Drs. Robert Petre and Frank Marshall in collaboration with Drs. Cassey Lisse (U. Maryland), Konrad Dennerl (MPE), and others have produced results which opened a surprising new area of X-ray astronomy: the study of X-ray emission from comets. X-ray emission from Comet Hyakutake was discovered using the ROSAT HRI and there have been follow up observations of Hyakutake and observations of Comets Hale-Bopp and Tabur using ROSAT, RXTE and ASCA.
Drs. Michael Corcoran and Jean Swank with Drs. Ian Stevens (U. Birmingham), Andy Pollock (Computer & Scientific Co. Ltd), Steven Shore (Indiana U. South Bend), and Gayle Rawley (Applied Research Corporation) analyzed phase-variable X-ray emission from the WR+O binary V444 Cygni, and interpreted the variations using a simplified colliding wind model. Dr. Corcoran, with Drs. Stevens, Steven Skinner (U. Colorado), Allan Willis (University College London), Fumiaki Nagase (ISAS), and Katsuji Koyama (Kyoto U.) obtained and analyzed X-ray spectra of the colliding-wind WR+O binary Gamma2 Velorum using the ASCA satellite. These observations showed that the wind collision produces a high-temperature (kT=1.2 keV) region which suffers variable absorption. Dr. Corcoran also analyzed archival ROSAT X-ray observations of 18 O binary stars, and found clear evidence for phase-dependent variability in the X-ray emission for 4 of the stars, and factor-of-two sporadic variability in another 5 systems.
Drs. Corcoran, Rawley, Swank, Stephen Drake, and Robert Petre with Drs. Augusto Damineli (Instituto Astronomico e Geofisico da USP), Roberto Viotti (Istitutodi Astrofisica Spaziale), Stephen White and Douglas Curry (U. Maryland), and Mr. Kazunori Ishibashi (U. Minnesota) have continued monitoring X-ray emission from the peculiar massive star Eta Carinae. A multi-wavelength campaign has been organized, including X-ray, optical, UV and radio observations. Dr. Corcoran is taking the lead on obtaining and analyzing the X-ray observations, and currently observations of the star with every available X-ray observatory have either been obtained or are scheduled.
Dr. Corcoran has taken the lead in organizing a dedicated campaign of X-ray observations of important massive stars in order to better understand the connection of the X-ray emission with other phenomena in the outer atmospheres of these objects. Currently 37 astronomers from various institutions around the world are participating, either in direct analysis of the X-ray observations or by providing supporting observations in the optical, UV or radio.
Drs. Nicholas White, Lorella Angelini, and Drake, together with Drs. Steven Pravdo (JPL) and Robert Stern (Lockheed Martin) published a paper on the newly discovered planetary system 51 Peg. They showed that the low X-ray emission of the solar-type star was inconsistent with the 4-day orbital period of the companion, unless the latter has a mass of 10 Jupiter masses or less. This is independent evidence that 51 Peg B is indeed a planetary mass object.
Drs. Drake, White, and Kulinder Singh continued their research on the coronae of late-type stars using ROSAT PSPC and ASCA soft X-ray spectra. They also compared the X-ray emission properties of RS CVn binaries with Algol-type binaries. Some studies were done in collaboration with Drs. Theodore Simon (U. Hawaii), Rolf Mewe (SRON), Jelle Kaastra (SRON), Duane Liedahl (LLNL), Marek Siarkowski (Polish Academy of Sciences), and Pavel Pres (Wroclaw U.). These analyses of the X-ray spectra all continued to show that the elemental abundances in the coronae of active stars are inconsistent with their photospheric values, implying that either there is a problem with the current atomic physics used to model these spectra or that some kind of element separation process is occurring.
Dr. Drake has been working on a continuing collaboration with Stephen White (U. Maryland), Robert Duncan (ATNF, Australia), and several others on radio monitoring of the prototypical Luminous Blue Variable Star Eta Carinae. Observations over a two-year period indicate a tripling of the cm-emission from the star and its Homunculus Nebula suggestive of a pulse of ionization having passed through this region. This radio increase may be related to the increase in the hard X-ray emission from this star that the collaboration led by Dr. Michael Corcoran has recently discovered.
Drs. Drake and Simon, together with Philip Kim (U. Hawaii), have studied the ROSAT Archive in order to explore the X-ray properties of normal A-type stars. They have found that, while the very latest A stars appear to represent the hottest members of the coronal-type stars, there is no good explanation for the 20% fraction of early- and mid- A-type stars that are X-ray emitters. It may be the case that these are all binary stars with later-type coronal companions, but the alternate hypothesis that this is a new class of X-ray emitting stars is also a viable alternative.
