Extreme synchrotron BL Lac objects

Costamante, L.; Ghisellini, G.; Giommi, P.; Tagliaferri, G.; Celotti, A.; Chiaberge, M.; Fossati, G.; Maraschi, L.; Tavecchio, F.; Treves, A.; Wolter, A.

doi:10.1051/0004-6361:20010412

Cited by 236 publications

(240 citation statements)

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“…During these states 1ES 0414+009 is characterized by a SED typical of "extreme" BL Lac objects (i.e. objects with the synchrotron peak above a few keV, Costamante et al 2001). In the VHE γ-ray domain, the data from the HEGRA experiment were used to derive an upper limit on the flux for this source corresponding to 13.5 × 10 −12 cm −2 s −1 above 910 GeV .…”

Section: Introductionmentioning

confidence: 99%

Discovery of hard-spectrumγ-ray emission from the BL Lacertae object 1ES 0414+009

Abramowski¹,

Acero²,

Aharonian³

et al. 2012

A&A

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Context. 1ES 0414+009 (z = 0.287) is a distant high-frequency-peaked BL Lac object, and has long been considered a likely emitter of very-highenergy (VHE, E > 100 GeV) γ-rays due to its high X-ray and radio flux. Aims. Observations in the VHE γ-ray band and across the electromagnetic spectrum can provide insights into the origin of highly energetic particles present in the source and the radiation processes at work. Because of the distance of the source, the γ-ray spectrum might provide further limits on the level of the extragalactic background light (EBL). Methods. We report observations made between October 2005 and December 2009 with H.E.S.S., an array of four imaging atmospheric Cherenkov telescopes. Observations at high energies (HE, 100 MeV-100 GeV) with the Fermi-LAT instrument in the first 20 months of its operation are also reported. To complete the multi-wavelength picture, archival UV and X-ray observations with the Swift satellite and optical observations with the ATOM telescope are also used. Results. Based on the observations with H.E.S.S., 1ES 0414+009 is detected for the first time in the VHE band. An excess of 224 events is measured, corresponding to a significance of 7.8σ. The photon spectrum of the source is well described by a power law, with photon index of Γ VHE = 3.45 ± 0.25 stat ± 0.20 syst . The integral flux above 200 GeV is (1.88 ± 0.20 stat ± 0.38 syst ) ×10 −12 cm −2 s −1 . Observations with the Fermi-LAT in the first 20 months of operation show a flux between 200 MeV and 100 GeV of (2.3 ± 0.2 stat ) × 10 −9 erg cm −2 s −1 , and a spectrum well described by a power-law function with a photon index Γ HE = 1.85 ± 0.18. Swift/XRT observations show an X-ray flux between 2 and 10 keV of (0.8−1) × 10 −11 erg cm −2 s −1 , and a steep spectrum Γ X = (2.2−2.3). Combining X-ray with optical-UV data, a fit with a log-parabolic function locates the synchrotron peak around 0.1 keV. Conclusions. Although the GeV-TeV observations do not provide better constraints on the EBL than previously obtained, they confirm a low density of the EBL, close to the lower limits from galaxy counts. The absorption-corrected HE and VHE γ-ray spectra are both hard and have similar spectral indices (≈1.86), indicating no significant change of slope between the HE and VHE γ-ray bands, and locating the γ-ray peak in the SED above 1-2 TeV. As for other TeV BL Lac objects with the γ-ray peak at such high energies and a large separation between the two SED humps, this average broad-band SED represents a challenge for simple one-zone synchrotron self-Compton models, requiring a high Doppler factor and very low B-field.

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Section: Introductionmentioning

confidence: 99%

Discovery of hard-spectrumγ-ray emission from the BL Lacertae object 1ES 0414+009

Abramowski¹,

Acero²,

Aharonian³

et al. 2012

A&A

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“…Their SED is characterised by two broad peaks, now almost commonly in- terpreted as due to synchrotron and inverse Compton radiation. In this respect, the BL Lac objects reported here should have the synchrotron peak emission around 0.1 keV (see Figure 1 in Costamante et al 2001), indicating the presence of high energy electrons: these extreme BL Lacs are good candidates for TeV emission that, if detected at these redshifts, could provide important constraints both on the physical process at work in such sources, and on the intensity and spectrum of the diffuse Extragalactic Background Light (see e.g. Costamante 2013).…”

Section: The Bl Lacs In the Exo50 Samplementioning

confidence: 65%

“…Their position in the several plots discussed here deviates significantly from the rest of the sample. The very high f x / f o selection used here allows us to pick up the most extreme BL Lacs (HBL type) in the α OX -α RX plane (see also Costamante et al 2001). This is clearly evident in Figure 8, where we show the α OX -α Rx plane for the BL Lacs objects in the BZCAT catalog (Massaro et al 2009) with the position of the two EXO50 sources identified as BL Lacs overlaid; only less than 1 percent of the BL Lacs discovered so far have broad band spectral indices as extreme as the ones discovered here using the f x / f o > 50 selection.…”

Section: The Bl Lacs In the Exo50 Samplementioning

confidence: 99%

Exploring the active galactic nuclei population with extreme X-ray-to-optical flux ratios (fx/fo > 50)

