arXiv subjects

Greg Madejski

Publications and source records attributed to Greg Madejski.

At least 37 records · Page 2Linked to original sources

arXiv2017

Hitomi X-ray studies of Giant Radio Pulses from the Crab pulsar

To search for giant X-ray pulses correlated with the giant radio pulses (GRPs) from the Crab pulsar, we performed a simultaneous observation of the Crab pulsar with the X-ray satellite Hitomi in the 2 -- 300 keV band and the Kashima NICT radio observatory in the 1.4 -- 1.7 GHz band with a net exposure of about 2 ks on 25 March 2016, just before the loss of the Hitomi mission.The timing performance of the Hitomi instruments was confirmed to meet the timing requirement and about 1,000 and 100 GRPs were simultaneously observed at the main and inter-pulse phases, respectively, and we found no apparent correlation between the giant radio pulses and the X-ray emission in either the main or inter-pulse phases.All variations are within the 2 sigma fluctuations of the X-ray fluxes at the pulse peaks, and the 3 sigma upper limits of variations of main- or inter- pulse GRPs are 22\% or 80\% of the peak flux in a 0.20 phase width, respectively, in the 2 -- 300 keV band.The values become 25\% or 110\% for main or inter-pulse GRPs, respectively, when the phase width is restricted into the 0.03 phase.Among the upper limits from the Hitomi satellite, those in the 4.5-10 keV and the 70-300 keV are obtained for the first time, and those in other bands are consistent with previous reports.Numerically, the upper limits of main- and inter-pulse GRPs in the 0.20 phase width are about (2.4 and 9.3) $\times 10^{-11}$ erg cm$^{-2}$, respectively. No significant variability in pulse profiles implies that the GRPs originated from a local place within the magnetosphere and the number of photon-emitting particles temporally increases.However, the results do not statistically rule out variations correlated with the GRPs, because the possible X-ray enhancement may appear due to a $>0.02$\% brightening of the pulse-peak flux under such conditions.

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arXiv2017

Search for Thermal X-ray Features from the Crab nebula with Hitomi Soft X-ray Spectrometer

The Crab nebula originated from a core-collapse supernova (SN) explosion observed in 1054 A.D. When viewed as a supernova remnant (SNR), it has an anomalously low observed ejecta mass and kinetic energy for an Fe-core collapse SN. Intensive searches were made for a massive shell that solves this discrepancy, but none has been detected. An alternative idea is that the SN1054 is an electron-capture (EC) explosion with a lower explosion energy by an order of magnitude than Fe-core collapse SNe. In the X-rays, imaging searches were performed for the plasma emission from the shell in the Crab outskirts to set a stringent upper limit to the X-ray emitting mass. However, the extreme brightness of the source hampers access to its vicinity. We thus employed spectroscopic technique using the X-ray micro-calorimeter onboard the Hitomi satellite. By exploiting its superb energy resolution, we set an upper limit for emission or absorption features from yet undetected thermal plasma in the 2-12 keV range. We also re-evaluated the existing Chandra and XMM-Newton data. By assembling these results, a new upper limit was obtained for the X-ray plasma mass of <~ 1Mo for a wide range of assumed shell radius, size, and plasma temperature both in and out of the collisional equilibrium. To compare with the observation, we further performed hydrodynamic simulations of the Crab SNR for two SN models (Fe-core versus EC) under two SN environments (uniform ISM versus progenitor wind). We found that the observed mass limit can be compatible with both SN models if the SN environment has a low density of <~ 0.03 cm-3 (Fe core) or <~ 0.1 cm-3 (EC) for the uniform density, or a progenitor wind density somewhat less than that provided by a mass loss rate of 10-5 Mo yr-1 at 20 km s-1 for the wind environment.

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arXiv2015

The Corona of the Broad-Line Radio Galaxy 3C 390.3

We present the results from a joint Suzaku/NuSTAR broad-band spectral analysis of 3C 390.3. The high quality data enables us to clearly separate the primary continuum from the reprocessed components allowing us to detect a high energy spectral cut-off ($E_\text{cut}=117_{-14}^{+18}$ keV), and to place constraints on the Comptonization parameters of the primary continuum for the first time. The hard over soft compactness is 69$_{-24}^{+124}$ and the optical depth 4.1$_{-3.6}^{+0.5}$, this leads to an electron temperature of $30_{-8}^{+32}$ keV. Expanding our study of the Comptonization spectrum to the optical/UV by studying the simultaneous Swift-UVOT data, we find indications that the compactness of the corona allows only a small fraction of the total UV/optical flux to be Comptonized. Our analysis of the reprocessed emission show that 3C 390.3 only has a small amount of reflection (R~0.3), and of that the vast majority is from distant neutral matter. However we also discover a soft X-ray excess in the source, which can be described by a weak ionized reflection component from the inner parts of the accretion disk. In addition to the backscattered emission, we also detect the highly ionized iron emission lines Fe XXV and Fe XXVI.

