arXiv subjects

O. Kargaltsev

Publications and source records attributed to O. Kargaltsev.

At least 37 records · Page 2Linked to original sources

arXiv2013

New constraints on the cooling of the Central Compact Object in Cas A

To examine the previously claimed fast cooling of the Central Compact Object (CCO) in the Cas A supernova remnant (SNR), we analyzed two Chandra observations of this CCO, taken in a setup minimizing instrumental spectral distortions. We fit the two CCO X-ray spectra from 2006 and 2012 with hydrogen and carbon neutron star atmosphere models. The temperature and flux changes in the 5.5 years between the two epochs depend on the adopted constraints on the fitting parameters and the uncertainties of the effective area calibrations. If we allow a change of the equivalent emitting region size, R_Em, the effective temperature remains essentially the same. If R_Em is held constant, the best-fit temperature change is negative, but its statistical significance ranges from 0.8sigma to 2.5sigma, depending on the model. If we assume that the optical depth of the ACIS filter contaminant in 2012 was +/-10% different from its default calibration value, the significance of the temperature drop becomes 0.8sigma to 3.1sigma, for the carbon atmospheres with constant R_Em. Thus, we do not see a statistically significant temperature drop in our data, but the involved uncertainties are too large to firmly exclude the previously reported fast cooling. Our analysis indicate a decrease of 4%-6% (1.9-2.9sigma significance) for the absorbed flux in the energy range 0.6-6keV between 2006 and 2012, most prominent in the 1.4-1.8 keV energy range. It could be caused by unaccounted changes of the detector response or contributions from unresolved SNR material along the line of sight to the CCO.

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arXiv2012

Discovery of TeV Gamma-ray Emission from CTA 1 by VERITAS

We report the discovery of TeV gamma-ray emission coincident with the shell-type radio supernova remnant (SNR) CTA 1 using the VERITAS gamma-ray observatory. The source, VER J0006+729, was detected as a 6.5 standard deviation excess over background and shows an extended morphology, approximated by a two-dimensional Gaussian of semi-major (semi-minor) axis 0.30 degree (0.24 degree) and a centroid 5' from the Fermi gamma-ray pulsar PSR J0007+7303 and its X-ray pulsar wind nebula (PWN). The photon spectrum is well described by a power-law dN/dE = N_0 (E/3 TeV)^(-Γ), with a differential spectral index of Γ= 2.2 +- 0.2_stat +- 0.3_sys, and normalization N_0 = (9.1 +- 1.3_stat +- 1.7_sys) x 10^(-14) cm^(-2) s^(-1) TeV^(-1). The integral flux, F_γ= 4.0 x 10^(-12) erg cm^(-2) s^(-1) above 1 TeV, corresponds to 0.2% of the pulsar spin-down power at 1.4 kpc. The energetics, co-location with the SNR, and the relatively small extent of the TeV emission strongly argue for the PWN origin of the TeV photons. We consider the origin of the TeV emission in CTA 1.

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arXiv2012

XMM-Newton observation of the very old pulsar J0108-1431

We report on an X-ray observation of the 166 Myr old radio pulsar J0108-1431 with XMM-Newton. The X-ray spectrum can be described by a power-law model with a relatively steep photon index Gamma~3 or by a combination of thermal and non-thermal components, e.g., a power-law component with fixed photon index Gamma~2 plus a blackbody component with a temperature of kT=0.11 keV. The two-component model appears more reasonable considering different estimates for the hydrogen column density. The non-thermal X-ray efficiency in the single power-law model is eta^PL (1-10 keV) = L^PL (1-10 keV) / Edot ~ 0.003, higher than in most other X-ray detected pulsars. In the case of the combined model, the non-thermal and thermal X-ray efficiencies are even higher, eta^PL (1-10 keV) ~ eta^bb ~ 0.006. We detected X-ray pulsations at the radio period of P=0.808s with significance of 7sigma. The pulse shape in the folded X-ray lightcurve (0.15-2 keV) is asymmetric, with statistically significant contributions from up to 5 leading harmonics. Pulse profiles at two different energy ranges differ slightly: the profile is asymmetric at low energies, 0.15-1 keV, while at higher energies, 1-2 keV, it has a nearly sinusodial shape. The radio pulse peak leads the 0.15-2 keV X-ray pulse peak by delta phi = 0.06 +/- 0.03.

