Discrete X-ray Sources Research Articles

On the detection of pre-low-mass X-ray binaries

We explore the population of candidate pre-low-mass X-ray binaries in which a neutron star accretes mass from the wind of a low-mass companion (mass < 2Msun) in the framework of a binary population synthesis study. The simulated accretion-luminosity distribution shows a primary peak close to 1e31 erg/s and a secondary peak near 1e28 erg/s. The relative contribution of the two peaks depends primarily on the magnitude of the kick velocity imparted to the neutron star at birth. The secondary peak is negligible for average kick velocities larger than 200 km/s, but becomes dominant for average kick velocities smaller than 50 km/s. Regardless of the relative contributions of the two peaks, our calculations suggest that pre-low-mass X-ray binaries may provide a non-negligible contribution to the population of discrete low-luminosity X-ray sources in the Galaxy.

ChandraObservation of the X‐Ray Source Population of NGC 6946

We present the results of a study of discrete X-ray sources in NGC 6946 using a deep Chandra ACIS observation. Based on the slope of the log N- log S distribution and the general correlation of sources with the spiral arms, we infer that the overall discrete source sample in NGC 6946 is dominated by high-mass X-ray binaries, in contrast to the source distributions in M31 and the Milky Way. This is consistent with the higher star formation rate in NGC 6946 than in those galaxies. We find that the strong X-ray sources in the region of the galactic center do not correlate in detail with images of the region in the near-IR, although one of them may be coincident with the galactic center. The noncentral ultraluminous X-ray source in NGC 6946, previously identified with a supernova remnant, has an X-ray spectrum and luminosity that is inconsistent with either a traditional pulsar wind nebula or a blast wave remnant.

Review of Discrete X-Ray Sources in the Small Magellanic Cloud: Summary of the ASCA Results and Implication on the Recent Star-Forming Activity

Abstract We made 22 observations on the Small Magellanic Cloud (SMC) and covered full regions of the main body and the eastern wing by the end of the ASCA mission. We detected 106 discrete sources with a criterion of $\mathrm{S/N} \gt 5$$\mathrm{S/N} \gt 5$ and performed systematic analyses on all of the sources. We determined the source positions with an $\sim {40{}^{\mathrm {\prime \prime }}}$ error radius (90% confidence) for sources detected in the central ${20{}^{\mathrm {\prime }}}$ radius of the GIS. We detected coherent pulsations from 17 sources. Among them, eight were newly discovered during this study. We classified most of these pulsars as X-ray binary pulsars (XBPs) based on their properties, such as the flux variability and the existence of an optical counterpart. We detected X-ray emission from eight supernova remnants (SNRs). Among them, five SNRs showed emission lines in their spectra, hence we regarded the five as thermal SNRs. We found that XBPs and thermal SNRs in the SMC can be clearly separated by their spectral hardness ratio. Applying this empirical law to faint (thus unclassified) sources, we found 19 XBP candidates and four thermal SNR candidates. We also found several tens of candidates for active galactic nuclei, both from the hardness ratio and the $\log N \hbox{--}\log S$ relation of extragalactic sources. Based on these ASCA results and further information from ROSAT, SAX, RXTE, CGRO, Chandra, and XMM-Newton, we compiled comprehensive catalogues of discrete X-ray sources in the Small Magellanic Cloud. Using the catalogues, we derived the spatial distributions of XBPs and SNRs. XBPs and SNRs were found to be concentrated in the main body and eastern wing, which resembles the distribution of young stars with ages of $\sim 2 \times 10^7 \,\mathrm{yr}$. By comparing the source populations in the SMC and our Galaxy, we suggest that the star-forming rate (per unit mass) in the SMC was much higher than the Galaxy $\sim 10^7 \,\mathrm{yr}$ ago. We also discuss the recent change of the star-forming rate in the SMC.

