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NASA NTRS1999

Disentangling the Hard X-ray Background ROSAT HRI

The goal of our investigation was to improve our understanding of the properties of the discrete X-ray sources that produce the X-ray background (XRB). Many surveys have shown that the XRB at energies of 0.5-3.0 keV is dominated by emission from extragalactic point sources and that a significant fraction of the XRB at higher energies also is produced by discrete sources. In spite of the fact that the bulk of the 0.5-10 keV XRB was demonstrated to arise from extragalactic point sources, the spectral shape of the background presented a difficulty, referred to as the "spectral paradox". Spectra for classes of individual sources generally have been found to be incompatible with the observed energy index, alpha = 0.4 of the XRB over the 2-10 keV energy range. For the 0.3-3.0 keV Einstein band, Macca'caro et al. (1988) derived a mean energy spectral index of a approx. 0.95 for 599 extragalactic sources and for a subset of X-ray selected AGN, found alpha = 1.03(sup +0-05, sub -0.06) . Wilkes and Elvis showed that radio "loudness" was strongly correlated with the source spectrum, such that radio-load quasars exhibited flatter spectra (alpha approx. 0.5), while radio-quiet quasars had steeper spectra (alpha approx. 1). Studies of moderately faint sources in the 0.1-2.0 keV ROSAT band also found rather steep spectra (alpha = 0.96 +/- 0.11 for sources with an average flux of 1.5 x 10(exp -14) ergs/sq cm sec). At higher energies and much higher fluxes, energy spectra of individual AGN suggested a "canonical" alpha = 0.7 energy spectrum. Thus, the best evidence suggested that known classes of AGN could not readily explain the observed X-ray background spectrum. In our ROSAT PSPC analysis, we studied not only the traditional log N - log S, but also the spectral properties of the sources. We computed hardness ratios for individual sources and performed spectral fits to the summed source spectra, averaged in flux bins from 10(exp -15) to 10(exp -12) ergs/sq cm sec. We found that the hardness ratios and source spectra consistently show a hardening in the source spectra as the source fluxes decrease, confirming the suggestion by Hasinger et al. (1993).

Murray, Stephen S.↗
NASA NTRS1999

X-Ray Spectral Studies of AGN with the ASCA Satellite

This project involved the interpretation of Advanced Satellite for Cosmology and Astrophysics (ASCA) observations of Active Galactic Nuclei (AGN), with emphasis on four different aspects of the AGN phenomenon: (1) Absorption by hot gas in blazars: This anomalous absorption was detected in several objects, notably the BL Lacertae (BL Lac) object 1426+428. (2) Separation of blazar and Seyfert components in superluminal radio galaxies and quasars: Both components were found in the radio galaxies samples, but with no clear trend in which dominates. (3) Detection of high energy Compton components in blazars: Both BL Lacs and quasars show hard X-ray spectra that represent the onset of the Compton-scattered gamma-ray component. (4) Correlation of X- and gamma-ray emission in blazars: Several multiwavelength monitoring campaigns showed a correlation between X- and gamma-rays in blazars.

Urry, C. Megan↗
NASA NTRS2000

Beamed and Unbeamed X-Ray Emission in FR1 Radio Galaxies

The research exploited ROSAT's sensitivity, together with its spatial and spectral resolution, to separate X-ray emission components in the sources. Prior to ROSAT, the dominant X-ray emission mechanism in radio galaxies as a class was unclear, with correlations between the X-ray and radio emission used on one hand to argue for a nuclear origin for the X-rays, and on the other hand for a thermal origin. Our observations (normally between 10 and 25 ks in length) routinely detected the target sources, and demonstrated that both resolved (thermal) and unresolved X-ray emission are typically present. Highlights of our work included two of the first detections of high-power radio galaxies at high redshift, 3C 280 and 3C 220.1. When combined with the work of two other groups, we find that of the 38 radio galaxies at z > 0.6 in the 3CRR sample, 12 were observed in ROSAT pointed observations and 9 were detected with the four most significant detections exhibiting source extent, including 3C 280 and 3C 220.1. Moreover, we discovered extended emission around five 3CRR quasars at redshift greater than about 0.4, one of which is at z > 0.6. Unification predicts that the X-ray environments of powerful radio galaxies and quasars should be similar, and our results show that powerful radio sources are finding some of the highest-redshift X-ray clusters known to date, pointing to deep gravitational potential wells early in the Universe.

