Candidate Counterparts Research Articles

An infrared imaging survey of the faint Chandra sources near the Galactic Centre

We present near-IR imaging of a sample of the faint, hard X-ray sources discovered in the 2001 Chandra ACIS-I survey towards the Galactic Centre (GC) (Wang et al. 2002). These ~800 discrete sources represent an important and previously undetected population within the Galaxy. From our VLT observations of 77 X-ray sources, we identify candidate K-band counterparts to 75% of the Chandra sources in our sample. The near-IR magnitudes and colours of the majority of candidate counterparts are consistent with highly reddened stars, indicating that most of the Chandra sources are likely to be accreting binaries at or near the GC.

ChandraHRC Localization of the Low‐Mass X‐Ray Binaries X1624−490 and X1702−429: The Infrared Counterparts

We report on the precise localization of the low-mass X-ray binaries X1624-490 and X1702-429 with the Chandra HRC-I. We determine the best positions to be 16h28m02825, - 49°11'5461 and 17h06m15314, - 43°02'0869 (J2000.0) for X1624-490 and X1702-429, respectively, with the nominal Chandra positional uncertainty of 06. We also obtained deep IR observations of the fields of these sources in an effort to identify the IR counterparts. A single, faint (Ks = 18.3 ± 0.1) source is visible inside the Chandra error circle of X1624-490, and we propose this source as its IR counterpart. For X1702-429, a Ks = 16.5 ± 0.07 source is visible at the edge of the Chandra error circle. The brightness of both counterpart candidates is comparable to that of other low-mass X-ray binary IR counterparts when corrected for extinction and distance.

Precise Localization of the Soft Gamma Repeater SGR 1627−41 and the Anomalous X‐Ray Pulsar AXP 1E1841−045 withChandra

We present precise localizations of AXP 1E1841-045 and SGR 1627-41 with Chandra. We obtained new infrared observations of SGR 1627-41 and reanalyzed archival observations of AXP 1E1841-045 in order to refine their positions and search for infrared counterparts. A faint source is detected inside the error circle of AXP 1E1841-045. In the case of SGR 1627-41, several sources are located within the error radius of the X-ray position and we discuss the likelihood of one of them being the counterpart. We compare the properties of our candidates to those of other known AXP and SGR counterparts. We find that the counterpart candidates for SGR 1627-41 and SGR 1806-20 would have to be intrinsically much brighter than AXPs to have detectable counterparts with the observational limits currently available for these sources. To confirm the reported counterpart of SGR 1806-20, we obtained new IR observations during the July 2003 burst activation of the source. No brightening of the suggested counterpart is detected, implying that the counterpart of SGR 1806-20 remains yet to be identified.

The isolated neutron star X-ray pulsars RX J0420.0–5022 and RX J0806.4–4123: New X-ray and optical observations

We report on the analysis of new X-ray data obtained with XMM-Newton and Chandra from two ROSAT-discovered X-ray dim isolated neutron stars (XDINs). RX J0806.4−4123 was observed with XMM-Newton in April 2003, 2.5 years after the first observation. The EPIC-pn data confirm that this object is an X-ray pulsar with 11.371 s neutron star spin period. The X-ray spectrum is consistent with absorbed black-body emission with a temperature kT = 96 eV and N H = 4 × 10 19 cm −2 without significant changes between the two observations. Four XMM-Newton observations of RX J0420.0−5022 between December 2002 and July 2003 did not confirm the 22.7 s pulsations originally indicated in ROSAT data, but clearly reveal a 3.453 s period. A fit to the X-ray spectrum using an absorbed black-body model yields kT = 45 eV, the lowest value found from the small group of XDINs and N H = 1.0 × 10 20 cm −2. Including a broad absorption line improves the quality of the spectral fits considerably for both objects and may indicate the presence of absorption features similar to those reported from RBS1223, RX J1605.3+3249 and RX J0720.4−3125. For both targets we derive accurate X-ray positions from the Chandra data and present an optical counterpart candidate for RX J0420.0−5022 with B = 26.6 ± 0.3 mag from VLT imaging.