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arXiv2015

First study of Mrk 501 through the eyes of NuSTAR, VERITAS and the LIDAR-corrected eyesight of MAGIC

The blazar Mrk 501 is among the brightest X-ray and TeV sources in the sky, and among the few sources whose spectral energy distributions can be characterized by current instruments with relatively short observations (minutes to hours). In 2013, we organized an extensive multi-instrument campaign including Fermi-LAT, MAGIC, VERITAS, F-GAMMA, Swift, GASP-WEBT, and other groups and instruments which provided the most detailed temporal and energy coverage on Mrk 501 to date. This campaign included, for the first time, observations with the Nuclear Stereoscopic Telescope Array (NuSTAR), a satellite mission launched in 2012. NuSTAR provides unprecedented sensitivity in the hard X-ray range 3-79 keV, which, together with very high energy (VHE; >100 GeV) observations, is crucial to probe the highest energy electrons in Mrk 501. The campaign covered a few day long flaring activity in July 2013 which could be studied with strictly simultaneous NuSTAR and MAGIC observations. A large fraction of the MAGIC data during this activity was affected by hazy atmospheric conditions, due to a sand layer from the Saharan desert. These data would have been removed in any standard Cherenkov Telescope data analysis. MAGIC has developed a technique to correct for adverse atmospheric conditions, making use of information from the LIDAR facility at the MAGIC site, and applies an event-by-event correction to recover data affected by adverse weather conditions. This is the first time that LIDAR information has been used to produce a physics result with Cherenkov Telescope data taken during adverse atmospheric conditions, and hence sets a precedent for current and future ground-based gamma-ray instruments. In this contribution we report the observational results, focusing on the LIDAR-corrected MAGIC data and the strictly simultaneous NuSTAR and MAGIC/VERITAS data, and discuss the scientific implications.

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arXiv2010

Soft Gamma-ray Detector for the ASTRO-H Mission

ASTRO-H is the next generation JAXA X-ray satellite, intended to carry instruments with broad energy coverage and exquisite energy resolution. The Soft Gamma-ray Detector (SGD) is one of ASTRO-H instruments and will feature wide energy band (40-600 keV) at a background level 10 times better than the current instruments on orbit. SGD is complimentary to ASTRO-H's Hard X-ray Imager covering the energy range of 5-80 keV. The SGD achieves low background by combining a Compton camera scheme with a narrow field-of-view active shield where Compton kinematics is utilized to reject backgrounds. The Compton camera in the SGD is realized as a hybrid semiconductor detector system which consists of silicon and CdTe (cadmium telluride) sensors. Good energy resolution is afforded by semiconductor sensors, and it results in good background rejection capability due to better constraints on Compton kinematics. Utilization of Compton kinematics also makes the SGD sensitive to the gamma-ray polarization, opening up a new window to study properties of gamma-ray emission processes. The ASTRO-H mission is approved by ISAS/JAXA to proceed to a detailed design phase with an expected launch in 2014. In this paper, we present science drivers and concept of the SGD instrument followed by detailed description of the instrument and expected performance.

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arXiv2010

The Nuclear Spectroscopic Telescope Array (NuSTAR)

The Nuclear Spectroscopic Telescope Array (NuSTAR) is a NASA Small Explorer mission that will carry the first focusing hard X-ray (5 -- 80 keV) telescope to orbit. NuSTAR will offer a factor 50 -- 100 sensitivity improvement compared to previous collimated or coded mask imagers that have operated in this energy band. In addition, NuSTAR provides sub-arcminute imaging with good spectral resolution over a 12-arcminute field of view. After launch, NuSTAR will carry out a two-year primary science mission that focuses on four key programs: studying the evolution of massive black holes through surveys carried out in fields with excellent multiwavelength coverage, understanding the population of compact objects and the nature of the massive black hole in the center of the Milky Way, constraining explosion dynamics and nucleosynthesis in supernovae, and probing the nature of particle acceleration in relativistic jets in active galactic nuclei. A number of additional observations will be included in the primary mission, and a guest observer program will be proposed for an extended mission to expand the range of scientific targets. The payload consists of two co-aligned depth-graded multilayer coated grazing incidence optics focused onto solid state CdZnTe pixel detectors. To be launched in early 2012 on a Pegasus rocket into a low-inclination Earth orbit. Data will be publicly available at GSFC's High Energy Astrophysics Science Archive Research Center (HEASARC) following validation at the science operations center located at Caltech.

