Peaked BL Lac Research Articles

Detection of the blazar S4 0954+65 at very-high-energy with the MAGIC telescopes during an exceptionally high optical state

Aims. The very high energy (VHE ≳100 GeV) γ-ray MAGIC observations of the blazar S4 0954+65, were triggered by an exceptionally high flux state of emission in the optical. This blazar has a disputed redshift of z = 0.368 or z ≥ 0.45 and an uncertain classification among blazar subclasses. The exceptional source state described here makes for an excellent opportunity to understand physical processes in the jet of S4 0954+65 and thus contribute to its classification. Methods. We investigated the multiwavelength (MWL) light curve and spectral energy distribution (SED) of the S4 0954+65 blazar during an enhanced state in February 2015 and have put it in context with possible emission scenarios. We collected photometric data in radio, optical, X-ray, and γ-ray. We studied both the optical polarization and the inner parsec-scale jet behavior with 43 GHz data. Results. Observations with the MAGIC telescopes led to the first detection of S4 0954+65 at VHE. Simultaneous data with Fermi-LAT at high energy γ-ray (HE, 100 MeV < E < 100 GeV) also show a period of increased activity. Imaging at 43 GHz reveals the emergence of a new feature in the radio jet in coincidence with the VHE flare. Simultaneous monitoring of the optical polarization angle reveals a rotation of approximately 100°. Conclusions. The high emission state during the flare allows us to compile the simultaneous broadband SED and to characterize it in the scope of blazar jet emission models. The broadband spectrum can be modeled with an emission mechanism commonly invoked for flat spectrum radio quasars (FSRQs), that is, inverse Compton scattering on an external soft photon fieldfrom the dust torus, also known as external Compton. The light curve and SED phenomenology is consistent with an interpretation of a blob propagating through a helical structured magnetic field and eventually crossing a standing shock in the jet, a scenario typically applied to FSRQs and low-frequency peaked BL Lac objects (LBL).

The broad-band properties of the intermediate synchrotron peaked BL Lac S2 0109+22 from radio to VHE gamma-rays

The MAGIC telescopes observed S2 0109+22 in 2015 July during its flaring activity in high energy gamma rays observed by Fermi-LAT. We analyse the MAGIC data to characterise the very high energy (VHE) gamma-ray emission of S2 0109+22, which belongs to the subclass of intermediate synchrotron peak (ISP) BL Lac objects. We study the multi-frequency emission in order to investigate the source classification. Finally, we compare the source long-term behaviour to other VHE gamma-ray emitting (TeV) blazars. We performed a temporal and spectral analysis of the data centred around the MAGIC interval of observation (MJD 57225-57231). Long-term radio and optical data have also been investigated using the discrete correlation function. The redshift of the source is estimated through optical host-galaxy imaging and also using the amount of VHE gamma-ray absorption. The quasi-simultaneous multi-frequency spectral energy distribution (SED) is modelled with the conventional one-zone synchrotron self-Compton (SSC) model. MAGIC observations resulted in the detection of the source at a significance level of $5.3\,\sigma$. The VHE gamma-ray emission of S2 0109+22 is variable on a daily time scale. VHE gamma-ray luminosity of the source is lower than the average of TeV BL Lacs. The optical polarization, and long-term optical/radio behaviour of the source are different from the general population of TeV blazars. All these findings agree with the classification of the source as an ISP BL Lac object. We estimate the source redshift as $z = 0.36 \pm 0.07$. The SSC parameters describing the SED are rather typical for blazars.

Neutrinos and Ultra-high-energy Cosmic-ray Nuclei from Blazars

Abstract We discuss the production of ultra-high-energy cosmic-ray (UHECR) nuclei and neutrinos from blazars. We compute the nuclear cascade in the jet for both BL Lac objects and flat-spectrum radio quasars (FSRQs), and in the ambient radiation zones for FSRQs as well. By modeling representative spectral energy distributions along the blazar sequence, two distinct regimes are identified, which we call “nuclear survival” (typically found in low-luminosity BL Lacs) and “nuclear cascade” (typically found in high-luminosity FSRQs). We quantify how the neutrino and cosmic-ray (CR) emission efficiencies evolve over the blazar sequence, and we demonstrate that neutrinos and CRs come from very different object classes. For example, high-frequency-peaked BL Lacs (HBLs) tend to produce CRs, and high-luminosity FSRQs are the more efficient neutrino emitters. This conclusion does not depend on the CR escape mechanism, for which we discuss two alternatives (diffusive and advective escape). Finally, the neutrino spectrum from blazars is shown to significantly depend on the injection composition into the jet, especially in the nuclear cascade case: Injection compositions heavier than protons lead to reduced neutrino production at the peak, which moves at the same time to lower energies. Thus, these sources will exhibit better compatibility with the observed IceCube and UHECR data.