Worrall, Diana M.↗
NASA NTRS1990

STS-31: Hubble HST Science

The primary objective of STS-31 was to deploy the Hubble Space Telescope(HST). This videotape presents a press briefing about the scientific goals of the HST program. The panel members were Dr. Weiler, HST program scientist; Dr. Boggess from NASA Goddard, the Center managing the HST Program; and Dr. Bahcall, President Elect of the American Astronomical Union. Dr. Weiler opened the panel discussion by introducing other HST scientists who were in the audience. Dr. Bahcall explained the four major areas that astronomers hope to better understand using the HST data: (1) The size and age of the universe; (2) quasars as flashlights to understanding other features of the universe; (3) planets around other stars; and (4) weather on the other planets of our Solar System. Other areas in which he hopes to have some understanding are galaxies under quasars, black holes, and missing matter. After his remarks, Dr. Bahcall presented a plaque to Charles Pellerin, who helped initiate the series of astrophysics telescopes. The HST is the first of these "Great Observatories". After the presentation, questions from the press were answered.

Source record↗
NASA NTRS2000

The 3C 279 Campaign of Winter 1999: A Gamma-Optical Correlation?

Preliminary results are presented from the gamma-optical campaign of January-February 1999 on 3C 279. During this period we obtained good optical sampling of the source, the best ever for a gamma-bright OVV quasar. Its large and fast variations have been compared with the gamma-ray fluxes obtained simultaneously by Energy Gamma Ray Experiment Telescope (EGRET), on Compton Gamma Ray Observatory (CGRO). Despite rather poor counting statistics in the gamma-ray data, a fair correlation is found, with the gamma variations following those in the optical by 3-4 days. This is the first time such a significant day-scale correlation has been observed between the optical and gamma emissions from a OVV quasar. Its implications are currently under study.

Hartman, R. C.↗
NASA NTRS2000

Analysis of the PKS0637-752 X-Ray Jet System

The X-ray jet emitted from the quasar PKS0637-752 is the largest and most luminous detected to date. It extends 10 arcsec west of the nucleus, and is coincident with GHz radio jet emission in this region. If the individual X-ray/optical/radio knots in this jet were detected as isolated objects, they might be classified as BL Lac objects or quasars. We present a detailed analysis of the Chandra observations of this system, including a search for an intra-cluster medium to confine the jet, and limits to X-ray emission from the eastern radio knot and past the bend of the radio jet to the northwest. The projection in the plane of the sky implies that the X-ray jet is an aligned extension of the pc-scale jet, for which VLBI/VSOP observations show super-luminal motion with an apparent velocity of 11c. In this case, the X-ray jet is of order 1 Mpc in length, and explaining the X-ray emission mechanism presents new challenges.

Schwartz, D. A.↗
NASA NTRS2000

Continued RXTE Observations of Three Blazars

This project continued (during Cycle 3 of the RXTE mission) weekly observations of the quasars PKS 1510-089 and 3C 454.3 and the BL Lac object OJ 287. The data were analyzed by the PI and his graduate student. In the middle of Cycle 3, the PI changed the mode of observation of 3C 454.3, based on a report from another astronomer that observations with the Rosat satellite had found a strong confusing X-ray source about 45 arcmin away. The RXTE observations, in which the flux was measured while pointing directly at 3C 454.3 and then 15' from 3C 454.3, in the direction away from the confusing source. The flux decreased considerably, and so it appears that nearly all the X-ray flux measured by RXTE comes from the confusing source rather than the quasar. OJ 287 was detected only at one epoch of the Cycle 3 observations. This corresponded to a particularly quiet period for this source, when its flux at other wavebands was also low. From the single epoch of detection, no conclusions can be drawn concerning the general nature of the X-ray emission from this object. In contrast to the other two sources, the results of the observations of PKS 1510-089, when combined with data from cycles 3 and 4, are very interesting. The RXTE (2.4-20 keV) abd 14.5 GHz University of Michigan Radio Astronomy Observatory (UMRAO) light curves through the end of RXTE Cycle 4 are shown for PKS 1510-089. Visual inspection reveals an obvious correlation between the X-ray and 14.5 GHz light curves. While this is as expected in the SSC model in a source with simultaneous mm-wave (the photons thought to be scattered to X-ray energies) and cm-wave variations, the X-ray spectrum is flat ("energy" spectral index alpha = 0.2-0.5, where F (sub v) proportional to v (sup - alpha)) compared with the mm-wave synchrotron spectrum (alpha approximately equal to 0.8). The PI is currently engaged in theoretical work to determine whether this is possible for an SSC model.