A Synoptic X‐Ray Study of M31 with theChandraHigh Resolution Camera

We have obtained 17 epochs of Chandra High Resolution Camera (HRC) snapshot images, each covering most of the M31 disk. The data cover a total baseline of ~2.5 yr and contain a mean effective exposure of 17 ks. We have measured the mean fluxes and long-term light curves for 166 objects detected in these data. At least 25% of the sources show significant variability. The cumulative luminosity function (CLF) of the disk sources is well fitted by a power law with a slope comparable to those observed in typical elliptical galaxies. The CLF of the bulge is a broken power law similar to measurements made by previous surveys. We note several sources in the southwestern disk with LX > 1037 ergs s-1. We cross-correlate all of our sources with published optical and radio catalogs, as well as new optical data, finding counterpart candidates for 55 sources. In addition, 17 sources are likely X-ray transients. We analyze follow-up Hubble Space Telescope (HST) WFPC2 data of two X-ray transients, finding F336W (U-band equivalent) counterparts. In both cases, the counterparts are variable. In one case, the optical counterpart is transient with F336W = 22.3 ± 0.1 mag. The X-ray and optical properties of this object are consistent with a ~10 M☉ black hole X-ray nova with an orbital period of 23 days. In the other case, the optical counterpart varies between F336W = 20.82 ± 0.06 and 21.11 ± 0.02 mag. Ground-based and HST observations show that this object is bright (V = 18.8 ± 0.1) and slightly extended. Finally, the frequency of bright X-ray transients in the M31 bulge suggests that the ratio of neutron star to black hole primaries in low-mass X-ray binaries (NS/BH) is ~1.

PSR B1951+32: A Bow Shock-confined X-Ray Nebula, a Synchrotron Knot, and an Optical Counterpart Candidate

The radio pulsar PSR B1951+32 and the supernova remnant CTB 80 provide a rich laboratory for the study of neutron stars and supernova remnants. Here, we present ground-based optical and near-infrared observations of them, along with X-ray observations with Chandra and a reanalysis of archival data obtained with the Hubble Space Telescope. The X-ray observations reveal a cometary pulsar wind nebula that appears to be confined by a bow shock produced by the high-velocity motion of the pulsar, making PSR B1951+32 a rare pulsar exhibiting both an Hα bow shock and a shocked X-ray pulsar wind nebula. The distribution of Hα and radio continuum emission is indicative of a contact discontinuity of the shocked pulsar winds and shocked ambient medium at ~0.05 pc. On the other hand, the optical synchrotron knot of PSR B1951+32 likely has a flat spectrum in the optical and near-infrared wave bands, and our astrometry is consistent with only one of the two reported optical counterpart candidates for the pulsar.

Near-Infrared Colors of Submillimeter-selected Galaxies

We report on deep near-infrared (NIR) observations of submillimeter-selected galaxies (SMGs) with the Near Infrared Camera (NIRC) on the Keck I telescope. We have identified K-band candidate counterparts for 12 out of 15 sources in the SCUBA Cluster Lens Survey. Three SMGs remain non-detections with K-band limits of K>23 mag, corrected for lensing. Compensating for lensing we find a median magnitude of K=22+/-1 mag for the SMG population, but the range of NIR flux densities spans more than a factor of 400. For SMGs with confirmed counterparts based on accurate positions from radio, CO, and/or millimeter continuum interferometric observations, the median NIR color is J-K=2.6+/-0.6 mag. The NIR-bright SMGs (K 3 mag). We argue that a color selection criterion of J-K>~3 mag can be used to help identify counterparts of SMGs that are undetected at optical and radio wavelengths. The number density of sources with J-K>3 mag is 5 arcmin^{-2} at K 2 mJy. It is not clear if the excess represents less luminous infrared-bright galaxies with S(850um)<~2 mJy, or if the faint extremely red NIR galaxies represent a different population of sources that could be spatially related to the SMGs.