Statistical Analysis of the Microvariable AGN Source Mrk 501

We report on the optical observations and analysis of the high-energy peaked BL Lac object (HBL), Mrk 501, at redshift z = 0.033. We can confirm microvariable behavior over the course of minutes on several occasions per night. As an alternative to the commonly understood dynamical model of random variations in intensity of the AGN, we develop a relativistic beaming model with a minimum of free parameters, which allows us to infer changes in the line of sight angles for the motion of the different relativistic components. We hope our methods can be used in future studies of beamed emission in other active microvariable sources, similar to the one we explored.

A Multiwavelength Study of Flaring Activity in the High-energy Peaked BL Lac Object 1ES 1959+650 During 2015–2016

Abstract We present the results from a multiwavelength study of the flaring activity in the high-energy peaked BL Lac object 1ES 1959+650 during 2015 January–2016 June. The source underwent two major outbursts, during 2015 March and 2015 October, across the whole electromagnetic spectrum (EMS). We used data from Fermi-LAT and Swift-XRT/UVOT, and optical data from Mt. Abu InfraRed Observatory and Steward Observatory to look for possible correlations between the emissions at different energies and the nature of the variability during the flaring state. During the 2015 October outburst, the nightly averaged V-band brightest magnitude, 14.45(0.03), and faintest magnitude, 14.45(0.03), were recorded. Apart from long-term flares, rapid and short-term variabilities were noticed at all energies. Our study suggests that the flaring activities at all frequencies, with diverse flare durations and time lags, are correlated. The magnetic field strength is estimated to be 4 G using the synchrotron-cooling timescale (2.3 hr), and the upper limits on the sizes of both emission regions, γ-ray and optical, are estimated to be of the order of 1016 cm. The quasi-simultaneous flux enhancements in 15 GHz and VHE γ-ray emissions indicate a fresh injection of plasma into the jet, which interacts with a standing submillimeter core, resulting in co-spatial emissions across the EMS. The synchrotron peak shifts to higher frequency in the spectral energy distribution while the γ-ray spectra softens during the flaring. The shape of the inverse-Compton spectra indicates a change in the particle energy distribution pre- and post-flare.

Expected signatures from hadronic emission processes in the TeV spectra of BL Lacertae objects

Context. The wealth of recent data from Imaging Air Cherenkov telescopes (IACTs), ultra-high energy cosmic-ray experiments and neutrino telescopes have fuelled a renewed interest in hadronic emission models for γ-loud blazars. Aims. We explore physically plausible solutions for a lepto-hadronic interpretation of the stationary emission from high-frequency peaked BL Lac objects (HBLs). The modelled spectral energy distributions are then searched for specific signatures at very high energies that could help to distinguish the hadronic origin of the emission from a standard leptonic scenario. Methods. By introducing a few basic constraints on parameters of the model, such as assuming the co-acceleration of electrons and protons, we significantly reduced the number of free parameters. We then systematically explored the parameter space of the size of the emission region and its magnetic field for two bright γ-loud HBLs, PKS 2155-304 and Mrk 421. For all solutions close to equipartition between the energy densities of protons and of the magnetic field, and with acceptable jet power and light-crossing timescales, we inspected the spectral hardening in the multi-TeV domain from proton-photon induced cascades and muon-synchrotron emission inside the source. Very-high-energy spectra simulated with the available instrument functions from the future Cherenkov Telescope Array (CTA) were evaluated for detectable features as a function of exposure time, source redshift, and flux level. Results. A range of hadronic scenarios are found to provide satisfactory solutions for the broad band emission of the sources under study. The TeV spectrum can be dominated either by proton-synchrotron emission or by muon-synchrotron emission. The solutions for HBLs cover a parameter space that is distinct from the one found for the most extreme BL Lac objects in an earlier study. Over a large range of model parameters, the spectral hardening due to internal synchrotron-pair cascades, the “cascade bump”, should be detectable for acceptable exposure times with the future CTA for a few nearby and bright HBLs.