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arXiv2012

Chandra observations of the old pulsar PSR B1451-68

We present 35 ks Chandra ACIS observations of the 42 Myr old radio pulsar PSR B1451-68. A point source is detected 0.32" +/- 0.73" from the expected radio pulsar position. It has ~200 counts in the 0.3-8 keV energy range. We identify this point source as the X-ray counterpart of the radio pulsar. PSR B1451-68 is located close to a 2MASS point source, for which we derive 7% as the upper limit on the flux contribution to the measured pulsar X-ray flux. The pulsar spectrum can be described by either a power-law model with photon index Gamma=2.4 (+0.4/-0.3) and a unrealistically high absorbing column density N(H)= (2.5 (+1.2/-1.3)) * 10^(21) cm^-2, or by a combination of a kT=0.35 (+0.12/-0.07) keV blackbody and a Gamma = 1.4 +/- 0.5 power-law component for N(H)[DM]= 2.6 * 10^(20) cm^-2, estimated from the pulsar dispersion measure. At the parallactic, Lutz-Kelker bias corrected distance of 480 pc, the non-thermal X-ray luminosities in the 0.3-8 keV energy band are either Lx(nonth)= (11.3 +/- 1.7) * 10^(29) erg/s or Lx(nonth)= (5.9 (+4.9/-5.0)) * 10^(29) erg/s, respectively. This corresponds to non-thermal X-ray efficiencies of either eta(nonth)= Lx(nonth) / (dE/dt) ~ 0.005 or 0.003, respectively.

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arXiv2011

The spectrum of the recycled PSR J0437-4715 and its white dwarf companion

We present extensive spectral and photometric observations of the recycled pulsar/white-dwarf binary containing PSR J0437-4715, which we analyzed together with archival X-ray and gamma-ray data, to obtain the complete mid-infrared to gamma-ray spectrum. We first fit each part of the spectrum separately, and then the whole multi-wavelength spectrum. We find that the optical-infrared part of the spectrum is well fit by a cool white dwarf atmosphere model with pure hydrogen composition. The model atmosphere (Teff = 3950pm150K, log g=6.98pm0.15, R_WD=(1.9pm0.2)e9 cm) fits our spectral data remarkably well for the known mass and distance (M=0.25pm0.02Msun, d=156.3pm1.3pc), yielding the white dwarf age (tau=6.0pm0.5Gyr). In the UV, we find a spectral shape consistent with thermal emission from the bulk of the neutron star surface, with surface temperature between 1.25e5 and 3.5e5K. The temperature of the thermal spectrum suggests that some heating mechanism operates throughout the life of the neutron star. The temperature distribution on the neutron star surface is non-uniform. In the X-rays, we confirm the presence of a high-energy tail which is consistent with a continuation of the cut-off power-law component (Gamma=1.56pm0.01, Ecut=1.1pm0.2GeV) that is seen in gamma-rays and perhaps even extends to the near-UV.

