Jorge Ariel Combi

Supernova remnants and γ-ray sources

Abstract A review of the possible relationship between γ -ray sources and supernova remnants (SNRs) is presented. Particular emphasis is given to the analysis of the observational status of the problem of cosmic ray acceleration at SNR shock fronts. All positional coincidences between SNRs and unidentified γ -ray sources listed in the Third EGRET Catalog at low Galactic latitudes are discussed on a case by case basis. For several coincidences of particular interest, new CO( J =1−0) and radio continuum maps are shown, and the mass content of the SNR surroundings is determined. The contribution to the γ -ray flux observed that might come from cosmic ray particles (particularly nuclei) locally accelerated at the SNR shock fronts is evaluated. We discuss the prospects for future research in this field and remark on the possibilities for observations with forthcoming γ -ray instruments.

Optical microvariability of EGRET blazars

We present results of a photometric CCD study of the incidence of microvariability in the optical emission of a sample of 20 blazars detected at gamma-ray energies by the EGRET instrument of the Compton Gamma-Ray Observatory. We have observed strong outbursts in some sources, but many others displayed no significant variability on timescales of hours. The typical minimum timescale results to be of ∼ several hours, not tens of minutes as claimed by some authors. The duty cycle for optical intranight microvariations of gamma- ray blazars, as estimated from our observations, seems to be ∼ 50 %, lower than what is usually assumed. For night-to-night variations, instead, the duty cycle approaches to what is observed in radio-selected BL Lacs and flat-spectrum radio quasars (i.e. ∼ 70 %).

A variability analysis of low-latitude unidentified gamma-ray sources

We present a study of 40 low-latitude unidentied 3EG gamma-ray sources which were found to be not positionally coincident with any known class of potential gamma-ray emitters in the Galaxy (Romero et al. 1999). We have performed a variability analysis which reveals that many of these 40 sources are variable. These sources have, in addition, a steep mean value of the gamma-ray spectral index, =2 :41 0:2, which, combined with the high level of variability, seems to rule out a pulsar origin. The positional coincidences with uncatalogued candidates to supernova remnants were also studied. Only 7 sources in the sample are spatially coincident with these candidates, a result that is shown to be consistent with the expected level of pure chance association. A complementary search for weak radio counterparts was also conducted and the results are presented as an extensive table containing all signicant point-like radio sources within the 40 EGRET elds. We argue that in order to produce the high variability, steep gamma-ray spectra, and absence of strong radio counterparts observed in some of the gamma-ray sources of our sample, a new class of objects should be postulated, and we analyze a viable candidate.

THE INCIDENCE OF THE HOST GALAXY IN MICROVARIABILITY OBSERVATIONS OF QUASARS

The incidence of a host galaxy in aperture photometry of active galactic nuclei is studied by means of actual and simulated CCD observations. Our goal is to evaluate the importance of spurious variations, introduced by seeing —uctuations during the observations, in the diUerential light curves used to study optical microvariability. Repeated CCD observations during two consecutive nights were used to obtain time-resolved aperture photometry for the BL Lac object PKS 2316(423, which is located at the center of a conspicuous elliptical galaxy, and for several —eld stars. The blazar seems to be variable according to standard variability criteria; however we show that the observed diUerential magnitude variations are strongly correlated with seeing —uctuations during the nights. Moreover, another galaxy within the same CCD —eld shows nearly identical variations, clearly indicating that such variations are artifacts of the aperture photometry. Simulated observations of quasars within host galaxies of diUerent morphologies and spanning a wide range of luminosities were also used to evaluate the eUects of changing seeing con- ditions. The results show that spurious diUerential magnitude variations due to seeing —uctuations are larger for active nuclei within brighter hosts, particularly when small photometric apertures (about the seeing FWHM in radius) are used. According to our results, several recommendations are given to future observers.

Chandra/Very Large Array Follow-Up of TeV J2032+4131, the Only Unidentified TeV Gamma-Ray Source

The High Energy Gamma Ray Astronomy (HEGRA) Cerenkov telescope array group recently reported a steady and extended unidentified TeV gamma-ray source lying at the outskirts of Cygnus OB2. This is the most massive stellar association known in the Galaxy, estimated to contain � 2600 OB-type members alone. It has been previously argued that the large-scale shocks and turbulence induced by the multiple interacting supersonic winds from the many young stars in such associations may play a role in accelerating Galactic cosmic rays. Indeed, Cyg OB2 also coincides with the nonvariable MeV–GeV range unidentified EGRET source, 3EG 2033+4118. We report on the near-simultaneous follow-up observations of the extended TeV source region with the Chandra X-Ray Observatory and the Very Large Array radio telescope, obtained in order to explore this possibility. Analysis of the CO, H i, and IRAS 100 lm emissions shows that the TeV source region coincides with an outlying subgroup of powerful OB stars that have evacuated or destroyed much of the ambient atomic, molecular, and dust material and that may be related to the very high energy emissions. An interesting supernova-remnant–like structure is also revealed near the TeV source region in the CO, H i, and radio emission maps. Applying a numerical simulation that accurately tracks the radio to gamma-ray emission from primary hadrons as well as primary and secondary e � , we find that the broadband spectrum of the TeV source region favors a predominantly nucleonic—rather than electronic—origin for the high-energy flux, although deeper X-ray and radio observations will help confirm this. A very reasonable, � 0.1%, conversion efficiency of Cyg OB2’s extreme stellar wind mechanical luminosity to nucleonic