Dr. Keith Arnaud, in collaboration with Dr. Kazimierz Borkowski (North Carolina State U.) and Prof. Patrick Harrington (U. Maryland), analyzed ASCA and HST observations of the planetary nebula BD+30 3639. The ASCA spectrum shows the presence of 3 million degree gas, several times lower than that expected from the stellar wind velocity. This may be due to mixing of stellar wind gas and nebular material which is also hinted at by the discovery of small dusty clumps in the HST [SIII] and [OII] narrow band images.
Drs. Nicholas White, Lorella Angelini, and Ken Ebisawa in collaboration with Drs. Y. Tanaka (ISAS) and P. Ghosh (TATA) used ASCA data and Einstein SSS archival data to investigate the properties of the 8.7 second X-ray pulsar 4U0142+61. The large blackbody flux measured implies a bigger emission region than expected for disk accretion onto a magnetized neutron star than can be explained by a substantial radial flow component. This evidence together with the measured spin down provides indirect support that 4U0142+61 is an isolated neutron star accreting material from a remnant of a Thorne-Zytkow object.
Dr. Eric Gotthelf, with collaborators Drs. Vicky Kaspi (CalTech) and Matthew Bailes (U. Melbourne) continued the study of a newly discovered millisecond radio pulsar J1105-6107. Recent X-ray observations of this object revealed the faint X-ray emission prediced for this young, energetic pulsar.
Drs. Ken Ebisawa, Lev Titarchuk, and Sandip Chakrabarti proposed a model to explain the difference of the slopes of the hard X-ray spectra in the high and low states of galactic black hole candidates.
Drs. Ebisawa, Charles Day, and Tim Kallman in collaboration with Drs. Fumiaki Nagase and Taro Kotani (ISAS), Drs. Kenji Kawashima and Shunji Kitamoto (Osaka U.), and Dr. Jonathan Woo (MIT), studied an orbital eclipse of Cen X-3 with ASCA, and determined the ionization structure of the photoionized plasma around the neutron star.
Drs. Ebisawa and Nicholas White in collaboration with Dr. Yoshihiro Ueda and Prof. Hajime Inoue (ISAS) and Prof. Yasuo Tanaka (MPE) studied iron line structure in the X-ray energy spectra of Cyg X-1 with ASCA, and found that the spectrum is explained by reflection by an outer part of the accretion disk which is moderately ionized.
Dr. Altan Baykal studied the torque and X-ray flux fluctuations of the HMXRB OAO 1657-415 using the BATSE archival data base at HEASARC. These were explained as the episodic formation of accretion disks.
Drs. Koji Mukai, Eric Schlegel, and Jean Swank (GSFC) and Drs. Janet Wood and Tim Naylor (Keele) have analyzed the ASCA data on the dwarf nova system, HT Cassiopeiae. They have shown that this system, which shows an eclipse of the white dwarf by the companion in visual light, also shows a deep eclipse in the X-rays. They have been able to locate the X-ray emission to the immediate vicinity of the white dwarf; using the measurement of the eclipse transitions, they have also constrained the size of the X-ray emitting region to be less than 1.15 times the size of the white dwarf in radius.
Dr. Mukai with Dr. Coel Hellier (Keele) and Drs. Manabu Ishida and Ryuichi Fujimoto (ISAS) have analyzed the ASCA data on the intermediate polar AO Psc; they show that electron scattering near the magnetic poles is the likely mechanism for the high energy spin modulation seen in this system. The same collaboration is now working on the ASCA observation of another intermediate polar, BG CMi.
Drs. Lorella Angelini and Nicholas White in collaboration with Dr. A. Parmar (ESTEC/ESA) have found two previously unreported outbursts from the Ultrasoft X-ray Transient 4U1630-47 using archival data from the ROSAT PSPC and Einstein SSS. The times of the outbursts are consistent with a previously reported 600 days outburst recurrence period which is by far the shortest known for this class of object. The recurrence of the outbursts appears stable over the last 24 years. An update of the ephemeris times and period was obtained including also the outbursts seen in the Ginga and ASCA archival data (also in collaboration with Drs. O.R. Williams (ESTEC/ESA) and E.Kuulkers (ESTEC/ESA)).
Drs. Day and Angelini in collaboration with Drs. S. Pravdo (JPL), B. A. Harmon (MSFC/NASA), A. Yoshida, and P. Saraswat (RIKEN), using BATSE/GRO observations covering 7.5 orbital periods of the X-ray binary pulsar GX301-2, detected both the known pre-periatron (PP) flare and a previously unknown periodic near-apatron (NA) flare. Both the asymmetry of the PP flare and the existence of the NA flare can be explained by an equatorially enhanced stellar wind or a circumstellar disk around the optical companion WRA 977 which GX 301-2 crosses twice per orbit. The iron line and edge energy, measured with the ASCA SIS and GIS data, are consistent with neutral or nearly neutral iron probably resulting from fluorescence in the circumstellar material much cooler than the X-ray emission region.