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arXiv2011

XMM-Newton Observations of Two Candidate Supernova Remnants

Candidate supernova remnants G23.5+0.1 and G25.5+0.0 were observed by XMM-Newton in the course of a snap-shot survey of plerionic and composite SNRs in the Galactic plane. In the field of G23.5+0.1, we detected an extended source, ~3' in diameter, which we tentatively interpret as a pulsar-wind nebula (PWN) of the middle-aged radio pulsar B1830-08. Our analysis suggests an association between PSR B1830-08 and the surrounding diffuse radio emission. If the radio emission is due to the SNR, then the pulsar must be significantly younger than its characteristic age. Alternatively, the radio emission may come from a relic PWN. In the field of G25.5+0.0, which contains the extended TeV source HESS J1837-069, we detected the recently discovered young high-energy pulsar J1838-0655 embedded in a PWN with extent of 1.3'. We also detected another PWN candidate (AX J1837.3-0652) with an extent of 2' and unabsorbed luminosity L_(2-10 keV) ~ 4 x 10^33 erg/s at d=7 kpc. The third X-ray source, located within the extent of the HESS J1837-069, has a peculiar extended radio counterpart, possibly a radio galaxy with a double nucleus or a microquasar. We did not find any evidence of the SNR emission in the G25.5+0.0 field. We provide detailed multiwavelength analysis and identifications of other field sources and discuss robustness of the G25.5+0.0 and G23.5+0.1 classifications as SNRs. (abstract abridged)

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arXiv2010

Optical-uv spectrum and proper motion of the middle-aged pulsar b1055-52

PSRB1055-52 is a middle-aged (~535 kyr) radio, X-ray, and gamma-ray pulsar showing X-ray thermal emission from the neutron star (NS) surface. A candidate optical counterpart to PSRB1055-52 was proposed by Mignani and coworkers based on Hubble Space Telescope (HST) observations performed in 1996, in one spectral band only. We report on HST observations of this field carried out in 2008, in four spectral bands. The astrometric and photometric analyses of these data confirm the identification of the proposed candidate as the pulsar's optical counterpart. Similarly to other middle-aged pulsars, its optical-UV spectrum can be described by the sum of a power-law (PLO) component, presumably emitted from the pulsar magnetosphere, and a Rayleigh-Jeans (RJ) component emitted from the NS surface. The spectral index of the PLO component, alpha_O=1.05+/-0.34, is larger than for other pulsars with optical counterparts. The RJ component, with the brightness temperature TO=(0.66+/-0.10) d_350**2 R_O,13**-2 MK (where d_350 and R_O,13 are the distance to the pulsar in units of 350 pc and the radius of the emitting area in units of 13 km), shows a factor of 4 excess with respect to the extrapolation of the X-ray thermal component into the UV-optical. This hints that the RJ component is emitted from a larger, colder area, and suggests that the distance to the pulsar is smaller than previously thought. From the absolute astrometry of the HST images we measured the pulsar coordinates with a position accuracy of 0.15". From the comparison with previous observations we measured the pulsar proper motion, mu = 42+/-5 mas/yr, which corresponds to a transverse velocity V_t = (70+/-8) d_350 km/s.

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arXiv2009

Pulsar Wind Nebulae in the Chandra Era

Pulsar winds shocked in the ambient medium produce spectacular nebulae observable from the radio through gamma-rays. The shape and the spectrum of a pulsar wind nebula (PWN) depend on the angular distribution, magnetization and energy spectrum of the wind streaming from the pulsar magnetosphere, as well as on the pulsar velocity and the properties of the ambient medium. The advent of Chandra, with its unprecedented angular resolution and high sensitivity, has allowed us not only to detect many new PWNe, but also study their spatial and spectral structure and dynamics, which has significantly advanced our understanding of these objects. Here we overview recent observational results on PWNe, with emphasis on Chandra observations.