A Chandra observation of the interacting pair of galaxies NGC 4485/44901

We report the results of a 20-ks Chandra ACIS-S observation of the galaxy pair NGC 4485/4490. This is an interacting system containing a late-type spiral with an enhanced star formation rate (NGC 4490), and an irregular companion that possesses a disturbed morphology. A total of 29 discrete X-ray sources are found coincident with NGC 4490, but only one is found within NGC 4485. The sources range in observed X-ray luminosity from ∼ 2 × 10 37 to 4 × 10 39 erg s −1 . The more luminous sources appear, on average, to be spectrally harder than the fainter sources, an effect that is attributable to increased absorption in their spectra. Extensive diffuse X-ray emission is detected coincident with the disc of NGC 4490, and in the tidal tail of NGC 4485, which appears to be thermal in nature and hence the signature of a hot interstellar medium in both galaxies. However, the diffuse component accounts for only ∼10 per cent of the total X-ray luminosity of the system (2 × 10 40 erg s −1 , 0.5‐8 keV), which arises predominantly in a handful of the brightest discrete sources. This diffuse emission fraction is unusually low for a galaxy pair which has many characteristics that would lead it to be classified as a starburst system, possibly as a consequence of the small gravitational potential well of the system. The discrete source population, on the other hand, is similar to that observed in other starburst systems, possessing a flat luminosity function slope of ∼− 0.6 and a total of six ultraluminous X-ray sources (ULX). Five of the ULX are identified as probable black hole X-ray binary systems, and the sixth (which is coincident with a radio continuum source) is identified as an X-ray luminous supernova remnant. The ULX all lie in star formation regions, providing further evidence of the link between the ULX phenomenon and active star formation. Importantly, this shows that even in star-forming regions, the ULX population is dominated by accreting systems. We discuss the implications of this work for physical models of the nature of ULX, and in particular how it argues against the intermediate-mass black hole hypothesis.

Luminous X-ray sources in spiral and star-forming galaxies.

For studies of discrete X-ray source populations in nearby galaxies, high spatial resolution is a key to making progress. Now, for the first time, using the Chandra X-ray observatory, we are able to study these source populations in detail for galaxies beyond M31 and our local group galaxies. Analysis of accretion-driven and supernova-related discrete sources provides a new perspective on the evolution of galactic stellar populations, as well as giving insights into the physical mechanisms operating in individual cases. A particularly intriguing area, which we are only just beginning to address, is the nature of the most X-ray-luminous sources that are being discovered in many spiral and star-forming galaxies.

On the Population of Wind-accreting Neutron Stars in the Galaxy

We explore the possibility that neutron stars accreting from the winds of main-sequence stellar companions account for a significant fraction of low-luminosity, hard X-ray sources (LX 1035 ergs s-1; 1-10 keV) in the Galaxy. This work was motivated by recent Chandra observations of the Galactic center by Wang, Gotthelf, & Lang. Our calculations indicate that many of the discrete X-ray sources detected in this survey may be wind-accreting neutron stars and that many more may be discovered with deeper X-ray observations. We propose that an infrared observing campaign be undertaken to search for the stellar counterparts of these X-ray sources. If future observations reveal that a large fraction of the X-ray sources are wind-accreting neutron stars, this should provide an important calibration point for massive binary population synthesis studies.

Three LINERs under theChandraX‐Ray Microscope

We use observations of three galaxies hosting low-ionization nuclear emission-line regions (LINERs; NGC 404, NGC 4736, and NGC 4579) with the Chandra X-Ray Observatory to study their power sources. We find very diverse properties within this small group: NGC 404 has an X-ray faint nucleus with a soft, thermal spectrum; NGC 4736 harbors a plethora of discrete X-ray sources in and around its nucleus; and NGC 4579 has a dominant nuclear point source embedded in a very extended, diffuse nebulosity. From their multiwavelength properties we conclude the following about the power sources in these LINERs: the nucleus of NGC 404 is the site of a weak, compact starburst, whose X-ray emission is due to gas heated by stellar winds and supernovae; NGC 4736 appears to be in a recent or aging starburst phase, where the X-ray emission is dominated by a dense cluster of X-ray binaries; and NGC 4579 is powered by accretion onto a supermassive black hole. We detect 39 discrete sources in NGC 4736 and 21 in NGC 4579, most with LX > 1037 ergs s-1. One source in the disk of NGC 4579 appears to be an ultraluminous X-ray binary with LX(2-10 keV) = 9 × 1039 ergs s-1, but it could also be a background quasar. The most luminous discrete sources have simple power-law spectra, which along with their luminosities suggest that these are X-ray binaries accreting near or above the Eddington rate for a neutron star. By comparing the luminosity functions of discrete X-ray sources in these and other galaxies, we find a potential connection between the age of the stellar population and the slope of the cumulative X-ray luminosity function: galaxies with primarily old stellar populations have steeper X-ray luminosity functions than starburst galaxies. We suggest that this difference results from the contribution of high-mass X-ray binaries from the young stellar population to the upper end of the luminosity function.