Centaurus A as a source of extragalactic cosmic rays with arrival energies well beyond the GZK cutoff

Abstract The ultra—high energy cosmic rays recently detected by several air shower experiments could have an extragalactic origin. In this case, the nearest active galaxy Centaurus A might be the source of the most energetic particles ever detected on Earth. We have used recent radio observations in order to estimate the arrival energy of the protons accelerated by strong shock fronts in the outer parts of this southern radio source. We expect detections corresponding to particles with energies up to ∼ 2.2 × 10 21 eV and an arrival direction of ( l ≈ 310°, b ≈ 20°) in galactic coordinates. The future Southern Hemisphere Pierre Auger Observatory might provide a decisive test for extragalactic models of the origin of the ultra—high energy cosmic rays.

Supernova-remnant origin of cosmic rays?

It is thought that Galactic cosmic-ray nuclei are gradually accelerated to high energies (up to about 300 TeV per nucleon, where 1 TeV is 1012 eV) in the expanding shock waves connected with the remnants of powerful supernova explosions. However, this conjecture has eluded direct observational confirmation since it was first proposed in 1953 (ref. 3). Enomoto et al. claim to have finally found definitive evidence that corroborates this model, proposing that very-high-energy, TeV-range, γ-rays from the supernova remnant (SNR) RX J1713.7 – 3946 are due to the interactions of energetic nuclei in this region. Here we argue that their claim is not supported by the existing multiwavelength spectrum of this source. The search for the origin(s) of Galactic cosmic-ray nuclei may be closing in on the long-suspected supernova-remnant sources, but it is not yet over.

Radio Detection of the Supernova Remnant RX J0852.0?4622

The X-ray source RX J0852.0-4622 has been recently proposed as a candidate for a young nearby supernova remnant on the basis of its X-ray (≥1.3 keV) morphology, inferred internal shock velocities, and the clear detection of 44Ti emission lines. In this Letter, we report its detection at radio wavelengths (2.4 and 1.42 GHz). The radio images match the X-ray morphology very well and show a limb-brightened source with some elongated features protruding from the outer shell. These features could be explosion fragments similar to those detected in the Vela supernova remnant. At radio frequencies the source appears to be nonthermal, with an index α ~ -0.3. This synchrotron emission seems to extend up to X-ray energies, implying the existence of very high energy electrons locally accelerated in the remnant.

Is the Supernova Remnant RX J1713.7–3946 a Hadronic Cosmic-Ray Accelerator?

The nonthermal supernova remnant (SNR) RX J1713.7-3946 has recently been shown to be a site of cosmic-ray (CR) electron acceleration to TeV energies. Here we present evidence that this remnant is also accelerating CR nuclei. Such energetic nuclei can interact with ambient interstellar gas to produce high-energy gamma rays via the decay of neutral pions. We associate the unidentified EGRET GeV gamma-ray source, 3EG J1714-3857, with a very massive (~3 × 105 M☉) and dense (~500 nucleons cm-3) molecular cloud interacting with SNR RX J1713.7-3946. Direct evidence for such interaction is provided by observations of the lowest two rotational transitions of CO molecules in the cloud; as in other clear cases of interaction, the CO (J = 2 → 1)/CO (J = 1 → 0) ratio is significantly enhanced. Since the cloud is of low radio and X-ray brightness, electrons cannot be responsible for the bulk of the GeV emission there. A picture thus emerges in which both electrons and nuclei are being accelerated by the SNR: whereas the relativistic electrons dominate the local nonthermal radio, X-ray, and TeV emission, the shock-accelerated CR protons and ions (hadrons) are exposed through their interactions in the adjacent massive cloud, leading to the observed GeV emission via the gamma decay of neutral pions.

Variable gamma-ray emission from the Be/X-ray transient A0535+26?

We present a study of the unidentied gamma-ray source 3EG J0542+2610. This source is spatially superposed on the supernova remnant G180.0-1.7, but its time variability makes a physical link unlikely. We have searched in the EGRET location error box for compact radio sources that could be the low energy counterpart of the gamma-ray source. Although 29 point-like radio sources were detected and measured, none of them is strong enough to be considered the counterpart of a background gamma-ray emitting AGN. We suggest that the only ob- ject within the 95% error box capable of producing the required gamma-ray flux is the X-ray transient A0535+26. We show that this Be/accreting pulsar can produce variable hadronic gamma-ray emission through the mecha- nism originally proposed by Cheng & Ruderman (1989), where a proton beam accelerated in a magnetospheric electrostatic gap impacts the transient accretion disk.