HRC-I Observations Research Articles

BURN OUT OR FADE AWAY? ON THE X-RAY AND MAGNETIC DEATH OF INTERMEDIATE MASS STARS

The nature of the mechanisms apparently driving X-rays from intermediate mass stars lacking strong convection zones or massive winds remains poorly understood, and the possible role of hidden, lower mass close companions is still unclear. A 20ks Chandra HRC-I observation of HR 4796A, an 8 Myr old main sequence A0 star devoid of close stellar companions, has been used to search for a signature or remnant of magnetic activity from the Herbig Ae phase. X-rays were not detected and the X-ray luminosity upper limit was L_X =< 1.3x10^27 erg/s. The result is discussed in the context of various scenarios for generating magnetic activity, including rotational shear and subsurface convection. A dynamo driven by natal differential rotation is unlikely to produce observable X rays, chiefly because of the difficulty in getting the dissipated energy up to the surface of the star. A subsurface convection layer produced by the ionisation of helium could host a dynamo that should be effective throughout the main-sequence but can only produce X-ray luminosities of order 10^25 erg/s. This luminosity lies only moderately below the current detection limit for Vega. Our study supports the idea that X-ray production in Herbig Ae/Be stars is linked largely to the accretion process rather than the properties of the underlying star, and that early A stars generally decline in X-ray luminosity at least 100,000 fold in only a few million years.

X-ray source variability study of the M 31 central field usingChandraHRC-I

[Abridged] The central field of the Andromeda galaxy (M 31) has been monitored, using the Chandra HRC-I detector (about 0.1-10 keV energy range) from 2006 to 2012 with the main aim to detect X-rays from optical novae. We present a systematic analysis of all X-ray sources found in the 41 nova monitoring observations, along with 23 M 31 central field HRC-I observations available from the Chandra data archive starting in December 1999. Based on these observations, we studied the X-ray long-term variability of the source population and especially of X-ray binaries in M 31. We created a catalogue of sources, detected in the 64 available observations, which add up to a total exposure of about 1 Ms. We present a point-source catalogue, containing 318 X-ray sources with detailed long-term variability information, 28 of which are published for the first time. The spatial and temporal resolution of the catalogue allows us to classify 115 X-ray binary candidates showing high X-ray variability or even outbursts in addition to 14 globular cluster X-ray binary candidates showing no significant variability. The analysis may suggest, that outburst sources are less frequent in globular clusters than in the field of M 31. We detected 7 supernova remnants, one of which is a new candidate and in addition resolved the first X-rays from a known radio supernova remnant. Besides 33 known optical nova/X-ray source correlations, we also discovered one previously unknown super-soft X-ray outburst and several new nova candidates. The catalogue contains a large sample of detailed long-term X-ray light curves in the M 31 central field, which helps to understand the X-ray population of our neighbouring spiral galaxy M 31.

Nova M31N 2007-12b: supersoft X-rays reveal an intermediate polar?

For the He/N nova M31N 2007-12b, we analyzed XMM-Newton EPIC and Chandra HRC-I observations of our monitoring program performed at intervals of ten days and added results of a XMM-Newton target of opportunity observation and Swift XRT observations. The supersoft source (SSS) emission started between 21 and 30 d after the optical outburst and ended between 60 and 120 d after outburst, making M31N 2007-12b one of the few novae with the shortest SSS phase known. The X-ray spectrum was supersoft and can be fitted with a white dwarf (WD) atmosphere model with solar abundances absorbed by the Galactic foreground. The temperature of the WD atmosphere seems to increase at the beginning of the SSS phase from ~70 to ~80 eV. The luminosity of M31N 2007-12b during maximum was at the Eddington limit of a massive WD and dropped by ~30% in the observation 60 d after outburst. The radius of the emission region is ~6x10^8 cm. In the four bright state observations, we detected a stable 1110 s pulsation, which we interpret as the WD rotation period. In addition, we detect dips in three observations that might represent a 4.9 h or 9.8 h binary period of the system. Nova envelope models with <50% mixing between solar-like accreted material and the degenerate core of the WD can be used to describe the data. We derive a WD mass of 1.2 Msun, as well as an ejected and burned mass of 2.0x10^{-6} Msun} and 0.2x10^{-6} Msun, respectively. The observed periodicities indicate that nova M31N 2007-12b erupted in an intermediate polar (IP) system. The WD photospheric radius seems to be larger than expected for a non-magnetic WD but in the range for magnetic WDs in an IP system. (abridged)