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arXiv2009

Constraining Emission Models of Luminous Blazar Sources

Many luminous blazars which are associated with quasar-type active galactic nuclei display broad-band spectra characterized by a large luminosity ratio of their high-energy (gamma-ray) and low-energy (synchrotron) spectral components. This large ratio, reaching values up to 100, challenges the standard synchrotron self-Compton models by means of substantial departures from the minimum power condition. Luminous blazars have also typically very hard X-ray spectra, and those in turn seem to challenge hadronic scenarios for the high energy blazar emission. As shown in this paper, no such problems are faced by the models which involve Comptonization of radiation provided by a broad line-region, or dusty molecular torus. The lack or weakness of bulk Compton and Klein-Nishina features indicated by the presently available data favors production of gamma-rays via up-scattering of infrared photons from hot dust. This implies that the blazar emission zone is located at parsec-scale distances from the nucleus, and as such is possibly associated with the extended, quasi-stationary reconfinement shocks formed in relativistic outflows. This scenario predicts characteristic timescales for flux changes in luminous blazars to be days/weeks, consistent with the variability patterns observed in such systems at infrared, optical and gamma-ray frequencies. We also propose that the parsec-scale blazar activity can be occasionally accompanied by dissipative events taking place at sub-parsec distances and powered by internal shocks and/or reconnection of magnetic fields. These could account for the multiwavelength intra-day flares occasionally observed in powerful blazars sources.

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arXiv2008

Suzaku Observations of Extreme MeV Blazar SWIFT J0746.3+2548

We report the Suzaku observations of the high luminosity blazar SWIFT J0746.3+2548 (J0746) conducted in November 2005. This object, with z = 2.979, is the highest redshift source observed in the Suzaku Guaranteed Time Observer (GTO) period, is likely to show high gamma-ray flux peaking in the MeV range. As a result of the good photon statistics and high signal-to-noise ratio spectrum, the $Suzaku$ observation clearly confirms that J0746 has an extremely hard spectrum in the energy range of 0.3-24 keV, which is well represented by a single power-law with a photon index of 1.17 and Galactic absorption. The multiwavelength spectral energy distribution of J0746 shows two continuum components, and is well modeled assuming that the high-energy spectral component results from Comptonization of the broad-line region photons. In this paper we search for the bulk Compton spectral features predicted to be produced in the soft X-ray band by scattering external optical/UV photons by cold electrons in a relativistic jet. We discuss and provide constraints on the pair content resulting from the apparent absence of such features.

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arXiv2008

MAXI and GLAST Studies of Jets in Active Galaxies

The recent launch of Fermi / GLAST - coinciding with the MAXI workshop - opens a new era for studies of jet-dominated active galaxies, known as blazars. While the emission processes operating in various spectral bands in blazars are reasonably well understood, the knowledge of the details of the structure of the jet, location of the dissipation region with respect to the accreting black hole, and coupling of the jet to the accretion process are known only at a rudimentary level. Blazars are variable, and this provides an opportunity to use the variability in various bands - and in particular, the relationship of respective time series to each other - to explore the relative location of regions responsible for emission in the respective bands. Observationally, this requires well-sampled time series in as many spectral bands as possible. To this end, with its all-sky, sensitive monitoring capability, the recently launched GLAST, and MAXI, to be deployed in 2009, are the most promising instruments bound to provide good sampling in respectively the energetic gamma-ray, and the soft X-ray band. This paper highlights the inferences regarding blazar jets that can be gleaned from such joint observations.

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arXiv2006

Analysis and Interpretation of Hard X-ray Emission from the Bullet Cluster (1E0657-56), the Most Distant Cluster of Galaxies Observed by RXTE

Evidence for non-thermal activity in clusters of galaxies is well established from radio observations of synchrotron emission by relativistic electrons. New windows in the Extreme Ultraviolet and Hard X-ray ranges have provided for more powerful tools for the investigation of this phenomenon. Detection of hard X-rays in the 20 to 100 keV range have been reported from several clusters of galaxies, notably from Coma and others. Based on these earlier observations we identified the relatively high redshift cluster 1E0657-56 (also known as RX J0658-5557) as a good candidate for hard X-ray observations. This cluster, also known as the bullet cluster, has many other interesting and unusual features, most notably that it is undergoing a merger, clearly visible in the X-ray images. Here we present results from a successful RXTE observations of this cluster. We summarize past observations and their theoretical interpretation which guided us in the selection process. We describe the new observations and present the constraints we can set on the flux and spectrum of the hard X-rays. Finally we discuss the constraints one can set on the characteristics of accelerated electrons which produce the hard X-rays and the radio radiation.