A Large, Uniform Sample of X-Ray-emitting AGNs: Selection Approach and an Initial Catalog from theROSATAll-Sky and Sloan Digital Sky Surveys

Many open questions in X-ray astronomy are limited by the relatively small number of objects in uniform optically identified and observed samples, especially when rare subclasses are considered or when subsets are isolated to search for evolution or correlations between wavebands. We describe the initial results of a new program aimed to ultimately yield ~104 fully characterized X-ray source identifications?a sample about an order of magnitude larger than earlier efforts. The technique is detailed and employs X-ray data from the ROSAT All-Sky Survey (RASS) and optical imaging and spectroscopic follow-up from the Sloan Digital Sky Survey (SDSS); these two surveys prove to be serendipitously very well matched in sensitivity. As part of the SDSS software pipelines, optical objects in the SDSS photometric catalogs are automatically positionally cross-correlated with RASS X-ray sources. Then priorities for follow-on SDSS optical spectra of candidate counterparts are automatically assigned using an algorithm based on the known ratios of fx/fopt for various classes of X-ray emitters at typical RASS fluxes of ~10-13 ergs cm-2 s-1. SDSS photometric parameters for optical morphology, magnitude, and colors, plus FIRST radio information, serve as proxies for object class. Initial application of this approach to RASS/SDSS data from 1400 deg2 of sky provides a catalog of more than 1200 spectroscopically confirmed quasars and other AGNs that are probable RASS identifications. Most of these are new identifications, and only a few percent of the AGN counterparts are likely to be random superpositions. The magnitude and redshift ranges of the counterparts are very broad, extending over 15 < m < 21 and 0.03 < z < 3.6, respectively. Although most identifications are quasars and Seyfert 1 galaxies, a variety of other AGN subclasses are also sampled. Substantial numbers of rare AGN types are found, including more than 130 narrow-line Seyfert 1 galaxies and 45 BL Lac candidates. These early results already provide a very sizable set of source identifications, demonstrate the utility of the sample in multiwaveband investigations, and show the capability of the joint RASS/SDSS approach to efficiently proceed toward the largest homogeneously selected/observed sample of X-ray?emitting quasars and other kinds of AGNs.

An XMM-Newton observation of IGR J16320-4751 = AX J1631.9-4752

The hard X-ray sensitivity and arcminute position accuracy of the recently launched International Gamma-Ray Laboratory (INTEGRAL) has lead to the (re-)discovery of a class of heavily absorbed hard X-ray sources lying in the Galactic plane. We report on the analysis of an XMM observation of such a source IGR J16320 4751 = AX J1631.9 4752. Our analysis allowed us to obtain the most accurate X-ray position to date (Rodriguez et al. 2003), and to identify a likely infrared counterpart (Tomsick et al. 2003). We present the detailed analysis of the IGR J16320 4751 XMM spectra. The PN spectrum can be well represented by a single powerlaw or a comptonized spectrum with a high equivalent absorption column density of2 10 23 cm 2 . The current analysis and the comparison with the properties of other sources favor the possibility that the source is a Galactic X-Ray Binary (XRB). The identification of two candidate IR counterparts is in good agreement with this identification. The hard spectrum previously seen with ASCA, and the brightness of the candidate counterparts indicate that IGR J16320 4751 is most probably a highly absorbed High Mass X-ray Binary, hosting a neutron star.

Spectroscopic identification of the infrared counterpart to GX5-1

Using CGS4 on UKIRT, we have obtained a 1.95–2.45 μm infrared (IR) spectrum of the primary candidate counterpart to the bright Z LMXB GX5-1.IR photometry by Naylor, Charles & Longmore and the astrometry of Jonker et al.had previously identified this star as the most likely counterpart to GX5-1.The spectrum presented here clearly shows Brackett γ and He lines in emission, for the first time confirming the identity of the counterpart.Similar to our previous spectroscopy of the Z source LMXBs Sco X-1 and Sco X-2 (Bandyopadhyay et al.1999), the K-band spectrum of GX5-1 shows emission lines only.We briefly discuss the implications of this spectrum for the nature of the Z sources.