HRC-I/ChandraX-ray observations towardsσ Orionis

Aims: We investigated the X-ray emission from young stars and brown dwarfs in the sigma Orionis cluster (tau~3 Ma, d~385 pc) and its relation to mass, presence of circumstellar discs, and separation to the cluster centre by taking advantage of the superb spatial resolution of the Chandra X-ray Observatory. Methods: We used public HRC-I/Chandra data from a 97.6 ks pointing towards the cluster centre and complemented them with X-ray data from IPC/Einstein, HRI/ROSAT, EPIC/XMM-Newton, and ACIS-S/Chandra together with optical and infrared photometry and spectroscopy from the literature and public catalogues. On our HRC-I/Chandra data, we measured count rates, estimated X-ray fluxes, and searched for short-term variability. We also looked for long-term variability by comparing with previous X-ray observations. Results: Among the 107 detected X-ray sources, there were 70 cluster stars with known signposts of youth, two young brown dwarfs, 12 cluster member candidates, four field dwarfs, and two galaxies with optical-infrared counterpart. The remaining sources had extragalactic nature. Based on a robust Poisson-chi^2 analysis, nine cluster stars displayed flares or rotational modulation during the HRC-I observations, while other eight stars and one brown dwarf showed long-term X-ray flux variations. We constructed a cluster X-ray luminosity function from O9.5 (~18 Msol) to M6.5 (~0.06 Msol). We found: a tendency of early-type stars in multiple systems or with spectroscopic peculiarities to display X-ray emission, that the two detected brown dwarfs and the least-massive star are among the sigma Orionis objects with the highest L_X/L_J ratios, and that a large fraction of known classical T Tauri stars in the cluster are absent in this and other X-ray surveys. We concluded that dozens X-ray sigma Orionis stars and brown dwarfs are still to be detected [abridged].

Rigid Field Hydrodynamic simulations of the magnetosphere of σ Orionis E

AbstractWe present Rigid Field Hydrodynamic simulations of the magnetosphere of σ Ori E. We find that the X-ray emission from the star's magnetically confined wind shocks is very sensitive to the assumed mass-loss rate. To compare the simulations against the measured X-ray emission, we first disentangle the star from its recently discovered late-type companion using Chandra HRC-I observations. This then allows us to place an upper limit on the mass-loss rate of the primary, which we find to be significantly smaller than previously imagined.

X‐ray emission from optical novae in M 31

AbstractThe first supersoft source (SSS) identification with an optical nova in M 31 was based on ROSAT observations. Twenty additional X‐ray counterparts (mostly identified as SSS by their hardness ratios) were detected using archival ROSAT, XMM‐Newton and Chandra observations obtained before July 2002. Based on these results optical novae seem to constitute the major class of SSS in M 31. An analysis of archival Chandra HRC‐I and ACIS‐I observations obtained from July 2004 to February 2005 demonstrated that M 31 nova SSS states lasted from months to about 10 years. Several novae showed short X‐ray outbursts starting within 50 d after the optical outburst and lasting only two to three months. The fraction of novae detected in soft X‐rays within a year after the optical outburst was more than 30%. Ongoing optical nova monitoring programs, optical spectral follow‐up and an up‐to‐date nova catalogue are essential for the X‐ray work. Re‐analysis of archival nova data to improve positions and find additional nova candidates are urgently needed for secure recurrent nova identifications. Dedicated XMM‐Newton/Chandra monitoring programs for X‐ray emission from optical novae covering the centre area of M 31 continue to provide interesting new results (e.g. coherent 1105 s pulsations in the SSS counterpart of nova M31N 2007‐12b). The SSS light curves of novae allow us – together with optical information – to estimate the mass of the white dwarf, of the ejecta and the burned mass in the outburst. Observations of the central area of M 31 allow us – in contrast to observations in the Galaxy – to monitor many novae simultaneously and proved to be prone to find many interesting SSS and nova types (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

