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Title: New ALMA and Fermi /LAT Observations of the Large-scale Jet of PKS 0637−752 Strengthen the Case Against the IC/CMB Model

Abstract

The Chandra X-ray observatory has discovered several dozen anomalously X-ray-bright jets associated with powerful quasars. A popular explanation for the X-ray flux from the knots in these jets is that relativistic synchrotron-emitting electrons inverse-Compton scatter cosmic microwave background (CMB) photons to X-ray energies (the IC/CMB model). This model predicts a high gamma-ray flux that should be detectable by the Fermi /Large Area Telescope (LAT) for many sources. GeV-band upper limits from Fermi /LAT for the well-known anomalous X-ray jet in PKS 0637−752 were previously shown in Meyer et al. to violate the predictions of the IC/CMB model. Previously, measurements of the jet synchrotron spectrum, important for accurately predicting the gamma-ray flux level, were lacking between radio and infrared wavelengths. Here, we present new Atacama Large Millimeter/submillimeter Array (ALMA) observations of the large-scale jet at 100, 233, and 319 GHz, which further constrain the synchrotron spectrum, supporting the previously published empirical model. We also present updated limits from the Fermi /LAT using the new “Pass 8” calibration and approximately 30% more time on source. With these deeper limits, we rule out the IC/CMB model at the 8.7 σ level. Finally, we demonstrate that complete knowledge of the synchrotron SED is criticalmore » in evaluating the IC/CMB model.« less

Authors:
; ;  [1]; ;  [2]; ;  [3];  [4]
  1. University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250 (United States)
  2. Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ (United Kingdom)
  3. European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching-bei-München (Germany)
  4. ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo (Netherlands)
Publication Date:
OSTI Identifier:
22654558
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal Letters; Journal Volume: 835; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; APPROXIMATIONS; CALIBRATION; FORECASTING; GALAXIES; GAMMA RADIATION; GEV RANGE; GHZ RANGE; QUASARS; RELATIVISTIC RANGE; RELICT RADIATION; SPECTRA; TELESCOPES; WAVELENGTHS; X RADIATION