DeepBVRimaging of the field of the millisecond pulsar PSR J0030+0451 with the VLT

We report on deep BVR-imaging of the field of the nearby millisecond pulsar PSR J0030+0451 obtained with the ESO/VLT/FORS2. We do not detect any optical counterpart down to B > 27.3, V > 27. 0a ndR > 27.0 in the immediate vicinity of the radio pulsar position. The closest detected sources are offset by >3 �� , and they are excluded as counterpart candidates by our astrometry. Using our upper limits in the optical, and including recent XMM-Newton X-ray data we show that any nonthermal power-law spectral component of neutron star magnetospheric origin, as suggested by the interpretation of X-ray data, must be suppressed by at least a factor of ∼500 in the optical range. This either rules out the nonthermal interpretation or suggests a dramatic spectral break in the 0.003−0.1 keV range of the power-law spectrum. Such a situation has never been observed in the optical/X-ray spectral region of ordinary pulsars, and the origin of such a break is unclear. An alternative interpretation with a purely thermal X-ray spectrum is consistent with our optical upper limits. In this case the X-ray emission is dominated by hot polar caps of the pulsar.

Detection of new optical counterpart candidates to PSR B1951+32 with HST/WFPC2

There remain several definitive-ray pulsars that are as yet undetected in the optical regime. A classic case is the pulsar PSR B1951+32, associated with the complex CTB 80 SNR. Previous ground based high speed 2-d optical studies have ruled out candidates to mV 24. Hester (2000a) has reported an analysis of archival HST/WFPC2 observations of the CTB 80 complex which suggest a compact synchrotron nebula coincident with the pulsar's radio position. Performing a similar analysis, we have identified a possible optical counterpart within this synchrotron nebula at mV 25:5 26, and another optical counterpart candidate nearby at mV 24:5. We assess the reality of these counterpart candidates in the context of existing models of pulsar emission.

Possible Identifications for Southern EGRET Sources

We have made total flux density observations at high radio frequencies (90 and 230 GHz) of 12 southern AGNs that were classified as possible EGRET identifications in the Third EGRET Catalog. Our observations confirm the blazar nature of five of them. We have also studied sources that we considered good candidates for AGN counterparts of previously unidentified EGRET sources and that had not been observed in the millimeter domain before. Four of them showed millimeter range activity that may be related to their gamma-ray activity, making them good candidates for the EGRET source identification.

6.7 GHz methanol masers at sites of star formation

We report the results of an unbiased survey for 6.7 GHz methanol maser emission of a ∼21 deg 2 strip of the Galactic plane carried out with the 32 m Torun radio telescope. An area at 20° ≤ l < 40°, ‖b‖ ≤ 0°.52 was surveyed in an equilateral triangular grid with a sensitivity limit of about 1.6 Jy. We detected a total of 100 sources, 26 of which are new detections. All the new sources are of moderate intensity and their peak flux densities have median value of 6.5 Jy, i.e. about half that of previously known sources in the sample. About 80% of maser sources have IR counterpart candidates within a 1' radius but not all the IRAS counterparts of methanol masers have colours typical of ultracompact HII regions. An excess of masers unassociated with IR sources occurs at 30° < l < 35° because of incompleteness of IR catalogues due to strong confusion near the tangential region of the spiral arm. Our unbiased survey doubled the number of detections as compared to IRAS-based observations. Within the positional uncertainty of 1' about one third of the methanol sources have radio continuum counterparts at 5 GHz of a flux density greater than 2.5-10 mJy. The distribution of methanol sources appears to be consistent with a clustered mode of formation of massive stars.

NIR Narrow- and Broad-Band Study of the SSA 22 Field

Abstract Deep narrow- and broad-band near-infrared imaging observations of the central $ {2{}^{\mathrm {\prime }}} \times {2{}^{\mathrm {\prime }}}$ region of the SSA 22 field were made with the near-infrared camera (CISCO) attached to the Subaru Telescope. Using a narrow-band filter centered at $2.033\, {\mu \mathrm {m}}$, [O III] $\lambda~5007$ emitters at $z \sim 3.06 \pm 0.02$ were searched to examine star-forming activities in an over-density region where a clustering of Lyman Break Galaxies (LBGs) and Lyman $ \alpha$ emitter candidates around $ z = 3.09$ was reported, though the targeted redshift is slightly different from that of the peak of the over-density region. Although one emitter candidate at $ z = 3.06$ was detected, it is likely to be located at a redshift of between 1 and 2 judged based on multi-band photometry. Another emission-line object was detected in another narrow-band filter (“off band” filter) centered at $ 2.120\, {\mu \mathrm {m}}$, which is identified with a galaxy at $ z = 0.132$ (the emission line is Paschen $\alpha$). The $ K^{\prime}$-band imaging data revealed the presence of 12 Extremely Red Objects (EROs) with $I_{814}-K^{\prime} \geq4$. The distribution of the EROs does not seem to coincide with that of Lyman Break Galaxies or Lyman $\alpha $ emitters at $ z \sim3$. The magnitudes and colors of the EROs are not consistent with those of passively evolving massive elliptical galaxies at $ z \sim3$. Candidates for counterparts of the submm sources detected with SCUBA are found; no EROs around the submm sources are found in our magnitude limit.