THE DISCOVERY OF A VERY FAINT X-RAY TRANSIENT IN THE GLOBULAR CLUSTER M15

We have identified an X-ray transient (hereafter M15 X-3) in the globular cluster M15 from an archival Chandra grating observation. M15 X-3 appears at an X-ray luminosity of 6*10^{33} ergs/s with a spectrum consistent with an absorbed power law of photon index 1.51+-0.14. The object is identifiable in archival Chandra HRC-I observations with an X-ray luminosity of 2-6*10^{31} ergs/s and apparently soft colors, suggesting a neutron star low-mass X-ray binary in quiescence. We also observe it in outburst in a 2007 Chandra HRC-I observation, and in archival 1994-1995 ROSAT HRI observations. We identify a likely optical/UV counterpart with a (possibly transient) UV excess from archival HST data, which suggests a main sequence companion. We argue that M15 X-3's behavior is similar to that of the very faint X-ray transients which have been observed in the Galactic Center. We discuss several explanations for its very low X-ray luminosity, with the assumption that we have detected its companion. M15 X-3's uniquely low extinction and well-determined distance make it an excellent target for future studies.

High‐Resolution X‐Ray Imaging of the Center of IC 342

We presented the result of a high resolution (FWHM~0.5'') 12 ks Chandra HRC-I observation of the starburst galaxy IC342 taken on 2 April 2006. We identified 23 X-ray sources within the central 30' x 30' region of IC342. Our HRC-I observation resolved the historical Ultraluminous X-ray sources (ULX), X3, near the nucleus into 2 sources, namely C12 and C13, for the first time. The brighter source C12, with L(0.08-10keV)=(6.66\pm0.45)\times10^{38}ergs^-1, was spatially extended (~82 pc x 127 pc). From the astrometric registration of the X-ray image, C12 was at R.A.=03h:46m:48.43s, decl.=+68d05m47.45s, and was closer to the nucleus than C13. Thus we concluded that source was not an ULX and must instead be associated with the nucleus. The fainter source C13, with L(0.08-10keV)=(5.1\pm1.4) x 10^{37}ergs^-1 was consistent with a point source and located $6.51'' at P.A. 240 degree of C12. We also analyzed astrometrically corrected optical Hubble Space Telescope and radio Very Large Array images, a comparison with the X-ray image showed similarities in their morphologies. Regions of star formation within the central region of IC342 were clearly visible in HST H alpha image and this was the region where 3 optical star clusters and correspondingly our detected X-ray source C12 were observed. We found that a predicted X-ray emission from starburst was very close to the observed X-ray luminosity of C12, suggesting that nuclear X-ray emission in IC342 was dominated by starburst. Furthermore, we discussed the possibility of AGN in the nucleus of IC342. Although our data was not enough to give a firm existence of an AGN, it could not be discarded.

X‐Ray Flaring on the dMe Star, Ross 154

We present results from two Chandra imaging observations of Ross 154, a nearby flaring M dwarf star. During a 61 ks ACIS-S exposure, a very large flare occurred (the equivalent of a solar X3400 event, with LX = 1.8 × 1030 ergs s−1) in which the count rate increased by a factor of over 100. The early phase of the flare shows evidence for the Neupert effect, followed by a further rise and then a two-component exponential decay. A large flare was also observed at the end of a later 48 ks HRC-I observation. Emission from the nonflaring phases of both observations was analyzed for evidence of low-level flaring. From these temporal studies we find that microflaring probably accounts for most of the "quiescent" emission and that, unlike for the Sun and the handful of other stars that have been studied, the distribution of flare intensities does not appear to follow a power law with a single index. Analysis of the ACIS spectra, which was complicated by exclusion of the heavily piled-up source core, suggests that the quiescent Ne/O abundance ratio is enhanced by a factor of ~2.5 compared to the commonly adopted solar abundance ratio and that the Ne/O ratio and overall coronal metallicity during the flare appear to be enhanced relative to quiescent abundances. Based on the temperatures and emission measures derived from the spectral fits, we estimate the length scales and plasma densities in the flaring volume and also track the evolution of the flare in color-intensity space. Lastly, we searched for a stellar wind charge exchange X-ray halo around the star but without success; because of the relationship between mass-loss rate and the halo surface brightness, not even an upper limit on the stellar mass-loss rate can be determined.