Ceca

Carrera

Caccianiga

et al. 2015

Monthly Notices of the Royal Astronomical Society

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The cosmic history of the growth of supermassive black holes in galactic centers parallels that of star-formation in the Universe. However, an important fraction of this growth occurs inconspicuously in obscured objects, where ultraviolet/optical/near-infrared emission is heavily obscured by dust. Since the X-ray flux is less attenuated, a high X-ray-to-optical flux ratio ( f x / f o ) is expected to be an efficient tool to find out these obscured accreting sources. We explore here via optical spectroscopy, X-ray spectroscopy and infrared photometry the most extreme cases of this population (those with f x / f o > 50, EXO50 sources hereafter), using a well defined sample of 7 X-ray sources extracted from the 2XMM catalogue. Five EXO50 sources (∼ 70 percent of the sample) in the bright flux regime explored by our survey ( f (2−10keV) 1.5 × 10 −13 erg cm −2 s −1 ) are associated with obscured AGN (N H > 10 22 cm −2 ), spanning a redshift range between 0.75 and 1 and characterised by 2-10 keV intrinsic luminosities in the QSO regime (e.g. well in excess to 10 44 erg s −1 ). We did not find compelling evidence of Compton Thick AGN. Overall the EXO50 Type 2 QSOs do not seem to be different from standard X-ray selected Type 2 QSOs in terms of nuclear absorption; a very high AGN/host galaxy ratio seems to play a major role in explaining their extreme properties. Interestingly 3 out of 5 EXO50 Type 2 QSO objects can be classified as Extreme Dust Obscured Galaxies (EDOGs, f 24 µm / f R 2000), suggesting that a very high AGN/host ratios (along with the large amount of dust absorption) could be the natural explanation also for a part of the EDOG population. The remaining 2 EXO50 sources are classified as BL Lac objects, having rather extreme properties, and which are good candidates for TeV emission.

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“…The slight hardening of the XMM-Newton spectrum between MJD 53 534 and 53 536 is easily accounted for by only a minor variation in the high-energy index of the electron distribution. (Costamante et al 2001). In the VHE band, the black triangles and red circles are the time-average spectra observed in 2004 and 2005, respectively, and are not corrected for the effects of EBL absorption.…”

Section: Discussionmentioning

confidence: 99%

“…It is among the X-ray brightest of the 150 high-frequency peaked BL Lac objects (HBL) in the Sedentary Survey (Giommi et al 2005), which contains HBLs characterized by their exceptionally high X-ray-to-radio flux ratio. BeppoSAX observations (Costamante et al 2001) of H 2356-309 in June 1998 yielded an X-ray spectrum extending up to 50 keV, characterized by a broken power-law shape (Γ 1 = 0.78 +0.06 −0.07 ; Γ 2 = 1.10 ± 0.04) with a break at 1.8 ± 0.4 keV. A softer power-law spectrum between 2 and 10 keV with an X-ray photon index of (Γ X = 2.43 ± 0.11) was measured (Aharonian et al 2006b) during observations by the RXTE satellite on November 11, 2004.…”

Section: Introductionmentioning

confidence: 99%

Multi-wavelength observations of H 2356–309

Abramowski¹,

Acero²,

Aharonian³

et al. 2010

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Aims. The properties of the broad-band emission from the high-frequency peaked BL Lac H 2356-309 (z = 0.165) are investigated. Methods. Very high energy (VHE; E > 100 GeV) observations of H 2356-309 were performed with the High Energy Stereoscopic System ( Results. A strong VHE signal, ∼13σ total, was detected by HESS after the four years HESS observations (116.8 h live time). The integral flux above 240 GeV is I(>240 GeV) = (3.06 ± 0.26 stat ± 0.61 syst ) × 10 −12 cm −2 s −1 , corresponding to ∼1.6% of the flux observed from the Crab Nebula. A time-averaged energy spectrum is measured from 200 GeV to 2 TeV and is characterized by a power law (photon index of Γ = 3.06 ± 0.15 stat ± 0.10 syst ). Significant small-amplitude variations in the VHE flux from H 2356-309 are seen on time scales of months and years, but not on shorter time scales. No evidence for any variations in the VHE spectral slope are found within these data. The XMM-Newton X-ray measurements show a historically low X-ray state, characterized by a hard, broken-power-law spectrum on both nights. Conclusions. The broad-band spectral energy distribution (SED) of the blazar can be adequately fit using a simple one-zone synchrotron selfCompton (SSC) model. In the SSC scenario, higher VHE fluxes could be expected in the future since the observed X-ray flux is at a historically low level.

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Extreme synchrotron BL Lac objects

Cited by 236 publications

References 40 publications

Discovery of hard-spectrumγ-ray emission from the BL Lacertae object 1ES 0414+009

Discovery of hard-spectrumγ-ray emission from the BL Lacertae object 1ES 0414+009

Exploring the active galactic nuclei population with extreme X-ray-to-optical flux ratios (fx/fo > 50)

Multi-wavelength observations of H 2356–309

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