Astrometric Research Articles

VERA and ALMA observations of the H2O supermaser burst in Orion KL

Abstract The 22 GHz H2O maser in Orion KL has shown extraordinary burst events in 1979–1985 and 1998–1999, sometimes called supermaser. We have conducted monitoring observations of the supermaser in Orion KL using VERA (VLBI Exploration of Radio Astrometry) in the current third burst since 2011 March. Three flux maxima are detected in 2011 and 2012 with rising and falling timescales of 2–7 months. Time variations of the supermaser seem symmetric for all of the active phases. The maximum total flux density of 135000 Jy is observed in 2012 June while it is still one order of magnitude lower than those in previous bursts. The supermaser consists of two spatially different components at different velocities. They are elongated along a northwest–southeast direction perpendicular to the low-velocity outflow driven by Source I. Proper motions of the supermaser features with respect to Source I are measured toward west and southwest directions, almost parallel to the low-velocity outflow. The flux density and linewidth show an anti-correlation as expected for an unsaturated maser emission. The supermaser is located close to the methylformate (HCOOCH3) line and continuum emission peaks in the Orion Compact Ridge detected by ALMA (Atacama Large Millimeter/Submillimeter Array). The broader velocity range of the weak HCOOCH3 emission at the supermaser position would be evidence of a shock front. On the other hand, the 321 GHz H2O line is not detected at the position of the supermaser. It can be explained qualitatively by one of the theoretical H2O excitation models without extraordinary conditions. Our results support a scenario that the supermaser is excited in the dense gas interacting with the low-velocity outflow in the Compact Ridge. The extremely high flux density and its symmetric time variation for rising and falling phases could be explained by a beaming effect during the amplification process rather than changes in physical conditions.

Annual parallax measurements of a semi-regular variable star, RW Leporis

Abstract We have measured the annual parallax of water masers around the semi-regular variable star RW Leporis (RW Lep) from observations with VLBI Exploration of Radio Astrometry (VERA). The measured parallax is 1.62 ± 0.16 mas, corresponding to a distance of $617^{+68}_{-55}\:$pc. This is the first VLBI annual parallax of RW Lep, whose previously measured parallax is a negative value. Using the estimated distance we plot RW Lep on the period–luminosity plane. RW Lep is reported to have two different periods, the longer period shows that the star is located between the sequences C and C′. The shorter period shows that it is located on the sequence B+. We estimated the luminosity, radius, and mass of RW Lep to be 15400 L⊙, 428 R⊙, and 4.7 M⊙, respectively. We found 13 maser spots around the star, with a distribution ∼ 11 times larger than the star's radius.

VLBI observations of bright AGN jets with the KVN and VERA Array (KaVA): Evaluation of imaging capability

Abstract The Korean very-long-baseline interferometry (VLBI) network (KVN) and VLBI Exploration of Radio Astrometry (VERA) Array (KaVA) is the first international VLBI array dedicated to high-frequency (23–43 GHz bands) observations in East Asia. Here, we report the first imaging observations of three bright active galactic nuclei (AGNs) known for their complex morphologies: 4C 39.25, 3C 273, and M 87. This is one of the initial results of KaVA's early operation. Our KaVA images reveal extended outflows with complex substructures such as knots and limb brightening, in agreement with previous Very Long Baseline Array (VLBA) observations. Angular resolutions are better than 1.4 and 0.8 mas at 23 and 43 GHz, respectively. KaVA achieves a high dynamic range of ∼ 1000, more than three times the value achieved by VERA. We conclude that KaVA is a powerful array with a great potential for the study of AGN outflows, at least comparable to the best existing radio interferometric arrays.

VLBI observations of H2O maser annual parallax and proper motion in IRAS 20143+3634: Reflection on the Galactic constants