Marscher, Alan P.↗
NASA NTRS2001

Deep Asca Campaign to Probe one Typical Broad Absorption Line Qso

We present a deep ASCA observation of a broad absorption line quasar (BALQSO) PG 0946+301. The source was clearly detected in one of the gas imaging spectrometers, but not in any other detector. If BALQSOs have intrinsic X-ray spectra similar to normal radio-quiet quasars, our observations imply that there is Thomson thick X-ray absorption (NH(1024 per square centimeter) toward PG 0946+301. This is the largest column density estimated so far toward a BALQSO. The absorber must be at least partially ionized and may be responsible for attenuation in the optical and UV. If the Thomson optical depth toward BALQSOs is close to 1, as inferred here, then spectroscopy in hard X-rays with large telescopes like XMM would be feasible.

Green, P.↗
NASA NTRS2001

Search for the Identification of 3EG J1835+5918: Evidence for a New Type of High-Energy Gamma-Ray Source?

Most of the EGRET high-energy gamma-ray sources remain unidentified. It is highly likely that many of these are fainter blazars or pulsars, but there may also be new types of sources to be discovered. We have focussed our search for novel gamma-ray sources on 3EG 1835+5918, which is the brightest and most accurately positioned of the unidentified EGRET sources at high Galactic latitude (l, b = 89 deg, 25 deg). In this talk, we will summarize the results of X-ray, radio, and optical surveys of this location. In particular, we have made complete optical identifications of all of the ROSAT and ASCA sources in this region to a flux limit of approximately 1 x 10(exp -13) ergs/sq cm s. All of the X-ray sources within the EGRET error circle are radio-quiet quasars or coronally emitting stars. Previous radio pulsar searches have been unsuccessful. We set an upper limit of 3.8 mJy (at 1.4 GHz) on any possible radio counterpart to 3EG 1835+5918. We also find several quasars and white dwarfs using optical color selection, and we have monitored the entire field for variable optical objects on short and long time scales. Since no blazar-like or pulsar-like candidate has been found as a result of these searches, we assert that 3EG 1835+5918 must be lacking in one or more of the physically essential attributes of those classes of gamma-ray emitters. In particular, its radio flux is at least two orders of magnitude fainter than any of the securely identified EGRET blazars, and its soft X-ray flux is at least 30 times fainter than that of Geminga and other EGRET pulsars. If it is an AGN it lacks the beamed radio emission of blazars. If it is an isolated neutron star, it lacks both the thermal X-rays from a cooling surface and the magnetospheric non-thermal X-ray emission that is characteristic of all EGRET pulsars. As such, it is more problematic physically than Geminga, which is an ordinary pulsar that only lacks radio emission. As a pulsar, 3EG 1835+5918 would have to be either older or more distant than Geminga, and probably an even more efficient gamma-ray engine.

Mirabal, N.↗
NASA NTRS2001

Search for the Identification of 3EG J1835+5918: Evidence for a New Type of High-Energy Gamma-Ray Source