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arXiv2009

Chandra observation of the relativistic binary J1906+0746

PSR J1906+0746 is a 112-kyr-old radio pulsar in a tight relativistic binary with a compact high-mass companion, at the distance of about 5 kpc. We observed this unique system with the Chandra ACIS detector for 31.6 ks. Surprisingly, not a single photon was detected within the 3" radius from the J1906+0746 radio position. For a plausible range of hydrogen column densities, n_H=(0.5-1)\times10^{22} cm^{-2}, the nondetection corresponds to the 90% upper limit of (3-5)\times10^{30} erg s^{-1} on the unabsorbed 0.5-8 keV luminosity for the power-law model with Gamma=1.0-2.0, and ~10^{32} erg s^{-1} on the bolometric luminosity of the thermal emission from the NS surface. The inferred limits are the lowest known for pulsars with spin-down properties similar to those of PSR J1906+0746. We have also tentatively detected a puzzling extended structure which looks like a tilted ring with a radius of 1.6' centered on the pulsar. The measured 0.5-8 keV flux of the feature, 3.1\times10^{-14} erg cm^{-2} s^{-1}, implies an unabsorbed luminosity of 1.2\times10^{32} erg s^{-1} (4.5\times10^{-4} of the pulsar's spin-down power). Although all conventional interpretations of the ring appear to be problematic, the pulsar-wind nebula with an unusually underluminous pulsar remains the most viable interpretation.

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arXiv2009

The Behavior of Matter under Extreme Conditions

The cores of neutron stars harbor the highest matter densities known to occur in nature, up to several times the densities in atomic nuclei. Similarly, magnetic field strengths can exceed the strongest fields generated in terrestrial laboratories by ten orders of magnitude. Hyperon-dominated matter, deconfined quark matter, superfluidity, even superconductivity are predicted in neutron stars. Similarly, quantum electrodynamics predicts that in strong magnetic fields the vacuum becomes birefringent. The properties of matter under such conditions is governed by Quantum Chromodynamics (QCD) and Quantum Electrodynamics (QED), and the close study of the properties of neutron stars offers the unique opportunity to test and explore the richness of QCD and QED in a regime that is utterly beyond the reach of terrestrial experiments. Experimentally, this is almost virgin territory.

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arXiv2008

Detection of X-ray Emission from the Very Old Pulsar J0108-1431

PSR J0108-1431 is a nearby, 170 Myr old, very faint radio pulsar near the "pulsar death line" in the P-Pdot diagram. We observed the pulsar field with the Chandra X-ray Observatory and detected a point source (53 counts in a 30 ks exposure, energy flux (9+/-2)\times 10^{-15} ergs cm^{-2} s^{-1} in the 0.3-8 keV band) close to the radio pulsar position. Based on the large X-ray/optical flux ratio at the X-ray source position, we conclude that the source is the X-ray counterpart of PSR J0108-1431.The pulsar spectrum can be described by a power-law model with photon index Gamma \approx 2.2 and luminosity L_{0.3-8 keV} \sim 2\times 10^{28} d_{130}^2 ergs s^{-1}, or by a blackbody model with the temperature kT\approx 0.28 keV and bolometric luminosity L_{bol} \sim 1.3\times 10^{28} d_{130}^2 ergs s^{-1}, for a plausible hydrogen column density NH = 7.3\times 10^{19} cm^{-2} (d_{130}=d/130 pc). The pulsar converts \sim 0.4% of its spin-down power into the X-ray luminosity, i.e., its X-ray efficiency is higher than for most younger pulsars. From the comparison of the X-ray position with the previously measured radio positions, we estimated the pulsar proper motion of 0.2 arcsec yr^{-1} (V_\perp \sim 130 d_{130} km s^{-1}), in the south-southeast direction.