ROSATPSPC observations of nearby spiral galaxies - II. Statistical properties

We present a statistical analysis of the largest X-ray survey of nearby spiral galaxies in which diffuse emission has been separated from discrete source contributions. Regression and rank-order correlation analyses are used to compare X-ray properties, such as total, source and diffuse luminosities and diffuse emission temperature, with a variety of physical and multiwavelength properties, such as galaxy mass, type and activity, and optical and infrared luminosity. The results are discussed in terms of the way in which hot gas and discrete X-ray sources scale with the mass and activity of galaxies, and with the star formation rate. We find that the X-ray properties of starburst galaxies are dependent primarily on their star-forming activity, whilst for more quiescent galaxies, galaxy mass is the more important parameter. One of the most intriguing results is the tight linear scaling between far-infrared and diffuse X-ray luminosity across the sample, even though the hot gas changes from a hydrostatic corona to a free wind across the activity range sampled here.

ChandraX‐Ray Sources in M101

A deep (98.2 ks) Chandra Cycle 1 observation has revealed a wealth of discrete X-ray sources as well as diffuse emission in the nearby face-on spiral galaxy M101. From this rich data set we have created a catalog of the 110 sources from the S3 chip detected with a significance of greater than 3 σ. This detection threshold corresponds to a flux of ~10-16 ergs cm-2 s-1 and a luminosity of ~1036 ergs s-1 for a distance to M101 of 7.2 Mpc. The sources display a distinct correlation with the spiral arms of M101 and include a variety of X-ray binaries, supersoft sources, supernova remnants, and other objects of which only ~27 are likely to be background sources. There are only a few sources in the interarm regions, and most of these have X-ray colors consistent with that of background active galactic nuclei. The derived log N- log S relation for the sources in M101 (background subtracted) has a slope of -0.80 ± 0.05 over the range of 1036-1038 ergs s-1. The nucleus is resolved into two nearly identical X-ray sources, each with a 0.5-2.0 keV flux of 4 × 1037 ergs s-1. One of these sources coincides with the optical nucleus, and the other coincides with a cluster of stars 110 pc to the south. The field includes 54 optically identified supernova remnants (SNRs), of which 12 are detected by Chandra. Two of the SNR sources are variable and hence must be compact objects. In total, eight of the X-ray sources show evidence for short-term temporal variation during this observation. Two of these variable sources are now brighter than the ROSAT detection threshold, but they were not detected in the previous ROSAT observations taken in 1992 and 1996. There are also two variable sources previously seen with ROSAT that apparently have faded below the Chandra detection threshold. The brightest source in the field shows extreme long-term and short-term temporal variability. At its peak brightness it has a super-Eddington luminosity greater than 1039 ergs s-1. There are 10 supersoft sources (SSSs) in the field, which can be divided into two distinct subclasses: the brighter class (three objects) has a luminosity of ~1038 ergs s-1 and a blackbody temperature of ~70 eV, whereas the other class (seven objects) is an order of magnitude fainter and has a blackbody temperature of only ~50 eV.

X-ray luminosities of galaxies in groups

We have derived the X-ray luminosities of a sample of galaxies in groups, making careful allowance for contaminating intragroup emission. The L_X:L_B and L_X:L_{FIR} relations of spiral galaxies in groups appear to be indistinguishable from those in other environments, however the elliptical galaxies fall into two distinct classes. The first class is central-dominant group galaxies which are very X-ray luminous, and may be the focus of group cooling flows. All other early-type galaxies in groups belong to the second class, which populates an almost constant band of L_X/L_B over the range 9.8 < log L_B < 11.3. The X-ray emission from these galaxies can be explained by a superposition of discrete galactic X-ray sources together with a contribution from hot gas lost by stars, which varies a great deal from galaxy to galaxy. In the region where the optical luminosity of the non-central group galaxies overlaps with the dominant galaxies, the dominant galaxies are over an order of magnitude more luminous in X-rays. We also compared these group galaxies with a sample of isolated early-type galaxies, and used previously published work to derive L_X:L_B relations as a function of environment. The non-dominant group galaxies have mean L_X/L_B ratios very similar to that of isolated galaxies, and we see no significant correlation between L_X/L_B and environment. We suggest that previous findings of a steep L_X:L_B relation for early-type galaxies result largely from the inclusion of group-dominant galaxies in samples.