The EGRET source 3EG J1835+5918 is the brightest and most accurately positioned of the as-yet unidentified high-energy gamma-ray sources at high Galactic latitude (l, b = 89 deg, 25 deg). We present a multiwavelength study of the region around it, including X-ray, radio, and optical imaging surveys, as well as optical spectroscopic classification of most of the active objects in this area. Identifications are made of all but one of the ROSAT and ASCA sources in this region to a flux limit of approximately 5 x 10(exp -14) erg/sq cm s, which is 10(exp -4) of the gamma-ray flux. The identified X-ray sources in or near the EGRET error ellipse are radio-quiet QSOs, a galaxy cluster, and coronal emitting stars. We also find eight quasars using purely optical color selection, and we have monitored the entire field for variable optical objects on short and long time scales without any notable discoveries. The radio sources inside the error ellipse are all fainter than 4 mJy at 1.4 GHz. There are no flat-spectrum radio sources in the vicinity; the brightest neighboring radio sources are steep-spectrum radio galaxies or quasars. Since no blazar-like or pulsar-like candidate has been found as a result of these searches, 3EG J1835+5918 must be lacking one or more of the physically essential attributes of these known classes of gamma-ray emitters. If it is an AGN it lacks the beamed emission radio of blazars by at least a factor of 100 relative to identified EGRET blazars. If it is an isolated neutron star, it lacks the steady thermal X-rays from a cooling surface and the magnetospheric non-thermal X-ray emission that is characteristic of all EGRET pulsars. If a pulsar, 3EG J1835+5918 must be either older or more distant than Geminga, and probably an even more efficient or beamed gamma-ray engine. One intermittent ROSA T source falls on a blank optical field to a limit of B greater than 23.4, V greater than 23.3, and R greater than 22.5. In view of this conspicuous absence, RX J1836-2+5925 should be examined further as a candidate for identification with 3EG J1835+5918 and possibly the prototype of a new class of high-energy gamma-ray source.

Mirabal, N.↗
NASA NTRS1998

Discovery of Associated Absorption Lines in an X-Ray Warm Absorber: Hubble Space Telescope Observations of PG 1114+445

The unified picture of X-ray/UV absorbers offers a unique opportunity to probe the nuclear environment of active galactic nuclei. To test the unified absorber scenario and to understand the physical properties of the absorber, we obtained the first UV spectrum of PG 1114+445 using the Hubble Space Telescope (HST) Faint Object Spectrograph. PG 1114+445 is known to have an X-ray ionized ('warm') absorber,so that UV absorption is predicted. The HST spectrum clearly shows strong UV absorption lines due to Lyalpha, C iv, and N v, blueshifted by approximately 530 km s(exp -1) with respect to the quasar redshift. Since both X-ray and UV absorbers are rare in radio-quiet quasars, these observations argue strongly that the X-ray and UV absorbers are closely physically related. perhaps identical. We place constraints on the parameters of the absorber and conclude that the mass outflow rate is comparable to the accretion rate in PG 1114+445.

Mathur, Smita↗
NASA NTRS1997

X-Ray Absorption Toward the Einstein Ring Source PKS 1830-211

PKS 1830-211 is an unusually radio-loud gravitationally lensed quasar. In the radio spectrum, the system appears as two compact, dominant features surrounded by relatively extended radio emission that forms an Einstein ring. As the line of sight to it passes close to our Galactic center, PKS 1830-211 has not been detected in wave bands other than the radio and X-ray so far. Here we present X-ray data of PKS 1830-211 observed with ROSAT Position Sensitive Proportional Counter. The X-ray spectrum shows that absorption in excess of the Galactic contribution is highly likely, which at the redshift of the lensing galaxy (z(sub t)=0.886) corresponds to N(sub H)=3.5((sup 0.6)(sub -0.5))x10(exp 22) atoms sq cm. The effective optical extinction is large, A(sub V)(observed) is greater than or approximately 5.8. When corrected for this additional extinction, the two-point optical to X-ray slope alpha(sub ox) of PKS 1830-211 lies just within the observed range of quasars. It is argued here that both compact images must be covered by the X-ray absorber(s) that we infer to be the lensing galaxy (galaxies). The dust-to-gas ratio along the line of sight within the lensing galaxy is likely to be somewhat larger than for our Galaxy.