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arXiv2008

A possible optical counterpart to the old nearby pulsar J0108-1431

The multi-wavelength study of old (>100 Myr) radio pulsars holds the key to understanding the long-term evolution of neutron stars, including the advanced stages of neutron star cooling and the evolution of the magnetosphere. Optical/UV observations are particularly useful for such studies because they allow one to explore both thermal and non-thermal emission processes. In particular, studying the optical/UV emission constrains temperature of the bulk of the neutron star surface, too cold to be measured in X-ray observations.Aim of this work is to identify the optical counterpart of the very old (166 Myr) radio pulsar J0108-1431. We have re-analyzed our original VLT observations (Mignani et al. 2003), where a very faint object was tentatively detected close to the radio position, near the edge of a field galaxy. We found that the backward extrapolation of the PSR J0108-1431 proper motion recently measured by CHANDRA(Pavlov et al. 2008) nicely fits the position of this object. Based on that, we propose it as a viable candidate for the optical counterpart to PSR J0108-1431. The object fluxes (U =26.4+/-0.3; B =27.9; V >27.8) are consistent with a thermal spectrum with a brightness temperature of 9X10^4 K (for R = 13 km at a distance of 130 pc), emitted from the bulk of the neutron star surface. New optical observations are required to confirm the optical identification of PSR J0108-1431 and measure its spectrum.

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arXiv2008

Young energetic PSR J1617-5055, its underluminous nebula and unidentified TeV source HESS J1616-508

We observed the young energetic pulsar J1617-5055 with the Chandra ACIS detector for 60 ks. In addition to the pulsar, the X-ray images show a faint pulsar-wind nebula (PWN) seen up to ~1' from the pulsar. Deconvolution and reconstruction of the image reveal a brighter compact PWN component of ~1'' size, possibly with a jet-torus morphology. The PWN spectrum fits an absorbed power-law (PL) model with the photon index Gamma=1.5. The total PWN luminosity in the 0.5-8 keV band, L_{pwn}=3.2\times10^{33} ergs s^{-1} for d=6.5 kpc, is a fraction of 2\times 10^{-4} of the pulsar's spin-down power and a fraction of 0.2 of the pulsar's X-ray luminosity, which is a factor of 20 lower than one would expect from an average empirical relation found for a sample of PWNe observed with Chandra. The pulsar's spectrum can be described by an absorbed PL with n_{H}=3.5\times10^{22} cm^{-2} and Gamma=1.1, harder than any other pulsar spectrum reliably measured in the soft X-ray band. This non-thermal emission is 50% pulsed showing one peak per period. We have also investigated a possible connection between J1617 and the extended TeV source HESS J1616-508 whose center is located about 10' west of the pulsar. We find no preferential extension of the X-ray PWN toward the TeV source. Therefore, the Chandra data do not provide conclusive evidence for PSR J1617-5055 and HESS J1616-508 association. We have also analyzed archival X-ray, radio, and IR data on the HESS J1616-508 region and found traces of diffuse emission (resembling a shell in the radio) coinciding with the central part of HESS J1616-508. We speculate that the TeV source may be multiple, with most of the emission coming from an unknown SNR or a star-forming region, while some fraction of the TeV emission still may be attributed to the J1617 PWN.

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arXiv2008

X-ray Observations of Parsec-Scale Tails behind Two Middle-Aged Pulsars

Chandra and XMM-Newton resolved extremely long tails behind two middle-aged pulsars, J1509-5850 and J1740+1000. The tail of PSR J1509-5850 is discernible up to 5.6' from the pulsar (6.5 pc at a distance of 4 kpc), with a flux of 2*10^{-13} erg s^{-1} cm^{-2} in 0.5-8 keV. The tail spectrum fits an absorbed power-law (PL) model with the photon index of 2.3\pm0.2, corresponding to the 0.5-8 keV luminosity of 1*10^{33} ergs s^{-1}, for n_H= 2.1*10^{22} cm^{-2}. The tail of PSR J1740+1000 is firmly detected up to 5' (2 pc at a 1.4 kpc distance), with a flux of 6*10^{-14} ergs cm^{-2} s^{-1} in 0.4-10 keV. The PL fit yields photon index of 1.4-1.5 and n_H=1*10^{21} cm^{-2}. The large extent of the tails suggests that the bulk flow in the tails starts as mildly relativistic downstream of the termination shock, and then gradually decelerates. Within the observed extent of the J1509-5850 tail, the average flow speed exceeds 5,000 km s^{-1}, and the equipartition magnetic field is a few times 10^{-5} G. For the J1740+1000 tail, the equipartition field is a factor of a few lower. The harder spectrum of the J1740+1000 tail implies either less efficient cooling or a harder spectrum of injected electrons. For the high-latitude PSR J1740+1000, the orientation of the tail on the sky shows that the pulsar is moving toward the Galactic plane, which means that it was born from a halo-star progenitor. The comparison between the J1509 and J1740 tails and the X-ray tails of other pulsars shows that the X-ray radiation efficiency correlates poorly with the pulsar spin-down luminosity or age. The X-ray efficiencies of the ram-pressure confined pulsar wind nebulae (PWNe) are systematically higher than those of PWNe around slowly moving pulsars with similar spin-down parameters.