Gamma-ray blazar spectra with H.E.S.S. II mono analysis: The case of PKS 2155−304 and PG 1553+113

Context. The addition of a 28 m Cherenkov telescope (CT5) to the H.E.S.S. array extended the experiment’s sensitivityto lower energies. The lowest energy threshold is obtained using monoscopic analysis of data taken with CT5, providing access to gamma-ray energies below 100 GeV for small zenith angle observations. Such an extension of the instrument’s energy range is particularly beneficial for studies of active galactic nuclei with soft spectra, as expected for those at a redshift ≥0.5. The high-frequency peaked BL Lac objects PKS 2155−304 (z = 0.116) and PG 1553+113 (0.43 < z < 0.58) are among the brightest objects in the gamma-ray sky, both showing clear signatures of gamma-ray absorption at E > 100 GeV interpreted as being due to interactions with the extragalactic background light (EBL). Aims. The aims of this work are twofold: to demonstrate the monoscopic analysis of CT5 data with a low energy threshold, and to obtain accurate measurements of the spectral energy distributions (SED) of PKS 2155−304 and PG 1553+113 near their SED peaks at energies ≈100 GeV. Methods. Multiple observational campaigns of PKS 2155−304 and PG 1553+113 were conducted during 2013 and 2014 using the full H.E.S.S. II instrument (CT1–5). A monoscopic analysis of the data taken with the new CT5 telescope was developed along with an investigation into the systematic uncertainties on the spectral parameters which are derived from this analysis. Results. Using the data from CT5, the energy spectra of PKS 2155−304 and PG 1553+113 were reconstructed down to conservative threshold energies of 80 GeV for PKS 2155−304, which transits near zenith, and 110 GeV for the more northern PG 1553+113. The measured spectra, well fitted in both cases by a log-parabola spectral model (with a 5.0σ statistical preference for non-zero curvature for PKS 2155−304 and 4.5σ for PG 1553+113), were found consistent with spectra derived from contemporaneous Fermi-LAT data, indicating a sharp break in the observed spectra of both sources at E ≈ 100 GeV. When corrected for EBL absorption, the intrinsic H.E.S.S. II mono and Fermi-LAT spectrum of PKS 2155−304 was found to show significant curvature. For PG 1553+113, however, no significant detection of curvature in the intrinsic spectrum could be found within statistical and systematic uncertainties.

Evaluating Optical Classification for Fermi Blazar Candidates with a Statistical Method Using Broadband Spectral Indices

Abstract We aim to test whether a blazar candidate of uncertain type (BCU) in the third Fermi catalog of active galactic nuclei (3LAC) can be potentially classified as a BL Lac object or a flat-spectrum radio quasar (FSRQ) by performing a statistical analysis of its broadband spectral properties. We find that 34% of the radio-selected BCUs (583 BCUs) are BL Lac-like and 20% are FSRQ-like, at a 90% level of confidence. Similarly, 77.3% of the X-ray-selected BCUs (176 BCUs) are evaluated as BL Lac-like and 6.8% may be FSRQ-like sources. And 88.7% of the BL Lac-like BCUs that have synchrotron peak frequencies available are high synchrotron peaked BL Lacs in the X-ray-selected BCUs. The percentages are accordingly 62% and 7.3% in the sample of 124 optically selected BCUs. The high ratio of the number of BL Lac-like sources to the number of FSRQ-like BCUs in the X-ray-selected and optically selected BCU samples is due to selection effects. Examining the consistency between our evaluation and spectroscopic identification case by case with a sample of 78 radio-selected BCUs, it is found that the statistical analysis and its resulting classifications agree with the results of the optical follow-up spectroscopic observations. Our observation campaign for high- BCUs selected with our method, i.e., , is ongoing.