Mathur, Smita↗
NASA NTRS2004

Witnessing the Gradual Slowdown of Powerful Extragalactic Jets: The X-Ray-Optical-Radio Connection

A puzzling feature of the Chandra-detected quasar jets is that their X-ray emission decreases faster along the jet than their radio emission, resulting from an outward-increasing radio-to-X-ray ratio. In some sources this behavior is so extreme that the radio emission peak is located clearly downstream of that of the X-rays. This is a rather unanticipated behavior given that the inverse Compton nature of the X-rays and the synchrotron radio emission are attributed to roughly the same electrons of the jet's nonthermal electron distribution. In this letter we show that this morphological behavior can result from the gradual deceleration of a relativistic flow and that the offsets in peak emission at different wavelengths carry the imprint of this deceleration. This notion is consistent with another recent finding, namely, that the jets feeding the terminal hot spots of powerful radio galaxies and quasars are still relativistic with Lorentz factors GAMMA approximately 2-3. The picture of the kinematics of powerful jets emerging from these considerations is that they remain relativistic as they gradually decelerate from kiloparsec scales to the hot spots, where, in a final collision with the intergalactic medium, they slow down rapidly to the subrelativistic velocities of the hot spot advance speed.

Georganopoulos, Markos↗
NASA NTRS2005

The X-Ray Weakness of GPS Radio Galaxies: A Volume-Limited Complete Sample

The XMM-Newton observations of the GPS sample was completed last summer. We are in process of finalizing the paper describing the data and the results. The main goal of the project was to determine the X-ray spectra of the GPS galaxies in comparison to regular radio galaxies. Our XMM observations show evidence that the GPS galaxies are heavily obscured with the large absorbing columns exceeding N(H)greater than le22 cm^-2. Taking into account the obscuration we determined that the intrinsic X-ray luminosities of GPS galaxies are of order le43-le44 erg/s, comparable to low luminosity radio loud quasars. The large GPS samples can confirm the result, as at this moment our evidence is based only on 7 GPS galaxies observed with good S/N/ in X-rays. The first paper summarizing the results of the XMM observation of Mkn 668 has been published Astronomy & Astrophysics. We found soft X-ray signatures of a hot plasma (kT approximately 10^7~K) and a hard X-ray emission from the nucleus. The X-ray spectrum above 2.5-keV is characterized by a very flat (observed photon index, Gamma-0.5) power-law continuum, alongside with a strong Fe-K-alpha neutral iron fluorescent line (EW~600-eV). The best explanation for the origin of this high energy X-ray emission is in terms of the Compton-reflection of the nuclear emission. The primary X-ray emission is obscured by a Compton-thick (N_H ~ to 10^24 ~cm-2) matter which becomes transparent at higher energies. The observed above 2.5-keV X-rays are mostly due to reflection which is indicated by a strong Fe-K-alpha line. This source represented the second hard X-ray detection of the GPS galaxy ever (the first one being 1345+125; O'Dea et al. 2000). The observations of the other GPS galaxies in our sample confirmed the trend of the large obscuration present in the spectra. However, we do not have a compelling evidence for a hot gas in the nucleus. The two other GPS galaxies observed with Chandra were added to the total of 7 GPS galaxies. This GPS sample was compared against the sample of radio loud galaxies and quasar. The paper is about to be submitted. Our preliminarily results based on the XMM-Newton data for the entire sample were presented at the meeting in July 2004.

Mushotsky, Richard↗
NASA NTRS2005

X-ray Weak Broad-line Qquasars: Absorption or Intrinsic X-ray Weakness

XMM observations of X-ray weak quasars have been performed during 2003 and 2004. The data for all the observations have become available in 2004 (there has been a delay of several months on the initial schedule, due to high background flares which contaminated the observations: as a consequence, most of them had to be rescheduled). We have reduced and analyzed all the data, and obtained interesting scientific results. Out of the eight sources, 4 are confirmed to be extremely X-ray weak, in agreement with the results of previous Chandra observations. 3 sources are confined to be highly variable both in flux (by factor 20-50) and in spectral properties (dramatic changes in spectral index). For both these groups of objects we are completing a publication: 1) For the X-ray weak sources, a paper is submitted with a complete analysis of the X-ray spectra both from Chandra and XMM-Newton, and a comparison with optical and near-IR photometry obtained from all-sky surveys. Possible models for the unusual spectral energy distribution of these sources are also presented. 2) For the variable sources, a paper is being finalized where the X-ray spectra obtained with XMM-Newton are compared with previous X-ray observations and with observations at other wavelengths. It is shown that these sources are high luminosity and extreme cases of the highly variable class of narrow-line Seyfert Is. In order to further understand the nature of these X-ray weak quasars, we submitted proposals for spectroscopy at optical and infrared telescopes. We obtained time at the TNG 4 meter telescope for near-IR observations and at the Hobby-Eberly Telescope for optical high-resolution spectroscopy. These observations have been performed in early 2004. They will complement the XMM data and will lead to understanding of whether the X-ray weakness of these sources is an intrinsic property or is due to absorption by circum-nuclear material. The infrared spectra of the variable sources have been already analyzed and are discussed in the paper by Memola et al. which will be soon submitted.