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arXiv2005

Recent and Future Observations in the X-ray and Gamma-ray Bands: Chandra, Suzaku, GLAST, and NuSTAR

This paper presents a brief overview of the accomplishments of the Chandra satellite that are shedding light on the origin of high energy particles in astrophysical sources, with the emphasis on clusters of galaxies. It also discusses the prospects for the new data to be collected with instruments recently launched - such as Suzaku - or those to be deployed in the near future, and this includes GLAST and NuSTAR.

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arXiv2005

X-ray Emission from Megamaser Galaxy IC 2560

Observation of the H2O megamaser galaxy IC 2560 with the Chandra Observatory reveals a complex spectrum composed of soft X-ray emission due to multi-temperature thermal plasma, and a hard continuum with strong emission lines. The continuum is most likely a Compton reflection (reprocessing) of primary emission that is completely absorbed at least up to 7 keV. The lines can be identified with fluorescence from Si, S and Fe in the lowest ionization stages. The equivalent widths of the Si and S lines are broadly compatible with those anticipated for reprocessing by optically thick cold plasma of Solar abundances, while the large equivalent width of the Fe line requires some overabundance of iron. A contribution to the line from a transmitted component cannot be ruled out, but the limits on the strength of the Compton shoulder make it less likely. From the bolometric luminosity of the nuclear region, we infer that the source radiates at 1 - 10% of its Eddington luminosity, for an adopted central mass of 3 million Solar masses. The overall spectrum is consistent with the hypotheses that the central engines powering the detected megamsers in accretion disks are obscured from direct view by the associated accretion disk material itself, and that there is a correlation between the occurrence of megamaser emission and Compton-thick absorption columns. For the 11 known galaxies with both column density measurements and maser emission believed to arise from accretion disks, eight AGN are Compton thick.

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arXiv2005

Intrinsic Curvature in the X-ray Spectra of BL Lacertae Objects

We report results from {\it XMM-Newton} observations of thirteen X-ray bright BL Lacertae objects, selected from the {\it Einstein} Slew Survey sample. The survey was designed to look for evidence of departures of the X-ray spectra from a simple power law shape (i.e., curvature and/or line features), and to find objects worthy of deeper study. Our data are generally well fit by power-law models, with three cases having hard ($Γ<2; dN/dE \propto E^{-Γ}$) spectra that indicate synchrotron peaks at $E \gsim 5$ keV. Previous data had suggested a presence of absorption features in the X-ray spectra of some BL Lacs. In contrast, none of these spectra show convincing examples of line features, either in absorption or emission, suggesting that such features are rare amongst BL Lacs, or, more likely, artifacts caused by instrumental effects. We find significant evidence for intrinsic curvature (steepening by $dΓ/ d({\rm log} E) = 0.4 \pm 0.15$) in fourteen of the seventeen X-ray spectra. This cannot be explained satisfactorily via excess absorption, since the curvature is essentially constant from $0.5-6$ keV, an observation which is inconsistent with the modest amounts of absorption that would be required. We use the {\it XMM-Newton} Optical Monitor data with concurrent radio monitoring to derive broadband spectral energy distributions and peak frequency estimates. From these we examine models of synchrotron emission and model the spectral curvature we see as the result of episodic particle acceleration.

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arXiv2003

The X-ray Variability of NGC 4945: Characterizing the Power Spectrum through Light Curve Simulations

For light curves sampled on an uneven grid of observation times, the shape of the power density spectrum (PDS) includes severe distortion effects due to the window function, and simulations of light curves are indispensable to recover the true PDS. We present an improved method for comparing light curves generated from a PDS model to the measured data and apply it to a 50-day long RXTE observations of NGC 4945, a Seyfert 2 galaxy with well-determined mass from megamaser observations. The improvements over previously reported investigations include the adjustment of the PDS model normalization for each simulated light curve in order to directly investigate how well the chosen PDS shape describes the source data. We furthermore implement a robust goodness-of-fit measure that does not depend on the form of the variable used to describe the power in the periodogram. We conclude that a knee-type function (smoothly broken power law) describes the data better than a simple power law; the best-fit break frequency is ~10^-6 Hz.