Investigating the Puzzling Synchrotron Behaviour of Mrk 421

We investigate the multiwavelength behaviour of the high-energy peaked BL Lac object (HBL) Mrk 421 at redshift z = 0 . 031 in the period 2007–2015. We use optical photometric, spectroscopic, and polarimetric data and near-infrared data obtained by 35 observatories participating in the GLAST-AGILE Support Program (GASP) of the Whole Earth Blazar Telescope (WEBT), as well as by the Steward Observatory Support of the Fermi Mission. We also employ high-energy data from the Swift (UV and X-rays) satellite to study correlations among emission in different bands.

Parsec-Scale Structure and Kinematics of Faint TeV HBLs

We present new multi-epoch Very Long Baseline Array (VLBA) observations of a set of TeV blazars drawn from our VLBA program to monitor all TeV-detected high-frequency peaked BL Lac objects (HBLs) at parsec scales. Most of these sources are faint in the radio, so they have not been well observed with VLBI by other surveys. Our previous measurements of apparent jet speeds in TeV HBLs showed apparent jet speeds that were subluminal or barely superluminal, suggesting jets with velocity structures at the parsec-scale. Here we present apparent jet speed measurements for eight new TeV HBLs, which for the first time show a superluminal tail to the apparent speed distribution for the TeV HBLs.

An innovative blazar classification based on radio jet kinematics

Blazars are usually classified following their synchrotron peak frequency ($\nu F(\nu)$ scale) as high, intermediate, low frequency peaked BL Lacs (HBLs, IBLs, LBLs), and flat spectrum radio quasars (FSRQs), or, according to their radio morphology at large scale, FR~I or FR~II. However, the diversity of blazars is such that these classes seem insufficient to chart the specific properties of each source. We propose to classify a wide sample of blazars following the kinematic features of their radio jets seen in very long baseline interferometry (VLBI). For this purpose we use public data from the MOJAVE collaboration in which we select a sample of blazars with known redshift and sufficient monitoring to constrain apparent velocities. We selected 161 blazars from a sample of 200 sources. We identify three distinct classes of VLBI jets depending on radio knot kinematics: class I with quasi-stationary knots, class II with knots in relativistic motion from the radio core, and class I/II, intermediate, showing quasi-stationary knots at the jet base and relativistic motions downstream. A notable result is the good overlap of this kinematic classification with the usual spectral classification; class I corresponds to HBLs, class II to FSRQs, and class I/II to IBLs/LBLs. We deepen this study by characterizing the physical parameters of jets from VLBI radio data. Hence we focus on the singular case of the class I/II by the study of the blazar BL Lac itself. Finally we show how the interpretation that radio knots are recollimation shocks is fully appropriate to describe the characteristics of these three classes.

MAGIC observations of the February 2014 flare of 1ES 1011+496 and ensuing constraint of the EBL density

In February-March 2014, the MAGIC telescopes observed the high-frequency peaked BL Lac 1ES 1011+496 (z=0.212) in flaring state at very-high energy (VHE, E>100GeV). The flux reached a level more than 10 times higher than any previously recorded flaring state of the source. We present the description of the characteristics of the flare presenting the light curve and the spectral parameters of the night-wise spectra and the average spectrum of the whole period. From these data we aim at detecting the imprint of the Extragalactic Background Light (EBL) in the VHE spectrum of the source, in order to constrain its intensity in the optical band. For this we implement the method developed by the H.E.S.S. collaboration in which the intrinsic energy spectrum of the source is modeled with a simple function, and the EBL-induced optical depth is calculated using a template EBL model. The likelihood of the observed spectrum is then maximized, including a normalization factor for the EBL opacity among the free parameters. From the data collected differential energy spectra was produced for all nights of the observed period. Evaluating the changes in the fit parameters we conclude that the spectral shape for most of the nights were compatible, regardless of the flux level, which enabled us to produce an average spectrum from which the EBL imprint could be constrained. The likelihood ratio test shows that the model with an EBL density 1.07(-0.20,+0.24)_{stat+sys}, relative to the one in the tested EBL template (Dominguez et al.2011), is preferred at the 4.6 sigma level to the no-EBL hypothesis, with the assumption that the intrinsic source spectrum can be modeled as a log-parabola. This would translate into a constraint of the EBL density in the wavelength range [0.24 um,4.25 um], with a peak value at 1.4 um of F=12.27_{-2.29}^{+2.75} nW m^{-2} sr^{-1}, including systematics.