Abstract We report the results of very long baseline interferometry (VLBI) observations of H2O masers in the IRAS 20143+3634 star-forming region (SFR) using VERA (VLBI Exploration of Radio Astrometry). By tracking masers for a period of over two years we measured a trigonometric parallax of π = 0.367 ± 0.037 mas, corresponding to a source distance of $D = 2.72 ^{+0.31}_{-0.25}$ kpc and placing it in the Local spiral arm. Our trigonometric distance is just 60% of the previous estimation based on radial velocity, significantly impacting the astrophysics of the source. We measured proper motions of −2.99 ± 0.16 mas yr−1 and −4.37 ± 0.43 mas yr−1 in RA and Dec, respectively, which were used to estimate the peculiar motion of the source as (Us, Vs, Ws) = (−0.9 ± 2.9, −8.5 ± 1.6, +8.0 ± 4.3) km s−1 for R0 = 8 kpc and Θ0 = 221 km s−1, and (Us, Vs, Ws) = (−1.0 ± 2.9, −9.3 ± 1.5, +8.0 ± 4.3) km s−1 for R0 = 8.5 kpc and Θ0 = 235 km s−1. IRAS 20143+3634 was found to be located near the tangent point in the Cygnus direction. Using our observations we derived the angular velocity of Galactic rotation of the local standard of rest (LSR), Ω0 = 27.3 ± 1.6 km s−1 kpc−1, which is consistent with previous values derived using VLBI astrometry of SFRs at the tangent points and Solar circle. It is higher than the value recommended by the International Astronomical Union of Ω0 = 25.9 km s−1 kpc−1 which was calculated using the Galactocentric distance of the Sun and circular velocity of the LSR.

Trigonometric parallax of IRAS 22555+6213 with VERA: Three-dimensional view of sources along the same line of sight

Abstract We report the results of the measurement of the trigonometric parallax of an H2O maser source in IRAS 22555+6213 with the VLBI Exploration of Radio Astrometry (VERA). The annual parallax was determined to be 0.314 ± 0.070 mas, corresponding to a distance of $3.18^{+0.90}_{-0.66}\,{\rm kpc}$. Our results confirm IRAS 22555+6213 to be located in the Perseus arm. We computed the peculiar motion of IRAS 22555+6213 to be (Usrc, Vsrc, Wsrc) = (4 ± 4, −32 ± 6, 8 ± 6) km s−1, where Usrc, Vsrc, and Wsrc are directed toward the Galactic center, in the direction of Galactic rotation and toward the Galactic north pole, respectively. IRAS 22555+6213, NGC 7538, and Cepheus A lie along the same line of sight, and are within 2° on the sky. Their parallax distances, with which we derived their absolute position in the Milky Way, show that IRAS 22555+6213 and NGC 7538 are associated with the Perseus Arm, while Cepheus A is located in the Local Arm. We compared the kinematic distances of IRAS 22198+6336, G108.18+05.51, G108.20−00.58, G108.47+02.81, G108.59+00.49, Cepheus A, NGC 7538, G111.23−01.23, G111.25−00.76, and IRAS 22555+6213 derived with flat and non-flat rotation curves with their parallax distance and found the kinematic distance derived from the non-flat rotation assumption (−5 to −39 km s−1 lag) to be consistent with the parallax distance.

Uncertainty Analysis Method for Intersecting Process of U-Shaped Horizontal Wells

Accurate monitoring and determination of the trajectory and relative position are two very important concerns during drilling the connection of two wells to form a U-shaped horizontal well. The composite error circle based on trajectory uncertainty analysis is introduced. The timing of using the magnetic ranging tool is established by comparing the diameter of the composite error circle with that of the ranging scope of the magnetic ranging tool. The profile is designed, and the key extension formula is given according to the characteristics of the magnetic ranging tool. The measurement principle of a Rotating Magnetic Ranging System, which is widely used during the drilling of U-shaped wells, is introduced in order to analyze the relative position uncertainty. Through analysis of the error sources, a covariance matrix of the relative position uncertainty is constructed, and its error ellipsoid and error ellipse are determined. The error ellipse is found to be beneficial in using the magnetic ranging tool during the drilling of U-shaped horizontal wells by comparing the semi-axis of the error ellipse and the radius of the borehole.

A quiescent galaxy at the position of the long GRB 050219A

Long-duration gamma-ray bursts (LGRBs) are produced by the collapse of very massive stars. Due to the short lifetime of their progenitors, LGRBs pinpoint star-forming galaxies. We present here a multi-band search for the host galaxy of the long dark GRB 050219A within the enhanced Swift/XRT error circle. We used spectroscopic observations acquired with VLT/X-shooter to determine the redshift and star-formation rate of the putative host galaxy. We compared the results with the optical/IR spectral energy distribution obtained with different facilities. Surprisingly, the host galaxy is a old and quiescent early-type galaxy at z = 0.211 characterised by an unprecedentedly low specific star-formation rate. It is the first LGRB host to be also an early-type post-starburst galaxy. This is further evidence that GRBs can explode in all kind of galaxies, with the only requirement being an episode of star-formation.