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arXiv2007

Chandra Observation of PSR B1823-13 and its Pulsar Wind Nebula

We report on an observation of the Vela-like pulsar B1823-13 and its synchrotron nebula with Chandra.The pulsar's spectrum fits a power-law model with a photon index Gamma_PSR=2.4 for the plausible hydrogen column density n_H=10^{22} cm^{-2}, corresponding to the luminosity L_PSR=8*10^{31} ergs s^{-1} in the 0.5-8 keV band, at a distance of 4 kpc. The pulsar radiation likely includes magnetospheric and thermal components, but they cannot be reliably separated because of the small number of counts detected and strong interstellar absorption. The pulsar is surrounded by a compact, 25''x 10'', pulsar wind nebula (PWN) elongated in the east-west direction, which includes a brighter inner component, 7''x 3'', elongated in the northeast-southwest direction. The slope of the compact PWN spectrum is Gamma_comp=1.3, and the 0.5-8 keV luminosity is L_comp~3*10^{32} ergs s^{-1}. The compact PWN is surrounded by asymmetric diffuse emission (extended PWN) seen up to at least 2.4' south of the pulsar, with a softer spectrum (Gamma_ext=1.9), and the 0.5-8 keV luminosity L_ext~10^{33}-10^{34} ergs s^{-1}. We also measured the pulsar's proper motion using archival VLA data: μ_α=23.0+/-2.5 mas yr^{-1}, μ_δ=-3.9+/-3.3 mas yr^{-1}, which corresponds to the transverse velocity v_perp=440 km s^{-1}. The direction of the proper motion is approximately parallel to the elongation of the compact PWN, but it is nearly perpendicular to that of the extended PWN and to the direction towards the center of the bright VHE gamma-ray source HESS J1825-137, which is likely powered by PSR B1823-13.

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arXiv2007

X-ray emission from PSR J1809-1917 and its pulsar wind nebula, possibly associated with the TeV gamma-ray source HESS J1809-193

We detected X-ray emission from the 50-kyr-old pulsar J1809-1917 and resolved its pulsar wind nebula (PWN) with Chandra. The pulsar spectrum fits PL+BB model with the photon index of 1.2 and the BB temperature of 2 MK for n_{H}=0.7\times 10^{22} cm^{-2}. The luminosities are(4\pm 1)\times 10^{31} ergs s^{-1} for the PL component (in the 0.5-8 keV band) and ~1\times 10^{32} ergs s^{-1} for the BB component (bolometric) at a plausible distance of 3.5 kpc. The bright inner PWN component of a 3''\times12'' size is elongated in the north-south direction, with the pulsar close to its south end. This component is immersed in a larger (20''\times40''), similarly elongated outer PWN component of lower surface brightness. The elongated shape of the compact PWN can be explained by the ram pressure confinement of the pulsar wind due to the supersonic motion of the pulsar. The PWN spectrum fits a PL model with photon index of 1.4\pm0.1 and 0.5-8 keV luminosity of 4\times10^{32} ergs s^{-1}. The compact PWN appears to be inside a large-scale (~4'\times4') emission more extended to the south of the pulsar, i.e. in the direction of the alleged pulsar motion. To explain the extended X-ray emission ahead of the moving pulsar, one has to invoke strong intrinsic anisotropy of the pulsar wind or assume that this emission comes from a relic PWN swept by the asymmetrical reverse SNR shock. The pulsar and its PWN are located within the extent of the unidentified TeV source HESS J1809-193 whose brightest part is offset by ~8' to the south of the pulsar, i.e. in the same direction as the large-scale X-ray emission. Although the association between J1809-1917 and HESS J1809-193 is plausible, an alternative source of relativistic electrons powering HESS J1809-193 might be the serendipitously discovered X-ray source CXOU J180940.7-192544.