Probing the proper motion of the central compact object in Puppis-A with the Chandra high resolution camera

By making use of the sub-arcsecond angular resolution of the High Resolution Camera (HRC-I) aboard the Chandra X-ray satellite we have examined the central compact object RX J0822-4300 in the supernova remnant Puppis-A for a possible proper motion. Using data which span an epoch of 1952 days we found the position of RX J0822-4300 different by 0.574 ± 0.184 arcsec, implying a proper motion of ± 34.46 mas/yr. For a distance of 2.2 kpc, this proper motion is equivalent to a recoil velocity of 1121.79 ± 359.60 km s-1. The position angle is found to be ± . Since both the magnitude and direction of the proper motion are in agreement with the birth place of RX J0822-4300 being near to the optical expansion center of the supernova remnant, the result presented in this letter is a promising indication of a fast moving compact object in a supernova remnant. Although the positional shift inferred from the current data is significant at a ~ level only, one or more future HRC-I observations can obtain a much larger positional separation and further constrain the measurement.

ChandraX‐Ray Observation of the Orion Nebula Cluster. I. Detection, Identification, and Determination of X‐Ray Luminosities

In this first of two companion papers on the Orion Nebula Cluster (ONC), we present our analysis of a 63 Ksec Chandra HRC-I observation that yielded 742 X-ray detections within the 30'x30' field of view. To facilitate our interpretation of the X-ray image, here we collect a multi-wavelength catalog of nearly 2900 known objects in the region by combining 17 different catalogs from the recent literature. We define two reference groups: an infrared sample, containing all objects detected in the K band, and an optical sample comprising low extinction, well characterized ONC members. We show for both samples that field object contamination is generally low. Our X-ray sources are primarily low mass ONC members. The detection rate for optical sample stars increases monotonically with stellar mass from zero at the brown dwarf limit to ~100% for the most massive stars but shows a pronounced dip between 2 and 10 solar masses. We determine L_X and L_X/L_{bol} for all stars in our optical sample and utilize this information in our companion paper to study correlations between X-ray activity and other stellar parameters.

ITAL]Chandra[/ITAL] High Resolution Camera Imaging of GRS 1758−258

We observed the microquasar GRS 1758-258 four times with Chandra. Two observations with the imaging detector of the High Resolution Camera (HRC-I) were made in 2000 September-October spanning an intermediate-to-hard spectral transition (identified with the Rossi X-Ray Timing Explorer). Another HRC-I observation and an Advanced CCD Imaging Spectrometer/High Energy Transmission Grating (HETG) observation were made in 2001 March following a hard-to-soft transition to a very low flux state. Based on the three HRC images and the HETG zero-order image, the accurate position (J2000) of the X-ray source is R.A. = 18h01m1239, decl. = -25°44'361 (90% confidence radius = 045), consistent with the purported variable radio counterpart. All three HRC images are consistent with GRS 1758-258 being a point source, indicating that any bright jet is less than ~1 lt-month in projected length, assuming a distance of 8.5 kpc.

Deconvolving the Nucleus of Centaurus A Using Chandra PSF Library

We describe preliminary results from our study of multi-scale structures in Centaurus A (NGC 5128) obtained using the Chandra X-ray Observatory HRC-I observations. The high-angular resolution Chandra images reveal X-ray multi-scale structures in this object with unprecedented detail and clarity. The region surrounding the Cen A nucleus, believed to be associated with a supermassive black hole, shows structures on arcsecond scales clearly resolved from the central source.