A search forFermibursts associated with supernovae and their frequency of occurrence

Context: Observations suggest that the major fraction of long duration gamma-ray bursts (GRBs) are connected with broad-lines supernovae Ib/c, (SNe-Ibc). The presence of GRB-SNe is revealed by rebrightenings emerging from the optical GRB afterglow $10$--$15$ days, in the rest-frame of the source, after the prompt GRB emission. Aims: \textit{Fermi}-GBM has a field of view (FoV) which is about 6.5 times larger than the FoV of \textit{Swift}, therefore we expect that a number of GRB-SN connections have been missed due to lack of optical and X-ray instruments on board of \textit{Fermi}, which are essential to reveal SNe associated with GRBs. This fact has motivated our search in the \textit{Fermi} catalogue for possible GRB-SN events. Methods: The search for possible GRB-SN associations follows two requirements: (1) SN should fall inside the \textit{Fermi}-GBM error box of the considered long GRB, and (2) this GRB should occur within $20$ days before the SN event. Results: We have found $5$ cases, within $z<0.2$ fulfilling the above reported requirements. One of them, GRB 130702A-SN 2013dx, was already known as GRB-SN association. We have analyzed the remaining $4$ cases and we concluded that three of them are, very likely, just random coincidences, due to the \textit{Fermi}-GBM large error box associated with each GRB detection. We found one GRB possibly associated with a SN 1998bw-like, GRB 120121B/SN 2012ba. Conclusions: The very low redshift of GRB 120121B/SN 2012ba ($z = 0.017$) implies a low isotropic energy of this burst ($E_{iso} = 1.39 \times 10^{48}$) erg. We then compute the rate of \textit{Fermi} low-luminosity GRBs connected with SNe to be $\rho_{0,b} \leq 770\ $Gpc$^{-3}$yr$^{-1}$. We estimate that \textit{Fermi}-GBM could detect $1$--$4$ GRBs-SNe within $z \leq 0.2$ in the next 4 years.

A strong radio brightening at the jet base of M87 during the elevated very-high-energy γ-ray state in 2012

AbstractThe nearby radio galaxy M87 offers a unique opportunity for exploring the connection between γ-ray production and jet formation at an unprecedented linear resolution. However, the origin and location of the γ-rays in this source is still elusive. Based on previous radio/TeV correlation events, the unresolved jet base (radio core) and the peculiar knot HST-1 at &gt;120 pc from the nucleus are proposed as candidate site(s) of γ-ray production. Here we report our intensive, high-resolution radio monitoring observations of the M87 jet with the VLBI Exploration of Radio Astrometry (VERA) and the European VLBI Network (EVN) from February 2011 to October 2012. During this period, an elevated level of the M87 flux is reported at TeV with VERITAS. We detected a remarkable flux increase in the radio core with VERA at 22/43 GHz coincident with the VHE activity. Meanwhile, HST-1 remained quiescent in terms of its flux density and structure at radio. These results strongly suggest that the TeV γ-ray activity in 2012 originates in the jet base within 0.03 pc (projected) from the central supermassive black hole.

The nature of the unassociated 2FGL sources

AbstractThe majority of Fermi-LAT detected (2FGL) sources are AGN, mostly blazars. However, the second largest category in the 2FGL are unassociated sources (~30% or 575 sources), whose multi-wavelength counterpart is either inconclusive or absent. Follow-up observations and archival data at X-ray, optical, and radio frequencies suggest that many unassociated 2FGL sources are strong candidates to be AGN. Typical observed characteristics of 2FGL detected AGN include variability at all frequencies and a spectral energy distribution (SED) with two “bumps”; a low-frequency synchrotron peak in the radio to optical/X-ray region and a high-frequency peak, possibly due to synchrotron self-Compton or Inverse Compton processes, that extends up to TeV energies. We present optical follow-up observations of a sample of Fermi unassociated sources with one or more potential X-ray counterparts detected within the LAT error circle.

