sources we observed in the corresponding subset of our 2 × 1.5 degree Spitzer/IRAC Galactic Center Survey at 3.6-8.0 μm (see Stolovy et al.; this conference), using various spatial and brightness thresholds. The correlation was determined for source separations of less than 0.5, 1.0 or 2.0 arcsec. No significant correlation was found between the X-ray and infrared point sources on these scales. Only one compact infrared source, IRS 13, can be identiffed with any of the dozen prominent X-ray emission features in the 3 × 3 parsec region centered on Sgr A*, and the diffuse X-ray and infrared emission around Sgr A* seems to be anti-correlated on a few-arcsecond scale. We compare our results with previous identifications of near infrared companions to Chandra X-ray sources.

Confirmation of New Supernova Remnants near the Galactic Centre

We have made deep observations with the GMRT at 330 MHz of seven, yet- unclassified objects in the Molonglo Galactic Centre Survey (MGCS) in order to confirm the nature of these sources. Based on morphology and non-thermal spectral indices, five of them are now confirmed to be SNRs. This makes the number density of SNRs in the region a factor of two higher than expected from a smooth distribution of SNRs in the Galaxy.

Unveiling the nature of the highly absorbed X-ray source SAX J1748.2-2808 with XMM-Newton

We report on the results of an EPIC XMM-Newton observation of the faint source SAXJ1748.2-2808 and the surrounding field. This source was discovered during the BeppoSAX Galactic center survey performed in 1997-1998. A spatial analysis resulted in the detection of 31 sources within the EPIC field of view. SAXJ1748.2-2808 is clearly resolved into 2 sources in EPIC images with the brighter contributing almost 80% of the 2-10keV flux. Spectral fits to this main source are consistent with an absorbed power-law with a photon index of 1.4+/-0.5 and absorption equivalent to 14E22cm-2 together with an iron line at 6.6+/-0.2 keV with an equivalent width of ~780eV. The significantly better statistics of the XMM-Newton observation, compared with BeppoSAX,allows to exclude a thermal nature for the X-ray emission. A comparison with other observations of SAXJ1748.2-2808 does not reveal any evidence for spectral or intensity long-term variability. Based on these properties we propose that the source is a low-luminosity high-mass X-ray binary located in the Galactic center region.

High-energy observations of the state transition of the X-ray nova and black hole candidate XTE J1720-318

We report the results of extensive high-energy observations of the X-ray transient and black hole candidate XTE J1720-318 performed with INTEGRAL, XMM-Newton and RXTE. The source, which underwent an X-ray outburst in 2003 January, was observed in February in a spectral state dominated by a soft component with a weak high-energy tail. The XMM-Newton data provided a high column density Nh of 1.2*e22 cm^{-2} which suggests that the source lies at the Galactic Centre distance. The simultaneous RXTE and INTEGRAL Target of Opportunity observations allowed us to measure the weak and steep tail, typical of a black-hole binary in the so-called High/Soft State. We then followed the evolution of the source outburst over several months using the INTEGRAL Galactic Centre survey observations. The source became active again at the end of March: it showed a clear transition towards a much harder state, and then decayed to a quiescent state after April. In the hard state, the source was detected up to 200 keV with a power law index of 1.9 and a peak luminosity of 7*e36 erg s^{-1} in the 20-200 keV band, for an assumed distance of 8 kpc. We conclude that XTE J1720-318 is indeed a new member of the black hole X-ray novae class which populate our galactic bulge and we discuss its properties in the frame of the spectral models used for transient black hole binaries.

A New Population of X-ray Transients in the Galactic Center

A comparison of the XMM-Newton and Chandra Galactic Center Surveys has revealed two faint X-ray transients with contrasting properties. The X-ray spectrum of XMM J174544−2913.0 shows a strong iron line with an equivalent width of ∼2 keV, whereas that of XMM J174457−2850.3 is characterized by a very hard continuum with photon index ∼1.0. The X-ray flux of both sources varied by more than two orders of magnitude over a period of months with a peak X-ray luminosity of 5 × 1034 ergs s−1. We discuss the nature of these peculiar sources.