NICMOS imaging search for high-redshift damped Lyαgalaxies

We are engaged in a programme of imaging with the STIS and NICMOS (NIC2) instruments aboard the Hubble Space Telescope (HST), to search for the galaxy counterparts of 18 high-redshift z>1.75 damped Lyα absorption lines and five Lyman-limit systems seen in the spectra of 16 target quasars. This paper presents the results of the imaging campaign with the NIC2 camera. We describe the steps followed in reducing the data and combining in mosaics, and the methods used for subtracting the image of the quasar in each field, and for constructing error frames that include the systematic errors associated with the PSF subtraction. To identify candidate counterparts, which are either compact or diffuse, we convolved the image and variance frames with circular top-hat filters of diameter 0.45 and 0.90 arcsec respectively, to create frames of summed S/N within the aperture. For each target quasar we provide catalogues listing positions and aperture magnitudes of all sources within a square of side 7.5 arcsec centred on the quasar, detected at S/N>6. We find a total of 41 candidates, of which three have already been confirmed spectroscopically as the counterparts. We provide the aperture magnitude detection limits as a function of impact parameter, for both detection filters, for each field. The average detection limit for compact (diffuse) sources is HAB=25.0 (24.4) at an angular separation of 0.56 (0.79) arcsec from the quasar, improving to HAB=25.5 (24.8) at large angular separations. For the brighter sources we have measured the half-light radius and the n parameter of the best-fitting deconvolved Sersic-law surface brightness profile, and the ellipticity and orientation.

The Optical Counterparts of the Luminous X‐Ray Binary Stars in Globular Clusters

Ten percent of our GalaxyIs luminous ergs (L X 1036 s~1) X-ray point sources are located in globular clusters (GCs), but GCs contribute a far smaller fraction of normal stars to the Galaxy. X-ray bursts have been observed from nearly all GC sources, indicating that they are low-mass X-ray binary (LMXB) systems containing a neutron star and a low-mass companion star from which matter is being transferred. The understanding of LMXB overabundance in GCs may well lead to important insights into the formation and evolution of these exotic binary systems as well as the dynamics of GCs themselves. The goals of this dissertation are to identify the optical counterparts to some GC LMXBs lacking candidates, bring together and compare in the most homogeneous fashion all available Hubble Space T elescope (HST ) optical observations of the current group of GC LMXB counterparts, and discuss the implications for cluster dynamics and LMXB systems in general. In this work, candidates for three additional optical counterparts to luminous X-ray binaries in GCs are presented, thereby doubling the number of optical counterpart candidates. Two, those in NGC 1851 and NGC 6441, are very likely correct although they require additional work to conÐrm the identiÐcations, while the third, in NGC 6652, remains somewhat tentative because of the positional discrepancy with the X-ray coordinates and the fact that the entire error circle is not surveyed. A homogeneous set of HST photometric measurements for all the counterparts identiÐed thus far is presented, and their optical properties are compared with those of Ðeld LMXBs. The mean color of the cluster sources is (U[B)0 identical to that of the Ðeld sources, and the mean is M B0 similar in range, but fainter on average than bursters in the Ðeld. The ratio of optical to X-ray Nux of cluster sources seems to show a signiÐcantly larger dispersion than that of the Ðeld sources. The GC LMXBs seem to conform well to the previously suggested optical luminosity, X-ray luminosity, orbital period relation (J. van Paradijs & J. McClintock, A&A, 290, 133 [1994]). If the source in NGC 1851Efor which no orbital period has yet been determinedEfollows this pattern, then the period will likely be less than 0.8 hr. Similarly, if the object advanced here in NGC 6652 is indeed the correct optical counterpart, this relation suggests that its orbital period will also prove to be less than 1 hr. New and archival HST spectra and imaging data are analyzed to intercompare the UV/optical spectral energy distributions (SEDs) of GC LMXBs. A set of simple model SEDs is introduced and compared with the observations to infer accretion rates, disk diameters, and other properties of these systems. The GC LMXBs are particularly well suited for SED modeling as the distance and reddening of these systems is quite well understood, unlike most Ðeld LMXBs. The sources in NGC 6441 and M15 are found to be optically quite luminous, with comparatively shallow SEDs. The faint sources in NGC 6712 and NGC 1851 have signiÐcantly steeper SEDs and are most likely underluminous because their orbital separations and thus accretion disks are small. The case of NGC 6624 appears slightly di†erent from the others, with an average optical luminosity, yet with the steepest SED slope. This is most likely due to its high accretion rate isD10 times higher than any of the other (L X GC LMXBs) and extremely small size. From its luminosity, it is inferred that the source in NGC 6652 will have similar system parameters and SED to the one in NGC 1851. If these inferences prove correct, it will mean that twothirds of GC LMXBs for which orbital periods are known have periods less than 1 hr and are therefore quite likely double degenerate ultracompact binaries. Such a high fraction of ultracompact systems would be remarkable and may indicate that the processes responsible for creating such short-period systems are more common than previously thought.