The dark nature of GRB 130528A and its host galaxy

We study the dark nature of GRB 130528A through multi-wavelength observations and conclude that the main reason for the optical darkness is local extinction inside of the host galaxy. Automatic observations were performed at BOOTES-4/MET robotic telescope. We also triggered target of opportunity (ToO) observation at the OSN, IRAM PdBI and the GTC+OSIRIS. The host galaxy photometric observations in optical to near-infrared (nIR) wavelengths were achieved through large ground-based aperture telescopes, such as the 10.4m GTC, the 4.2m WHT, 6m BTA, and the 2m LT. Based on these observations, spectral energy distributions (SED) for the host galaxy and afterglow were constructed. Thanks to mm observations at PdBI, we confirm the presence of a mm source within the XRT error circle that faded over the course of our observations and identify the host galaxy. However, we do not find any credible optical source within early observations with BOOTES-4/MET and 1.5m OSN telescopes. Spectroscopic observation of this galaxy by GTC showed a single faint emission line that likely corresponds to [OII] 3727\{AA} at a redshift of 1.250+/-0.001 implying a SFR(M_sun/yr) > 6.18 M_sun/yr without correcting for dust extinction. The probable extinction was revealed through analysis of the afterglow SED, resulting in a value of AV >= ~ 0.9 at the rest frame, this is comparable to extinction levels found among other dark GRBs. The SED of the host galaxy is explained well (chi2/d.o.f.=0.564) by a luminous (MB=-21.16), low-extinction (AV =0, rest frame), and aged (2.6 Gyr) stellar population. We can explain this apparent contradiction in global and line-of-sight extinction if the GRB birth place happened to lie in a local dense environment. In light of having relatively small specific SFR (SSFR) ~ 5.3 M_sun/yr (L/L_star)-1, this also could explain the age of the old stellar population of host galaxy.

Multiwavelength search for counterparts of supersoft X-ray sources in M31

We searched optical/UV/IR counterparts of seven supersoft X-ray sources (SSS) in M31 in the Hubble Space Telescope (HST) "Panchromatic Hubble Andromeda Treasury" (PHAT) archival images and photometric catalog. Three of the SSS were transient, the other four are persistent sources. The PHAT offers the opportunity to identify SSS hosting very massive white dwarfs that may explode as type Ia supernovae in single degenerate binaries, with magnitudes and color indexes typical of symbiotic stars, high mass close binaries, or systems with optically luminous accretion disks. We find evidence that the transient SSS were classical or recurrent novae; two likely counterparts we identified are probably symbiotic binaries undergoing mass transfer at a very high rate. There is a candidate accreting white dwarf binary in the error circle of one of the persistent sources, r3-8. In the spatial error circle of the best studied SSS in M31, r2-12, no red giants or AGB stars are sufficiently luminous in the optical and UV bands to be symbiotic systems hosting an accreting and hydrogen burning white dwarf. This SSS has a known modulation of the X-ray flux with a 217.7 s period, and we measured an upper limit on its derivative, 0.82 x 10(-11). This limit can be reconciled with the rotation period of a white dwarf accreting at high rate in a binary with a few-hours orbital period. However, there is no luminous counterpart with color indexes typical of an accretion disk irradiated by a hot central source. Adopting a semi-empirical relationship, the upper limit for the disk optical luminosity implies an upper limit of only 169 minutes for the orbital period of the white dwarf binary.

Are both BL Lacs and pulsar wind nebulae the astrophysical counterparts of IceCube neutrino events?

IceCube has recently reported the discovery of high-energy neutrinos of astrophysical origin, opening up the PeV (10^15 eV) sky. Because of their large positional uncertainties, these events have not yet been associated to any astrophysical source. We have found plausible astronomical counterparts in the GeV -- TeV bands by looking for sources in the available large area high-energy gamma-ray catalogues within the error circles of the IceCube events. We then built the spectral energy distribution of these sources and compared it with the energy and flux of the corresponding neutrino. Likely counterparts include mostly BL Lacs and two Galactic pulsar wind nebulae. On the one hand many objects, including the starburst galaxy NGC 253 and Centaurus A, despite being spatially coincident with neutrino events, are too weak to be reconciled with the neutrino flux. On the other hand, various GeV powerful objects cannot be assessed as possible counterparts due to their lack of TeV data. The definitive association between high-energy astrophysical neutrinos and our candidates will be significantly helped by new TeV observations but will be confirmed or disproved only by further IceCube data. Either way, this will have momentous implications for blazar jets, high-energy astrophysics, and cosmic-ray and neutrino astronomy.