The extended jet of AP Librae: Origin of the very high-energyγ-ray emission?

The low-frequency peaked BL Lac object (LBL) AP Librae exhibits very-high-energy (VHE, $E>100$GeV) $\gamma$-ray emission and hosts an extended jet, which has been detected in radio and X-rays. The jet X-ray spectral index implies an inverse Compton origin. These observations are unusual for LBLs calling for a consistent explanation of this extraordinary source. The observationally constrained parameters necessary to describe the core emission within the standard one-zone model are unable to explain the broad-band spectrum, even if observationally unconstrained external photon fields are taken into account. We demonstrate that the addition of the extended jet emission successfully reproduces the total spectral energy distribution. In particular, the VHE radiation is produced in the $>100\,$kpc long extended jet via inverse Compton scattering of cosmic microwave background photons by highly relativistic electrons. We present several ways to test this theory. The extended jet is weakly magnetized ($B_0 = 2.5\,\mu$G), while its minimum and maximum electron Lorentz factors are $\gamma_{min}=60$ and $\gamma_{max}=5\times 10^6$, respectively. The electron spectral index is $s=2.6$. These parameters are comparable to parameters of other blazars with extended X-ray jets dominated by inverse Compton scattering.

Search for hard lags with intra‐night optical observations of BL Lacertae

AbstractIt is generally believed that the high energy end of synchrotron emission, generated by the most energetic tail of relativistic electrons in the jets, account for the X‐ray emission of high‐energy peaked BL Lac objects (HBLs) and the optical emission of intermediate‐energy peaked BL Lac objects (IBLs). It is thus expected that both should show similar variability characteristics. One of the important variability parameters is the inter‐band time lag which probes the acceleration and cooling of relativistic particles responsible for the emission. The switches between soft and hard lags have been detected in the intra‐day X‐ray variability of a few HBLs, which is not the case for the intra‐day optical variability of IBLs yet. We present the results of our intra‐night optical observations for BL Lacertae, aiming at searching for hard lags of its optical variations, performed with the 80 cm telescope in fourteen nights of 2010 September‐November. Intra‐night changes of ∼0.2 mag were detected in most of nights. The intra‐night variability amplitude tends to become larger from red to blue wavelength, and the optical spectrum hardens with increasing brightness. The intra‐night variations correlate between different wavebands, but we did not find significant time lags, either soft or hard. Nevertheless, on November 2, the B band variations showed a sign of lagging the R band ones by 317±214 s. The claim of this hard lag is strongly limited by the photometric precision and time resolution. Therefore, the switches between soft and hard lags of IBLs in the optical bands needs further demonstration with more higher quality observations. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Testing the blazar sequence with the least luminous BL Lacertae objects

In a previous paper, we proposed a new method to select low-power BL Lacs (LPBLs) based on mid-infrared emission and flux contrast through the Ca II spectral break; that study led to the selection of a complete sample formed by 34 LPBLs with 0.05<z<=0.15 and radio luminosities spanning the range log(L_r) = 39.2-41.5 [erg/s]. We now assemble the broadband spectral energy distributions (SEDs) of these sources to investigate their nature and compare them with brighter BL Lacs. We find that the ratios between the X-ray and radio luminosities range from ~20 to ~30000 and that the synchrotron peak frequencies span a wide energy interval, from log(nu_peak)~13.5 to ~20 [Hz]. This indicates a broad variety of SED shapes and a mixture of BL Lac flavors. Indeed, although the majority of our LPBLs are high-energy peaked BL Lacs (HBLs), we find that a quarter of them are low-energy peaked BL Lacs (LBLs), despite the fact that the sample is biased against the selection of LBLs. The analysis of the median LPBL SED confirms disagreement with the blazar sequence at low radio luminosities. Furthermore, if we limit the sample to the LBLs subsample, we find that their median SED shape is essentially indistinguishable from that of the most luminous BL Lacs. We conclude that the observed radio power is not the main driving parameter of the multiwavelength properties of BL Lacs.

The broadband spectral energy distributions of SDSS blazars**Supported by the National Natural Science Foundation of China.