Apparent Double SiO Maser Sources

During the course of a SiO maser survey of galactic disk IRAS sources at 43 GHz, we found two cases of doubly peaked SiO maser sources in the 40″ half-power beam of the Nobeyama 45-m telescope. The double peaks in the spectra of these sources (IRAS 18019-1822 and 18465-0717) have velocity separations of 73 and 147 km s−1, indicating that each peak comes from a separate source. Infrared imaging observations found pairs of candidate counterparts for each source with angular separations of about 20″. Combining the observational data, we conclude that these objects are apparent-double SiO sources, which are by chance seen in the same direction. The discovery rate for pairs of SiO maser sources in a single telescope beam is found to be consistent with the surface density of SiO maser sources in the galactic disk. These sources can be used to measure the parallaxes and proper motions with VLBI in the future.

Bok Globules in the Large Magellanic Cloud

We report the discovery of small, isolated dust clouds in the Large Magellanic Cloud (LMC) that are excellent candidates for counterparts to the Bok globules observed in the Galaxy. We detect these small clouds silhouetted against diffuse Hα emission on the basis of parallel imaging with the Wide Field Planetary Camera 2 on the Hubble Space Telescope. The clouds we identify as Bok globule candidates have typical sizes of approximately 1'', corresponding to a linear diameter of about 0.25 pc at the distance of the LMC. We derive lower limits for the optical depth within the dark clouds and masses, assuming that the clouds have density distributions similar to Galactic Bok globules. The sizes and estimated masses for LMC globules are comparable to those estimated for Galactic globules. An extended sample of such objects would be excellent targets for high-resolution infrared and millimeter observations to study low-mass star formation in such clouds in low-metallicity environments where the distance is well known.

Gamma‐Ray Burst Counterparts: Optical Data

The surest solution of the gamma-ray burst (GRB) mystery is to find an unambiguous low-energy quiescent counterpart. To this end, we have intensively searched within the smallest GRB error boxes for any counterpart candidates. This paper reports on 255 hr of exposure with ground-based telescopes since 1981. We report our results in the U, B, V, R. I, J, H, and K bands. We find the usual array of mildly unusual sources in the boxes, but none is sufficiently unusual to suggest a causal connection. We find that the smallest boxes are empty to fairly deep magnitudes. This fact can be of significance since virtually all cosmological models place bright bursters inside normal host galaxies at moderate distances. To allow for quantitative evaluation of the predictions of these models, we have compiled a list of limits on the brighest galaxy inside each of the 26 regions in various bands. This list was compiled from our own results as well as from the published literature. The limits on host galaxy luminosities from these data are substantially more restrictive than the limits from recent optical transients because the bursts we report on are much brighter than the bursts with optical transients.