Citation Formats

Meyer, Eileen T., Breiding, Peter, Georganopoulos, Markos, Oteo, Iván, Ivison, R. J., Zwaan, Martin A., Laing, Robert, and Godfrey, Leith, E-mail: meyer@umbc.edu. New ALMA and Fermi /LAT Observations of the Large-scale Jet of PKS 0637−752 Strengthen the Case Against the IC/CMB Model. United States: N. p., 2017. Web. doi:10.3847/2041-8213/835/2/L35.
Meyer, Eileen T., Breiding, Peter, Georganopoulos, Markos, Oteo, Iván, Ivison, R. J., Zwaan, Martin A., Laing, Robert, & Godfrey, Leith, E-mail: meyer@umbc.edu. New ALMA and Fermi /LAT Observations of the Large-scale Jet of PKS 0637−752 Strengthen the Case Against the IC/CMB Model. United States. doi:10.3847/2041-8213/835/2/L35.
Meyer, Eileen T., Breiding, Peter, Georganopoulos, Markos, Oteo, Iván, Ivison, R. J., Zwaan, Martin A., Laing, Robert, and Godfrey, Leith, E-mail: meyer@umbc.edu. Wed . "New ALMA and Fermi /LAT Observations of the Large-scale Jet of PKS 0637−752 Strengthen the Case Against the IC/CMB Model". United States. doi:10.3847/2041-8213/835/2/L35.
@article{osti_22654558,
title = {New ALMA and Fermi /LAT Observations of the Large-scale Jet of PKS 0637−752 Strengthen the Case Against the IC/CMB Model},
author = {Meyer, Eileen T. and Breiding, Peter and Georganopoulos, Markos and Oteo, Iván and Ivison, R. J. and Zwaan, Martin A. and Laing, Robert and Godfrey, Leith, E-mail: meyer@umbc.edu},
abstractNote = {The Chandra X-ray observatory has discovered several dozen anomalously X-ray-bright jets associated with powerful quasars. A popular explanation for the X-ray flux from the knots in these jets is that relativistic synchrotron-emitting electrons inverse-Compton scatter cosmic microwave background (CMB) photons to X-ray energies (the IC/CMB model). This model predicts a high gamma-ray flux that should be detectable by the Fermi /Large Area Telescope (LAT) for many sources. GeV-band upper limits from Fermi /LAT for the well-known anomalous X-ray jet in PKS 0637−752 were previously shown in Meyer et al. to violate the predictions of the IC/CMB model. Previously, measurements of the jet synchrotron spectrum, important for accurately predicting the gamma-ray flux level, were lacking between radio and infrared wavelengths. Here, we present new Atacama Large Millimeter/submillimeter Array (ALMA) observations of the large-scale jet at 100, 233, and 319 GHz, which further constrain the synchrotron spectrum, supporting the previously published empirical model. We also present updated limits from the Fermi /LAT using the new “Pass 8” calibration and approximately 30% more time on source. With these deeper limits, we rule out the IC/CMB model at the 8.7 σ level. Finally, we demonstrate that complete knowledge of the synchrotron SED is critical in evaluating the IC/CMB model.},
doi = {10.3847/2041-8213/835/2/L35},
journal = {Astrophysical Journal Letters},
number = 2,
volume = 835,
place = {United States},
year = {Wed Feb 01 00:00:00 EST 2017},
month = {Wed Feb 01 00:00:00 EST 2017}
}
  • The X-ray emission mechanism in large-scale jets of powerful radio quasars has been a source of debate in recent years, with two competing interpretations: either the X-rays are of synchrotron origin, arising from a different electron energy distribution than that producing the radio to optical synchrotron component, or they are due to inverse Compton scattering of cosmic microwave background photons (IC/CMB) by relativistic electrons in a powerful relativistic jet with bulk Lorentz factor Γ ∼ 10-20. These two models imply radically different conditions in the large-scale jet in terms of jet speed, kinetic power, and maximum energy of the particlemore » acceleration mechanism, with important implications for the impact of the jet on the large-scale environment. A large part of the X-ray origin debate has centered on the well-studied source 3C 273. Here we present new observations from Fermi which put an upper limit on the gamma-ray flux from the large-scale jet of 3C 273 that violates at a confidence greater that 99.9% the flux expected from the IC/CMB X-ray model found by extrapolation of the UV to X-ray spectrum of knot A, thus ruling out the IC/CMB interpretation entirely for this source when combined with previous work. Further, this upper limit from Fermi puts a limit on the Doppler beaming factor of at least δ < 9, assuming equipartition fields, and possibly as low as δ < 5, assuming no major deceleration of the jet from knots A through D1.« less
  • We present 18 GHz Australia Telescope Compact Array imaging of the megaparsec-scale quasar jet PKS 0637-752 with angular resolution {approx}0.''58. We draw attention to a spectacular train of quasi-periodic knots along the inner 11'' of the jet, with average separation {approx}1.1 arcsec (7.6 kpc projected). We consider two classes of model to explain the periodic knots: those that involve a static pattern through which the jet plasma travels (e.g., stationary shocks) and those that involve modulation of the jet engine. Interpreting the knots as re-confinement shocks implies the jet kinetic power Q{sub jet} {approx} 10{sup 46} erg s{sup -1}, butmore » the constant knot separation along the jet is not expected in a realistic external density profile. For models involving modulation of the jet engine, we find that the required modulation period is 2 Multiplication-Sign 10{sup 3} yr < {tau} < 3 Multiplication-Sign 10{sup 5} yr. The lower end of this range is applicable if the jet remains highly relativistic on kiloparsec scales, as implied by the IC/CMB model of jet X-ray emission. We suggest that the periodic jet structure in PKS 0637-752 may be analogous to the quasi-periodic jet modulation seen in the microquasar GRS 1915+105, believed to result from limit cycle behavior in an unstable accretion disk. If variations in the accretion rate are driven by a binary black hole, the predicted orbital radius is 0.7 pc {approx}< a {approx}< 30 pc, which corresponds to a maximum angular separation of {approx}0.1-5 mas.« less