Spectral and temporal properties of RX J0520.5-6932 (LXP 8.04) during a type-I outburst

We observed RX J0520.5-6932 in the X-rays and studied the optical light curve of its counterpart to verify it as a Be/X-ray binary. We performed an XMM-Newton anticipated target of opportunity observation in January 2013 during an X-ray outburst of the source in order to search for pulsations and derive its spectral properties. We monitored the source with Swift to follow the evolution of the outburst and to look for further outbursts to verify the regular pattern seen in the optical light curve with a period of ~24.4 d. The XMM-Newton EPIC light curves show coherent X-ray pulsations with a period of 8.035331(15) s (1 sigma). The X-ray spectrum can be modelled by an absorbed power law with photon index of ~0.8, an additional black-body component with temperature of ~0.25 keV and an Fe K line. Phase-resolved X-ray spectroscopy reveals that the spectrum varies with pulse phase. We confirm the identification of the optical counterpart within the error circle of XMM-Newton at an angular distance of ~0.8 arcsec, which is an O9Ve star with known Halpha emission. By analyzing the combined data from three OGLE phases we derived an optical period of 24.43 d.The X-ray pulsations and long-term variability, as well as the properties of the optical counterpart, confirm that RX J0520.5-6932 is a Be/X-ray binary pulsar in the Large Magellanic Cloud. Based on the X-ray monitoring of the source we conclude that the event in January 2013 was a moderately bright type-I X-ray outburst, with a peak luminosity of 1.79e36 erg/s.

THE FIRST VERY LONG BASELINE INTERFEROMETRY IMAGE OF A 44 GHz METHANOL MASER WITH THE KVN AND VERA ARRAY (KaVA)

We have carried out the first very long baseline interferometry (VLBI) imaging of 44 GHz class I methanol maser (7_{0}-6_{1}A^{+}) associated with a millimeter core MM2 in a massive star-forming region IRAS 18151-1208 with KaVA (KVN and VERA Array), which is a newly combined array of KVN (Korean VLBI Network) and VERA (VLBI Exploration of Radio Astrometry). We have succeeded in imaging compact maser features with a synthesized beam size of 2.7 milliarcseconds x 1.5 milliarcseconds (mas). These features are detected at a limited number of baselines within the length of shorter than approximately 650 km corresponding to 100 Mlambda in the uv-coverage. The central velocity and the velocity width of the 44 GHz methanol maser are consistent with those of the quiescent gas rather than the outflow traced by the SiO thermal line. The minimum component size among the maser features is ~ 5 mas x 2 mas, which corresponds to the linear size of ~ 15 AU x 6 AU assuming a distance of 3 kpc. The brightness temperatures of these features range from ~ 3.5 x 10^{8} to 1.0 x 10^{10} K, which are higher than estimated lower limit from a previous Very Large Array observation with the highest spatial resolution of ~ 50 mas. The 44 GHz class I methanol maser in IRAS 18151-1208 is found to be associated with the MM2 core, which is thought to be less evolved than another millimeter core MM1 associated with the 6.7 GHz class II methanol maser.

A STRONG RADIO BRIGHTENING AT THE JET BASE OF M 87 DURING THE ELEVATED VERY HIGH ENERGY GAMMA-RAY STATE IN 2012

We report our intensive radio monitoring observations of the jet in M87 with the VLBI Exploration of Radio Astrometry (VERA) and the European VLBI Network (EVN) from February 2011 to October 2012, together with contemporaneous high-energy gamma-ray light curves obtained by the Fermi-LAT. During this period, an elevated level of the M87 flux is reported at VHE gamma rays. We detected a remarkable increase of the radio flux density from the unresolved jet base (radio core) with VERA at 22 and 43GHz coincident with the VHE activity. Meanwhile, we confirmed with EVN at 5GHz that HST-1 (an alternative gamma-ray production candidate site) remained quiescent in terms of its flux density and structure. These results in the radio bands strongly suggest that the VHE gamma-ray activity in 2012 originates in the jet base within 0.03pc or 56 Schwarzschild radii from the central supermassive black hole. We further conducted VERA astrometry for the M87 core during the flaring period, and detected core shifts between 22 and 43GHz. We also discovered a clear frequency-dependent evolution of the radio core flare at 43, 22 and 5GHz; the radio flux density increased more rapidly at higher frequencies with a larger amplitude, and the light curves clearly showed a time-lag between the peaks at 22 and 43GHz. This indicates that a new radio-emitting component was created near the black hole in the period of the VHE event, and then propagated outward with progressively decreasing synchrotron opacity. By combining these results, we estimated an apparent speed of the newborn component, and derived a sub-luminal speed of less than ~0.2c. This value is significantly slower than the super-luminal (~1.1c) features that appeared from the core during the prominent VHE flaring event in 2008, suggesting that the stronger VHE activity can be associated with the production of the higher Lorentz factor jet.