TheBeppoSAXView of the X‐Ray Active Nucleus of NGC 4258

BeppoSAX observed the Seyfert 1.9 galaxy NGC 4258 in 1998 December, when its 2-10 keV luminosity was about 1041 ergs s-1. Large amplitude (100%) variability is observed in the 3-10 keV band on timescales of a few tens of thousands of seconds, while variability of ~20% is observed on timescales as short as 1 hr. The nuclear component is visible above 2 keV only, being obscured by a column density of (9.5 ± 1.2) × 1022 cm-2; this component is detected at up to 70 keV with a signal-to-noise ratio of 3 and with a steep power-law energy spectral index of αE = 1.11 ± 0.14. Bremsstrahlung emission for the 2-70 keV X-ray luminosity, as expected in advection-dominated accretion flow models with strong winds, is ruled out by the data. The ratio between the nuclear radio (22 GHz) luminosity and the X-ray (5 keV) luminosity is consistent with that of radio-quiet quasars and Seyfert galaxies. X-ray variability, spectral shape, and radio/X-ray and near-IR/X-ray luminosity ratios suggest that the nucleus of NGC 4258 could be a scaled down version of a Seyfert nucleus and that the X-ray nuclear luminosity can be explained in terms of Comptonization in a hot corona. The soft (E 2 keV) X-ray emission is complex. There are at least two thermal-like components with temperatures of 0.6 ± 0.1 keV and 1.3 keV. The cooler (L0.1-2.4 keV~ 1040 ergs s-1) component is probably associated with the jet, resolved in X-rays by the ROSAT HRI (Cecil et al. 1994). The luminosity of the second component, which can be modeled equally well by an unobscured power-law model with αE = 0.2, is L0.1-2.4 keV~ 7 × 1039 ergs s-1, consistent with that expected from discrete X-ray sources (binaries and supernova remnants) in the host galaxy. Observations of NGC 4258 and other maser active galactic nuclei (AGNs) show strong nuclear X-ray absorption. We propose that this large column of gas might be responsible for shielding the regions of water maser emission from X-ray illumination. So a large column density absorbing gas may be a necessary property of masing AGNs.

Do LINER 2 galaxies harbour low-luminosity active galactic nuclei?

We use ROSAT HRI spatial data and ASCA spectral measurements for a sample of seven nearby, early type spiral galaxies, to address the question of whether a low-luminosity Active Galactic Nucleus (LLAGN) is present in galaxies that have a LINER 2 classification. The brightest discrete X-ray source in the ROAST HRI observations is invariably found to be positionally coincident with the optical galactic nucleus, and in most cases its flux dominates the X-ray emission from the central region of the galaxy. All seven galaxies have X-ray spectra consistent with a two-component, soft thermal plus hard power-law, spectral form. If we exclude the two galaxies with relatively hard X-ray spectra, NGC 3628 and NGC 4594, for which there is supporting evidence for a LLAGN (or alternatively in the case of NGC 3628 a dominant ultra-luminous X-ray binary), then the remaining galaxies show surprisingly similar X-ray spectral properties. Specifically the flux ratio F_x(0.5-1)/F_x(2-5), which measures the relative strengths of the thermal and non-thermal emission components, shows little scatter about a mean of 0.66, a value very similar to that measured in the classic starburst galaxy NGC 253. Since there is no obvious reason why the luminosity of the hard power-law continuum emanating from a putative LLAGN should be very closely correlated with the thermal emission of the surrounding region, this suggests that that the broad-band (0.5-5 keV) X-ray emission from these LINER 2 galaxies may originate in a common set of processes probably associated with the starburst phenomenon. Conversely, it appears that in many, perhaps the majority, of LINER 2 galaxies, the nuclear X-ray luminosity does not derive directly from the presence of a LLAGN.

ROSAT HRI catalogue of X-ray sources in the SMC region

During the operational phase of the ROSAT satellite between 1990 and 1998 the X-ray telescope pointed 71 times to the Small Magellanic Cloud (SMC) for observations with the High Resolution Imager (HRI), covering a field of 5° × 5°. From these data a catalogue of 121 discrete X-ray sources was derived. By cross-correlating the source catalogue with the SIMBAD data base and the TYCHO catalogue, the systematic positional error of the HRI source positions could be reduced. In total the X-ray position for 99 HRI sources was corrected yielding positional errors between 1” and 16”. The HRI catalogue was also cross-correlated with the catalogues derived from the ROSAT Position Sensitive Propotional Counter (PSPC) pointings (Kahabka et al. 1999; Haberl et al. 2000). For 75 HRI sources PSPC counterparts were found and thus hardness ratios are given. With the help of the information obtained from the cross-correlations 56 HRI sources were identified with objects of known or proposed nature. Four foreground stars, six supernova remnants, four supersoft sources, 12 X-ray binaries, and one AGN were detected by the HRI. Based on the existence of a likely optical counterpart or properties like hardness ratio and X-ray to optical flux ratio, further 15 HRI sources were classified into different source types.