  • We report the γ -ray detection of a young radio galaxy, PKS 1718−649, belonging to the class of compact symmetric objects (CSOs), with the Large Area Telescope (LAT) on board the Fermi satellite. The third Fermi Gamma-ray LAT catalog (3FGL) includes an unassociated γ -ray source, 3FGL J1728.0−6446, located close to PKS 1718−649. Using the latest Pass 8 calibration, we confirm that the best-fit 1 σ position of the γ -ray source is compatible with the radio location of PKS 1718−649. Cross-matching of the γ -ray source position with the positions of blazar sources from several catalogs yields negative results.more » Thus, we conclude that PKS 1718−649 is the most likely counterpart to the unassociated LAT source. We obtain a detection test statistics TS ∼ 36 (>5 σ ) with a best-fit photon spectral index Γ = 2.9 ± 0.3 and a 0.1–100 GeV photon flux density F {sub 0.1−100} {sub GeV} = (11.5 ± 0.3) × 10{sup −9} ph cm{sup −2} s{sup −1}. We argue that the linear size (∼2 pc), the kinematic age (∼100 years), and the source distance ( z = 0.014) make PKS 1718−649 an ideal candidate for γ -ray detection in the framework of the model proposing that the most compact and the youngest CSOs can efficiently produce GeV radiation via inverse-Compton scattering of the ambient photon fields by the radio lobe non-thermal electrons. Thus, our detection of the source in γ -rays establishes young radio galaxies as a distinct class of extragalactic high-energy emitters and yields a unique insight on the physical conditions in compact radio lobes interacting with the interstellar medium of the host galaxy.« less
  • Here, we report on multiwavelength observations of the blazar PKS 0537-441 (z = 0.896) obtained from microwaves through γ-rays by Submillimeter Array, Rapid Eye Mounting, Automatic Telescope for Optical Monitoring (ATOM), Swift and Fermi mostly during 2008 August–2010 April. Strong variability has been observed in γ-rays, with two major flaring episodes (2009 July and 2010 March) and a harder-when-brighter behaviour, quite common for flat spectrum radio quasars and low-synchrotron-peaked BL Lacertae objects (BL Lacs), in 2010 March. In the same way, the spectral energy distribution (SED) of the source cannot be modelled by a simple synchrotron self-Compton model, as opposedmore » to many BL Lacs, but the addition of an external Compton component of seed photons from a dust torus is needed. The 230 GHz light curve showed an increase simultaneous with the γ-ray one, indicating co-spatiality of the mm and γ-ray emission region likely at large distance from the central engine. The low, average, and high activity SED of the source could be fit changing only the electron distribution parameters, but two breaks in the electron distribution are necessary. The ensuing extra spectral break, located at near-infrared (NIR)–optical frequencies, together with that in γ-rays seem to indicate a common origin, most likely due to an intrinsic feature in the underlying electron distribution. An overall correlation between the γ-ray band with the R band and K band has been observed with no significant time lag. On the other hand, when inspecting the light curves on short time-scales some differences are evident. In particular, flaring activity has been detected in NIR and optical bands with no evident γ-ray counterparts in 2009 September and November. Moderate variability has been observed in X-rays with no correlation between flux and photon index. Finally, an increase of the detected X-ray flux with no counterpart at the other wavelengths has been observed in 2008 October, suggesting once more a complex correlation between the emission at different energy bands.« less
  • We re-analyze the Chandra X-ray spectrum of the kpc-scale jet in PKS 0637-752 to investigate the possible low energy cutoff in the relativistic electron spectrum producing the non-thermal radiation in the scenario of inverse Compton emission off the cosmic microwave background. This was among the first objects targeted by the Chandra Observatory and gives a unique opportunity to study the low energy X-ray emission free of contamination. As previously noted, the spectrum can be fit by a power law, with the slope predicted by the radio spectrum, modified by low energy absorption through the Galaxy as determined from the spectrummore » of the quasar core and by HI 21 cm observations. We report evidence for a broad excess of emission below 1 keV, but are unable to constrain the shape very well. If we assume that this soft excess is unrelated to the electron population responsible for the power law emission, and that the electron spectrum cuts off at an energy of {gamma}{sub min}m{sub e}c{sup 2}, then we must have {gamma}{sub min} ({Lambda}/10) {approx}< 75 due to the absence of any low energy turn-over in the X-ray spectrum. This predicts that the observed radio spectrum should extend unbroken down below 2 MHz for the estimated jet restframe magnetic field B = 10 {micro} G. In addition, the observed optical flux can be used to place a lower limit on {gamma}{sub min}; the constraint is not very strong, but does suggest that {gamma}{sub min} must be higher than 1 to avoid overproducing the optical emission. The implication of these limits on the jet luminosity is discussed. An alternative phenomenological description of the soft excess is offered where the low-energy end of the electron spectrum is modified to account for the excess.« less