DISTANCE AND PROPER MOTION MEASUREMENT OF WATER MASERS IN SHARPLESS 269 IRS 2w

We present astrometric analysis of archival data of water masers in the star-forming region Sharpless 269 (S269) IRS 2w, observed with the VLBI Exploration of Radio Astrometry. An annual parallax of one of the bright maser features in this region was previously reported to be 0.189+/-0.008 milliarcsecond (mas) using part of the same archival data as we used. However, we found that this maser feature is not the best to represent the annual parallax to S269 IRS 2w because the morphology is remarkably elongated in the east-west direction. For this study we have selected another maser feature showing simpler morphology. This makes the new annual parallax estimate more credible. Our newly obtained annual parallax is 0.247+/-0.034 mas, corresponding to 4.05+0.65-0.49 kpc. This value is well consistent with the 3.7-3.8 kpc obtained using the kinematic distance estimates and photometric distance modulus. We considered two hypotheses for the water maser spatial distribution, a bipolar outflow and an expanding ring, in a kinematic model fitting analysis with a radially expanding flow. At this stage, any conclusions about the systemic proper motion could not be drawn from the kinematic analysis. Alternatively, we evaluated the mean proper motion to be (0.39+/-0.92, -1.27+/-0.90) mas/yr eastward and northward, respectively, from the obtained proper motions of the detected water maser features. The newly obtained annual parallax and mean proper motion give the peculiar motion of S269 IRS 2w to be (Us, Vs, Ws) of (8+/-6, -21+/-17, 1+/-18) km/s.

Accurate parallax measurement toward the symbiotic star R Aquarii

Abstract Multiepoch phase-referencing VLBI (Very Long Baseline Interferometry) observations with VERA (VLBI Exploration of Radio Astrometry) were performed for the symbiotic star R Aquarii (R Aqr) from 2005 September to 2006 October. Tracing one of the v = 2, J = 1–0 SiO maser spots, we measured an annual parallax of π = 4.59 ± 0.24 mas, corresponding to a distance of $218_{-11}^{+12}\:$pc. Our result is consistent with earlier distance measurements, but yields the highest accuracy of ∼ 5% level. Applying our distance, we derived an absolute K-band magnitude of MK = −7.71 ± 0.11, which is consistent with the recent period–luminosity relation by VLBI parallax measurements for five OH-Mira variables. In addition, the expansion age of an inner nebula around R Aqr is found to be ∼ 240 yr, corresponding to about the year 1773.

Error Models of the Analog to Digital Converters

Abstract Error models of the Analog to Digital Converters describe metrological properties of the signal conversion from analog to digital domain in a concise form using few dominant error parameters. Knowledge of the error models allows the end user to provide fast testing in the crucial points of the full input signal range and to use identified error models for post correction in the digital domain. The imperfections of the internal ADC structure determine the error characteristics represented by the nonlinearities as a function of the output code. Progress in the microelectronics and missing information about circuital details together with the lack of knowledge about interfering effects caused by ADC installation prefers another modeling approach based on the input-output behavioral characterization by the input-output error box. Internal links in the ADC structure cause that the input-output error function could be described in a concise form by suitable function. Modeled functional parameters allow determining the integral error parameters of ADC. Paper is a survey of error models starting from the structural models for the most common architectures and their linkage with the behavioral models represented by the simple look up table or the functional description of nonlinear errors for the output codes.

ANNUAL PARALLAX DETERMINATION TOWARD A NEW X-RAY-EMITTING CLASS 0 CANDIDATE WITH THE WATER MASER IN THE NGC 2264 STAR-FORMING REGION

In our multi-epoch observation of the star-forming region NGC 2264 with the VLBI Exploration of Radio Astrometry, we detected two water maser features. We have measured the annual parallax of the maser sources for the first time in NGC 2264 and derived 1.356 ± 0.098 mas, corresponding to the distance of pc, which is consistent with its previously reported photometric observations. One of the maser features is located near a dust continuum core, CMM4S, positionally corresponding to the X-ray source FMS2-1269 listed in Flaccomio et al. Re-analyzing Chandra X-ray data, we found that FMS2-1269 is still surrounded by a dense envelope of a H2 column density, 5.4 × 1023 cm–2. We suggest that the maser detected near the CMM4S core is most likely associated with the X-ray-emitting Class 0 candidate FMS2-1269. The other maser feature is found to emerge from a high-velocity outflow of 150 km s–1 and is associated with the 3.6 cm radio continuum source NGC 2264 VLA 3 located close to IRS1.