A ROSAT PSPC catalogue of X-ray sources in the SMC region

We present a catalogue of 517 discrete X-ray sources in a field covering the Small Magellanic Cloud (SMC). The catalogue was derived from the pointed ROSAT PSPC observations performed between October 1991 and May 1994 and is complementary to the Large Magellanic Cloud (LMC) catalogue published by Haberl & Pietsch (1999). We followed the same identification scheme and used, among other information, X-ray hardness ratios and spatial extent to classify unknown sources as candidates for active galactic nuclei (AGN), foreground stars, supernova remnants (SNRs), supersoft sources (SSSs) and X-ray binaries. For 158 sources a likely source type is given, from which 46 sources are suggested as background AGN (including candidates resulting from a comparison of X-ray and radio images). Nearly all of the X-ray binaries known in the SMC were detected in ROSAT PSPC observations; most of them with luminosities below 1036 erg s-1 suggesting that the fraction of high luminosity X-ray binary systems in the Magellanic Clouds (MCs) is not significantly larger than in our galaxy. Seventeen X-ray sources are associated with SNRs found in earlier work and we suggest here two additional extended sources as SNR candidates. Three very soft sources are newly classified as SSSs from which one is identified with the symbiotic star LIN 358 in the SMC.

A ROSAT High Resolution Imager survey of bright nearby galaxies

We use the extensive public archive of ROSAT High Resolution Imager (HRI) observations to carry out a statistical investigation of the X-ray properties of nearby galaxies. Specifically we focus on the sample of 486 bright () northern galaxies studied by Ho, Filippenko and Sargent (HFS) in the context of their exploration of the optical spectroscopic properties of nearby galactic nuclei. Over 20 per cent of HFS galaxies are encompassed in ROSAT HRI fields of reasonable (10 ks) exposure. The X-ray sources detected within the optical extent of each galaxy are categorized as either nuclear or non-nuclear, depending on whether the source is positioned within or outside a 25-arcsec radius circle centred on the optical nucleus. A nuclear X-ray source is detected in over 70 per cent of the galaxies harbouring either a Seyfert or LINER nucleus, compared with a detection rate of only ∼40 per cent in less active systems. The correlation of the Hα luminosity with nuclear X-ray luminosity previously observed in QSOs and bright Seyfert 1 galaxies appears to extend down into the regime of ultra-low luminosity (LX∼1038−1040 erg s−1) active galactic nuclei (AGN). The inferred accretion rates for this sample of low-luminosity AGN are significantly sub-Eddington. In total, 142 non-nuclear sources were detected. In combination with published data for M31, this leads to a luminosity distribution (normalized to an optical blue luminosity of 1010 L⊙) for the discrete X-ray source population in spiral galaxies of the form where L38 is the X-ray luminosity in units of 1038 erg s−1. The implied LXLB ratio is ∼1.1×1039 erg s−1(1010 L⊙)−1. The nature of the substantial number of ‘superluminous’ non-nuclear objects detected in the survey is discussed.

Multifrequency observations of the interacting galaxy NGC 4922 (UCM 1259 + 2934)

Multifrequency observations of the interacting galaxy NGC 4922 (UCM 1259 + 2934)

The X-ray properties of the nearby LINER galaxy NGC 4736

NGC 4736 is a nearby Sab spiral galaxy, hosting one of the closest examples of a lowionization nuclear emission-line region (LINER) nucleus. We have utilized recent observations by ROSATand ASCA to characterize the X-ray properties of this galaxy. 12 discrete X-ray sources are detected within the region subtended by its optical disc, the majority of which are likely to be X-ray binaries associated with the galaxy. By far the brightest source in the X-ray band is positionally coincident with the nucleus of the galaxy and is spatially resolved into a component with a radial extent of ,3 kpc plus a point-like core. The broad-band (0.1±10 keV) spectrum of this nuclear source is composed of a hard continuum with a spectral slope characteristic of that observed in classical Seyfert nuclei (i.e. power-law photon index, G < 1:7), with thermal emission (kT ˆ 0:1±0.6 keV) dominant below 2 keV. An Fe Ka line may also be present at ,6:8 keV. There is no evidence for X-ray temporal variability on timescales of hours to years. A plausible model is that the hard continuum originates in a nearquiescent active galactic nucleus (with LX , 6 10 erg s, 0.5±10 keV) embedded in the LINER at the centre of NGC 4736. However, an alternative explanation, namely that the LINER is the site of a dense population of X-ray binary sources, cannot be completely