Jeonghee Rho, a U. Maryland graduate student working with Dr. Robert Petre submitted and successfully defended her Ph.D. dissertation entitled, "An X-Ray Study of Composite Supernova Remnants." Her study of supernova remnants with shell-like radio and centrally-filled X-ray morphology shows that they are invariably associated with complex regions of the interstellar medium, and thus likely to be the remnants of core-collapse supernovae. The most likely mechanism for producing the centrally filled X-ray morphology is the evaporation of interstellar clouds that have been left within the hot remnant interior by the SNR shock front.
One significant new direction in the X-ray study of supernova remnants is the search for non-thermal X-ray emission from the shells of SNR. This research was stimulated by the demonstration that the featureless X- ray spectrum from the shell of SN1006 is synchrotron emission from electrons with energy ~100 TeV. This result (Koyama et al. 1996), involving significant LHEA participation offers the strongest evidence of cosmic ray acceleration in supernova remnant shocks to energies near the "knee" of the cosmic ray spectrum at ~1,000 TeV. Followup work has led to the discovery of a possible cosmic ray acceleration site in IC 443, and the possible isolation of non-thermal spectral components from the shells of Cas A, W49B, and 3C 397.
Drs. Una Hwang and Eric Gotthelf produced X-ray emission line images of the Tycho supernova remnant using ASCA data. Relative spatial variations in these images were used to study spatial variations in the spectrum, and the Fe emission was shown to be interior to and much hotter than the emission from other elements. Dr. Hwang is also continuing her efforts to derive temperatures and abundances in SNR by fitting nonequilibrium ionization models to broadband ASCA X-ray spectra.
Drs. Gotthelf, Hwang, and Petre have used ASCA data to rediscoved the elusive Neutron Star candidate in the Supernova Remnant "RCW 103". This object may well be one of the few examples of an isolated, non-pulsed, cooling NS.
Dr. Michael Loewenstein is collaborating with Mr. Hironori Matsumoto and Dr. Hisamitsu Awaki (Kyoto U.), and other Japanese X-ray astronomers on a systematic investigation of the ASCA spectra of twelve early-type galaxies. Hot gas temperatures and (subsolar) metal abundances, and relative contributions of gaseous and x-ray binary components to the X-ray luminosity have been derived.
Drs. Peter Serlemitsos, Tahir Yaqoob, and Richard Mushoztky (GSFC), with Mr. Andrew Ptak (U. Maryland) have investigated the X-ray properties of the starburst galaxies M82 and NGC 253 using ASCA and ROSAT. These galaxies have complex spectra, requiring at two components in the ASCA bandpass. Strong line emission is observed in the ASCA bandpass of both galaxies allowing elemental abundances to be determined for the first time in the X-ray bandpass. Their analysis of the ASCA data from the starburst galaxy NGC 3628, in collaboration with Dr. Hideyo Kunieda and Mr. Yuichi Terashima (U. Nagoya), found that NGC 3628 may harbor a low-luminosity AGN and that the X-ray spectrum of NGC 3628 is one of the flattest observed to date in a galaxy.
Dr. Keith Arnaud, in collaboration with Dr. Peter Barthel (Kapteyn Astronomical Institute), demonstrated that radio galaxies inhabiting dense X-ray haloes have a larger ratio of radio to far-infrared luminosity than those not in haloes. This is the first straightforward observational proof that the radio luminosity of extragalactic radio sources is influenced by source environment, something that has long been hypothesized.
Dr. Greg Madejski was involved in a number of multi-wavelength campaigns to observe active galactic nuclei; his responsibility was the analysis of high-energy spectral data. This includes blazars 3C279 (led by Dr. Paola Grandi, STScI), PKS 2155-304 (led by Dr. Meg Urry, STScI), Mkn 421 (led by Dr. Tad Takahashi, ISAS/Tokyo), and AO 0235+164 (led by Madejski). This last object shows a component of X-ray absorption that is most likely due to an intervening galaxy at redshift z = 0.524, which, via comparison with radio and optical data, allows an estimate of elemental abundances in that galaxy. He also worked on the theoretical problems involving radiation drag in AGN jets (led by Dr. Marek Sikora, Copernicus Center). With Dr. Bozena Czerny (Copernicus Center), Madejski worked on the effect of dust on emergent spectra of AGN, and with Dr. Chris Done (Durham U.), he published data on Seyfert 2 galaxy NGC 4945, which is the brightest known Seyfert 2 on the sky at 100 keV.