X-ray Emission from Galactic Plane

We report several important results obtained from recent Galactic X-ray survey observations, in particular ASCA Galactic center and plane surveys and our Chandra deep survey on the (l,b) ≈ (255, 00) region. Strong hard X-ray diffuse components are observed from Galactic ridge, center and bulge, and they have both thermal and non-thermal spectral components. Dozens of discrete and extended sources have been discovered on the Galactic plane, which also indicate thermal and/or non-thermal X-ray energy spectra. They are often associated with radio sources and are considered to be SNR candidates. Most of the hard X-ray point sources in the outer part of the Galactic plane are considered to be background AGNs, while fraction of the Galactic hard X-ray sources (such as quiescent dwarf novae) increases toward the Galactic center. Most of the soft X-ray sources on the Galactic plane are presumably nearby active stars.

Galactic X‐ray surveys

AbstractWe review highlights of the results obtained from recent Galactic X‐ray survey observations, in particular ASCA Galactic center and plane surveys and our Chandra deep survey on the (l, b) ≈ (28°.5, 0°.0) region. Strong hard X‐ray diffuse components are observed from Galactic ridge, center and bulge, and they have both thermal and non‐thermal spectral components. Dozens of discrete and extended sources have been discovered on the Galactic plane, which also indicate thermal and/or non‐thermal X‐ray energy spectra. They are often associated with radio sources and are considered to be SNR candidates. Most of the hard X‐ray point sources in the outer part of the Galactic plane are considered to be background AGNs, while fraction of the Galactic hard X‐ray sources (such as quiescent dwarf novae) increases toward the Galactic center. Most of the soft X‐ray sources on the Galactic plane are presumably nearby active stars.

Discovery of a Slow X-Ray Pulsator, AX J1740.1-2847, in the Galactic Center Region

We report the discovery of an X-ray pulsar, AX J1740.1–2847, from the Galactic center region. This source was found as a faint hard X-ray object on 1998 September 7–8 with the ASCA Galactic center survey observation. Then, coherent pulsations of P = 729± 14 s period were detected. The X-ray spectrum is described by a flat power-law of ∼ 0.7 photon index. The large absorption column of log NH ∼ 22.4 cm−2 indicates that AX J1740.1–2847 is a distant source, larger than 2.4 kpc, and possibly near to the Galactic center region. The luminosity in the 2–10 keV band is larger than 2.5×1033 erg s−1, or likely to be 3.2×1034 erg s−1 at the Galactic-center distance. Although the slow-pulse period does not discriminate whether AX J1740.1–2847 is a white dwarf or neutron-star binary, the flat power-law and moderate luminosity strongly favor a neutron-star binary.

X-ray spectrum of the black hole candidate X1755-338

We report the first detection of a hard power-law tail in the X-ray spectrum of the black hole candidate (BHC) binary X1755-338, which was observed in 1989 March-September during the TTM Galactic Centre survey. In addition, an ultrasoft thermal component with a temperature of ~1.1-1.4 keV was also detected. We demonstrate that the soft and hard X-ray components of X1755-338 vary independently, as in the spectra of the well known BHCs LMC X-1, LMC X-3 and GX339-4 in their high (intensity) state. If the hard tail observed from X1755-338 is generated near the black hole by energetic electrons up-scattering low energy photons, the un-correlated variations imply that the soft X-rays from accretion disc may not be the main photon seeds needed for inverse Comptonization. The TTM observations strongly suggest that X1755-338 does indeed belong to the family of BHCs.

TTM Observations of X1755-338

X1755-338 is an X-ray binary source which displays X-ray dips with a 4.4 hour period (White et al. 1984). It was previously noted as an unusually soft X-ray source by Jones (1977) and was suggested later as a black-hole candidate (BHC) by White &amp; Marshall (1984), and White et al. (1984), based on the similarity of its location in an X-ray colour-colour diagram to that of a group of BHCs.The TTM is a coded-mask imaging spectrometer on board the KVANT module of the MIR space station. It is capable of producing images in the 2 – 30 keV band with an energy resolution of about 18% at 6 keV. The instrumental details are given in Brinkman et al. (1985).We observed X1755-338 in 1989 March-September during the period of the TTM Galactic Centre Survey.

The MOST Galactic Center Survey - II. New results on published supernova remnants and G2.4 + 1.4

The MOST Galactic Center Survey - II. New results on published supernova remnants and G2.4 + 1.4