RXTEMeasurement of the Diffuse X‐Ray Emission from the Galactic Ridge: Implications for the Energetics of the Interstellar Medium

The diffuse X-ray emission from the thin disk surrounding the Galactic midplane (the so-called Galactic ridge) was measured with the Rossi X-Ray Timing Explorer proportional counter array in order to determine the spatial extent, spectral nature, and origin of the emission. Spatial examination of the diffuse emission in the central 30? of the plane in Galactic longitude reveals the presence of two components: a thin disk of full width 05 centered roughly on the Galactic midplane and a broad component that can be approximated as a Gaussian distribution with FWHM of about 4?. Assuming an average distance of 16 kpc to the edge of the Galaxy, a scale height of about 70 pc and 500 pc is derived for the thin and broad disk components, respectively. Spectral examination of the emission clearly reveals the presence of a hard power-law tail above 10 keV and an emission line from He-like iron, indicating both thermal and possibly nonthermal origins for the diffuse emission. The averaged spectrum from the ridge in the 3-35 keV band can be modeled with a Raymond-Smith plasma component of temperature ~2-3 keV and a power-law component of photon index ~1.8. Based on this finding, we argue that the temperature of the hot phase of the interstellar medium (ISM) is less than the previously reported values of 5-15 keV. Motivated by the similarities between the characteristics of the thermal component of the Galactic ridge emission in our model and the thermal emission from supernova remnants (SNRs), we discuss the origin of the thermal emission in terms of a population of SNRs residing in the Galactic disk. We find that a supernova explosion rate of less than 5 century-1 is adequate to power the thermal emission from the ridge. The origin of the emission in the hard X-ray band modeled by a power law remains uncertain. Possible contributions from nonthermal bremsstrahlung of cosmic-ray electrons and protons; inverse Compton scattering of energetic electrons from ambient microwave, infrared, and optical photons; nonthermal emission from SNRs; and emission from discrete X-ray sources are discussed. We speculate that bremsstrahlung of accelerated electrons and protons in SNR sites can play a significant role in producing the hard tail of the spectrum. Moreover, their collisional losses can play a major role in the ionization of the ISM.

Interpretation of spectral paradox of cosmic X-ray background

The integrated spectrum of discrete X-ray sources (mainly the active galactic nuclei, AGN) is inconsistent with the observed spectrum of cosmic X-ray background (CXB), and it is so called CXB spectral paradox. The medium X-ray spectra of 68 AGNs are adopted, the evolution function of X-ray spectral indices is analyzed statistically, the fraction of CXB is calculated due to AGNs X-ray emission, which shows that almost 100% CXB comes from AGNs X-ray emission. Especially, the integrated spectrum in 2-10 keV is consistent with the observed spectrum of CXB. The spectral paradox of CXB can be interpreted by this result.

Detection of Excess Hard X-Ray Emission from the Optical Jet Galaxy NGC 1097

Using the ASCA observatory, X-ray observations were made of the spiral galaxy NGC 1097, which exhibits four optical jets. Hard X-ray emission from a point-like source at the nucleus was detected at a 2–10 keV luminosity of 1× 1041 erg s−1 for an assumed distance of 24 Mpc. The 2–10 keV spectrum was well fitted with a power-law of photon index ∼ 1.8. Although no X-ray variability was detected during a ∼ 40 ks pointing, the observed X-ray to optical luminosity ratio of NGC 1097 is too high for the X-ray emission to be explained by an assembly of discrete X-ray sources. From our results and optical activity, the existence of a low-luminosity active galactic nucleus (LLAGN) is strongly suggested. In the soft X-ray band (0.5−2 keV) a thin thermal emission with a temperature kT ∼ 0.6 keV was also detected, which is understood to be emission from hot gas associated with starburst activity.