Dr. Rita Sambruna continued her work on X-ray and multifrequency spectra of blazars. Together with Drs. Urry (STScI), Maraschi (U. Milano), and Ghisellini (Oss. Brera), she studied the radio to gamma-ray spectral distribution of the gamma-ray blazar PKS 0528+134, using simultaneous ASCA, EGRET, and ground-based observations. It was shown that the gamma-ray emission dominates the bolometric luminosity of this source, and that a most likely origin for the seed photons which are Compton-upscattered at gamma-rays is external to the relativistic jet.
Using proprietary ASCA and ROSAT data, together with archival BBXRT observations, Drs. Sambruna, Ian George, and Madejski studied the X-ray spectrum of the BL Lacertae object H1426+428. An absorption feature is present at 0.66 keV in the BBXRT data, consistent with absorption by highly ionized oxygen. There are indications that the feature energy varies with the source flux, possibly suggesting the presence of a warm absorber along the line of sight.
Using proprietary ASCA and ROSAT data, together with archival BBXRT observations, Dr. Sambruna and collaborators studied the X-ray emission of the BL Lacertae object H1426+428. An absorption feature is present at 0.66 keV in the BBXRT data, consistent with absorption by highly ionized oxygen. There are indications that the feature energy varies with the source flux, possibly suggesting the presence of a warm absorber along the line of sight.
Drs. Tahir Yaqoob, Peter Serlemitsos, Richard Mushotzky, and Steve Holt (GSFC) in collaboration with Dr. Edelson (Iowa State U.), Dr. Kimberly Weaver (Johns Hopkins U.), and Dr. Bob Warwick (Leicester U., UK) observed the active galaxy NGC 4151 with ASCA and found it to have an iron K emission line with a broad, complex profile. The discovery provides direct evidence of gravitational broadening due to a massive black hole and constrains the putative accretion disk in the system to be near face-on.
Drs. Michael Loewenstein and Mushotzky, in collaboration with Ms. Kyoko Matsushita (U. Tokyo), have discovered X-ray counterparts to micro-AGN discovered with the Hubble Space Telescope using ASCA and ROSAT data.
Drs. Serlemitsos, Yaqoob, and Mushotzky (GSFC) with Mr. Andrew Ptak (U. Maryland) are studying the X-ray properties of LINERs and low-luminosity AGN. Their analysis of M81 confirms the discovery of complex Fe line emission. In collaboration with Hideyo Kunieda and Yuichi Terashima (U. Nagoya), a typical AGN spectrum was observed in NGC 3147, previously classified as a normal galaxy. These results have strong implications for the connection between LINERs, low-luminosity AGN and "normal" AGN.
Drs. Paul Nandra, George and Jane Turner in collaboration with Y. Fukazawa (Tokyo U.) reported on the discovery of an iron K-alpha line in the quasar PG 1116+215. Unlike lower-luminosity AGN the evidence was for emisson from highly ionized iron. Such line emission is uncommon in quasars and this intermediate case is suggestive of evolution of the mass accretion rate towards high redshift and/or luminosity.
Drs. Richard Mushotzky, Michael Loewenstein, and Keith Arnaud continue to derive abundances of iron, silicon and other elements in intracluster media from ASCA data. These measurements have been used to place constraints on the early star formation history in elliptical galaxies and demonstrate the importance of feedback in the cluster formation process.
Drs. Loewenstein, Mushotzky, and Arnaud also continue to derive the total mass distributions in clusters using ASCA temperature profiles. They have shown that there is a discrepancy between the masses derived using the x-ray data and those derived using weak gravitational lensing in at least two clusters.
Drs. Loewenstein and Mushotzky have compared the baryon fraction distributions in the poor clusters Abell 1060 and AWM 7 and found them to be disjoint. Such variations are not predicted in standard models of cluster formation.
Drs. Hwang, Richard Mushotzky, and Michael Loewenstein, with Dr. Thomas H. Markert (MIT), Drs. Yasuo Fukazawa and Hironori Matsumoto (U. Tokyo) studied the robustness of the Fe abundances determined in X-rays with ASCA for the cooling flow galaxy M87 and five galaxy clusters. The consistency of Fe abundances determined independently for the L and K emission support X-ray deduced abundances for temperatures above 1 keV in optically thin thermal plasmas.
Drs. Caleb Scharf, Keith Jahoda, and Elihu Boldt searched for the presence of a Compton-Getting effect signature of the Sun's motion with respect to the flux distribution of gamma-ray bursts. Although no certain signal is detected, the search was more sensitive than previous searches since it included not just the number distribution but the flux distribution.
Dr. Eric Gotthelf is putting together an ASCA image catalog which consists of all the regions of the sky observed with the ASCA Observatory. This catalog includes images from the broad, soft, and hard band spectral bands. Using these images, Drs. Gotthelf and Nick White are creating an ASCA SIS source catalog which will be available to the general community this year.