Mushotzky, Richard↗
NASA NTRS2003

Measurement of Aberration of Gravity Using Jupiter's Motion

On September 8, 2002 Jupiter passed within 3.7 min of the bright radio source, J0842+1835. The deflection predicted by General Relativity (GR) of this source at closest approach contains two major terms: an outward radial deflection from Jupiter of 1190 arcsec, and a deflection of 51 arcsec associated with the aberration of gravity. We used the Very Long Baseline Array with the Effelsberg telescope to measure the position of J0842+1835 with respect to two quasars a few degrees away during the period of September 4-12. With the use of two quasars on opposites sides of the radio source, we removed most of the tropospheric delay variations, and obtained the deflection of J0842+1835 on September 8 to an accuracy less than 10 arcsec. The aberrational part of the deflection that we measured is 0.98 +/- 0.19 times that predicted by GR. The speed of gravity associated with the measured aberration of the gravitational field of Jupiter is (1.06 +\- 0.21) times the speed of light.

Fomalont, Edward B.↗
NASA NTRS2005

XMM-Newton Spectroscopy of the X-ray Detected Broad Absorption Line QSO CSO 755

We present the results from XMM-Newton observations of the highly optically polarized broad absorption line quasar (BALQSO) CSO 755. By analyzing its X-ray spectrum with a total of approximately 3000 photons we find that this source has an X-ray continuum of "typical" radio-quiet quasars, with a photon index of Gamma=1.83, and a rather flat (X-ray bright) intrinsic optical-to-X-ray spectral slope of alpha_ox=- 1.51. The source shows evidence for intrinsic absorption, and fitting the spectrum with a neutral-absorption model gives a column density of N_H approximately 1.2x10^22 cm^{-2}; this is among the lowest X-ray columns measured for BALQSOs. We do not detect, with high significance, any other absorption features in the X-ray spectrum. Upper limits we place on the rest-frame equivalent width of a neutral (ionized) Fe K-alpha line, less than =180 eV (less than =120 eV), and on the Compton-reflection component parameter, R less than =0.2, suggest that most of the X-rays from the source are directly observed rather than being scattered or reflected; this is also supported by the relatively flat intrinsic alpha ox we measure. The possibility that most of the X-ray flux is scattered due to the high level of UV-optical polarization is ruled out. Considering data for 46 BALQSOs from the literature, including CSO 755, we have found that the UV-optical continuum polarization level of BALQSOs is not correlated with any of their X-ray properties. A lack of significant short-term and long-term X-ray flux variations in the source may be attributed to a large black-hole mass in CSO 755. We note that another luminous BALQSO, PG 2112+059, has both similar shallow C IV BALs and moderate X-ray absorption.

Brandt, Niel↗
NASA NTRS2006

Multiwavelength Study of the Bright X-ray Source Population in the Interacting Galaxies NGC 5774/NGC 5775

The X-ray source population in the field of the interacting pair of galaxies NGC 5774/5775 is reported. A total of 49 discrete sources are detected, including 12 ultraluminous X-ray source candidates with lum inosities above 10(exp 39)erg/s in the 0.5 - 8.0 keV X-ray band. Several of these latter are transient X-ray sources that fall below detect ion levels in one of two X-ray observations spaced 15 months apart. X-ray source positions are mapped onto optical and radio images to sear ch for potential counterparts. Eleven sources have optically-bright c ounterparts. Optical colors are used to differentiate these sources, which are mostly located outside the optical extent of the interacting galaxies, as potential globular clusters (3 sources) and quasars (5) . Follow-up optical spectroscopy confirms two of the latter are background quasars.

Ghosh, Kajal K.↗