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arXiv2007

X-ray emission from the planet pulsar B1257+12

We report the detection of the millisecond pulsar B1257+12 with the Chandra X-ray Observatory. In a 20 ks exposure we detected 25 photons from the pulsar, with energies between 0.4 and 2.0 keV, corresponding to the flux F_X=(4.4+/- 0.9)*10^{-15} ergs s^{-1} cm^{-2} in this energy range. The X-ray spectrum can be described by a power-law model with photon index Gamma = 2.8 and luminosity L_X \approx 2.5*10^{29} ergs s^{-1} in the 0.3--8 keV band, for a plausible distance of 500 pc and hydrogen column density N_H=3*10^{20} cm^{-2}. Alternatively, the spectrum can be fitted by a blackbody model with kT ~ 0.22 keV and projected emitting area ~2000 m^2. If the thermal X-rays are emitted from two symmetric polar caps, the bolometric luminosity of the two caps is 2 L_bol ~ 3*10^{29} ergs s^{-1}. We compared our results with the data on other 30 millisecond pulsars observed in X-rays and found that the apparent X-ray efficiency of PSR B1257+12, L_X/Edot ~ 3*10^{-5} for d=500 pc, is lower than those of most of millisecond pulsars. This might be explained by an unfavorable orientation of the X-ray pulsar beam if the radiation is magnetospheric, or by strong asymmetry of polar caps if the radiation is thermal (e.g., one of the polar caps is much brighter than the other and remains invisible for most part of the pulsar period). Alternatively, it could be attributed to absorption of X-rays in circumpulsar matter, such as a flaring debris disk left over after formation of the planetary system around the pulsar.

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arXiv2007

TeV source HESS J1804-216 in X-rays and other wavelengths

The field of the extended TeV source HESS J1804-216 was serendipitously observed with the Chandra ACIS detector. The data reveal several X-ray sources within the bright part of HESS J1804-216. The brightest of these objects, CXOU J180432.4-214009, which has been also detected with Swift and Suzaku, is consistent with being a point-like source, with the 0.3-7 keV flux of (1.7\pm0.2)\times10^{-13} ergs s^{-1} cm^{-2}. Its hard and strongly absorbed spectrum can be fitted by the absorbed power-law model with the best-fit photon index of 0.45 and hydrogen column density of 4\times10^{22} cm^{-2}, both with large uncertainties due to the strong correlation between these parameters. A search for pulsations resulted in a 106 s period candidate, which however has a low significance of 97.9%. We found no infrared-optical counterparts for this source. The second brightest source, CXOU J180441.9-214224, which has been detected with Suzaku, is either extended or multiple, with the flux of about 10^{-13} ergs cm^{-2} s^{-1}. We found a nearby M dwarf within the X-ray source extension, which could contribute a fraction of the observed X-ray flux. The remaining sources are very faint (<3\times 10^{-14} ergs cm^{-2} s^{-1}), and at least some of them are likely associated with nearby stars. Although one or both of the two brighter X-ray sources could be faint accreting binaries or remote pulsars with pulsar wind nebulae (hence possible TeV sources), their relation to HESS J1804-216 remains elusive. The possibility that HESS J1804-216 is powered by the relativistic wind from the young pulsar B1800-21, located at a distance of about 10 pc from the TeV source, still remains a more plausible option.

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