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arXiv2002

Learning about Jets from Observations of Blazars

Jets, paving their way outward through the inner regions of active nuclei, Compton-interact with the UV radiation from an accretion disc and broad emission line region. We calculate the predicted properties of the resulting spectral signatures of this bulk-Compton process, noting that they are independent on the fractional proton content or kinetic power of the jet, and use the presence or absence of such signatures to put constraints on the structure of jets near their bases.

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arXiv2002

The Apparent Host Galaxy of PKS 1413+135: HST, ASCA and VLBA Observations

PKS 1413+135 (z=0.24671) is one of very few radio-loud AGN with an apparent spiral host galaxy. Previous authors have attributed its nearly exponential infrared cutoff to heavy absorption but have been unable to place tight limits on the absorber or its location in the optical galaxy. In addition, doubts remain about the relationship of the AGN to the optical galaxy given the observed lack of re-emitted radiation. We present new HST, ASCA and VLBA observations which throw significant new light on these issues. The HST observations reveal an extrremely red color (V-H = 6.9 mag) for the active nucleus of PKS 1413+135, requiring both a spectral turnover at a few microns due to synchrotron aging and a GMC-sized absorber. We derive an intrinsic column N_H = 4.6^{+2.1}_{-1.6} times 10^{22}cm^{-2} and covering fraction f = 0.12^{+0.07}_{-0.05}. As the GMC is likely in the disk of the optical galaxy, our sightline is rather unlikely (P ~ 2 times 10^{-4}). The properties of the GMC typical of GMCs in our own galaxy. The HI absorber appears centered 25 milliarcseconds away from the nucleus, while the X-ray and nearly all of the molecular absorbers must cover the nucleus, implying a complicated geometry and cloud structure, with a molecular core along our line of sight to the nucleus. Interestingly, the HST/NICMOS data require the AGN to be decentered relative to the optical galaxy by 13 +/- 4 milliarcseconds. This could be interpreted as suggestive of an AGN location far in the background compared to the optical galaxy, but it can also be explained by obscuration and/or nuclear structure, which is more consistent with the observed lack of multiple images.

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arXiv2002

On the Nature of MeV-blazars

Broad-band spectra of the FSRQ (flat-spectrum-radio quasars) detected in the high energy gamma-ray band imply that there may be two types of such objects: those with steep gamma-ray spectra, hereafter called MeV-blazars, and those with flat gamma-ray spectra, GeV-blazars. We demonstrate that this difference can be explained in the context of the ERC (external-radiation-Compton) model using the same electron injection function. A satisfactory unification is reachable, provided that: (a) spectra of GeV-blazars are produced by internal shocks formed at the distances where cooling of relativistic electrons in a jet is dominated by Comptonization of broad emission lines, whereas spectra of MeV-blazars are produced at the distances where cooling of relativistic electrons is dominated by Comptonization of near-IR radiation from hot dust; (b) electrons are accelerated via a two step process and their injection function takes the form of a double power-law, with the break corresponding to the threshold energy for the diffusive shock acceleration. Direct predictions of our model are that, on average, variability time scales of the MeV-blazars should be longer than variability time scales of the GeV-blazars, and that both types of the blazar phenomenon can appear in the same object.

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arXiv2002

Observation of X-ray variability in the BL Lac object 1ES1959+65

This paper reports X-ray spectral observations of a relatively nearby z = 0.048 BL Lacertae (BL Lac) object 1ES1959+650, which is a potential TeV emitter. The observations include 31 short pointings made by the Unconventional Stellar Aspect (USA) Experiment on board the Advanced Research and Global Observation Satellite (ARGOS), and 17 pointings by the PCA on board the Rossi X-ray Timing Explorer (RXTE). Most of these observations were spaced by less than 1 day. \es was detected by the ARGOS USA detector in the range 1-16 keV, and by the PCA in the 2-16 keV range but at different times. During the closely spaced RXTE observations beginning on 2000 July 28, an ending of one flare and a start of another are visible, associated with spectral changes, where the photon index Gamma ranges between ~ 1.4 and 1.7, and the spectrum is harder when the source is brighter. This implies that 1ES1959 is an XBL-type blazar, with the X-ray emission likely to originate via the synchrotron process. The USA observations reveal another flare that peaked on 2000 November 14 and doubled the flux within a few days, again associated with spectral changes of the same form. The spectral variability correlated with the flux and timing characteristics of this object that are similar to those of other nearby BL Lacs, and suggest relativistic beaming with a Doppler factor delta > 1.6 and magnetic fields of the order of a few mG. We also suggest that the steady component of the X-ray emission -- present in this object as well as in other XBLs -- may be due to the large-scale relativistic jet (such as measured by Chandra in many radio-loud AGN), but pointing very closely to our line of sight.

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