We compiled the radio, optical and X-ray data of blazars from the Sloan Digital Sky Survey database, and presented the distribution of luminosities and broadband spectral indices. The distribution of luminosities shows that the averaged luminosity of flat spectrum radio quasars (FSRQs) is larger than that of BL Lacertae (BL Lac) objects. On the other hand, the broadband spectral energy distribution reveals that FSRQs and low energy peaked BL Lac objects have similar spectral properties, but high energy peaked BL Lac objects have a distinct spectral property. This may be due to the fact that different subclasses of blazars have different intrinsic environments and are at different cooling levels. Even so, a unified scheme is also revealed from the color-color diagram, which hints that there are similar physical processes operating in all objects under a range of intrinsic physical conditions or beaming parameters.

An emerging population of BL Lacs with extreme properties: towards a class of EBL and cosmic magnetic field probes?

High energy observations of extreme BL Lac objects, such as 1ES 0229+200 or 1ES 0347-121, recently focused interest both for blazar and jet physics and for the implication on the extragalactic background light and intergalactic magnetic field estimate. However, the number of these extreme highly peaked BL Lac objects (EHBL) is still rather small. Aiming at increase their number, we selected a group of EHBL candidates starting from the BL Lac sample of Plotkin et al. (2011), considering those undetected (or only barely detected) by the Large Area Telescope onboard Fermi and characterized by a high X-ray vs. radio flux ratio. We assembled the multi-wavelength spectral energy distribution of the resulting 9 sources, profiting of publicly available archival observations performed by the Swift, Galex and Fermi satellites, confirming their nature. Through a simple one-zone synchrotron self-Compton model we estimate the expected VHE flux, finding that in the majority of cases it is within the reach of present generation of Cherenkov arrays or of the forthcoming Cherenkov Telescope Array (CTA).

THE 2012 FLARE OF PG 1553+113 SEEN WITH H.E.S.S. ANDFERMI-LAT

Very high energy (VHE, $E>$100 GeV) $\gamma$-ray flaring activity of the high-frequency peaked BL Lac object \pg\ has been detected by the \hess\ telescopes. The flux of the source increased by a factor of 3 during the nights of 2012 April 26 and 27 with respect to the archival measurements with hint of intra-night variability. No counterpart of this event has been detected in the \fla\ data. This pattern is consistent with VHE $\gamma$ ray flaring being caused by the injection of ultrarelativistic particles, emitting $\gamma$ rays at the highest energies. The dataset offers a unique opportunity to constrain the redshift of this source at \bestz\ using a novel method based on Bayesian statistics. The indication of intra-night variability is used to introduce a novel method to probe for a possible Lorentz Invariance Violation (LIV), and to set limits on the energy scale at which Quantum Gravity (QG) effects causing LIV may arise. For the subluminal case, the derived limits are $\textrm{E}_{\rm QG,1}>4.10\times 10^{17}$ GeV and $\textrm{E}_{\rm QG,2}>2.10\times 10^{10}$ GeV for linear and quadratic LIV effects, respectively.

A hadronic origin for ultra-high-frequency-peaked BL Lac objects

Current Cherenkov telescopes have identified a population of ultra-high-frequency peaked BL Lac objects (UHBLs), also known as extreme blazars, that exhibit exceptionally hard TeV spectra, including 1ES 0229+200, 1ES 0347-121, RGB J0710+591, 1ES 1101-232, and 1ES 1218+304. Although one-zone synchrotron-self-Compton (SSC) models have been generally successful in interpreting the high-energy emission observed in other BL Lac objects, they are problematic for UHBLs, necessitating very large Doppler factors and/or extremely high minimum Lorentz factors of the emitting leptonic population. In this context, we have investigated alternative scenarios where hadronic emission processes are important, using a newly developed (lepto-)hadronic numerical code to systematically explore the physical parameters of the emission region that reproduces the observed spectra while avoiding the extreme values encountered in pure SSC models. Assuming a fixed Doppler factor $\delta=30$, two principal parameter regimes are identified, where the high-energy emission is due to: 1) proton-synchrotron radiation, with magnetic fields $B \sim 1-100\ \textrm{G}$ and maximum proton energies $E_{p;max} \lesssim 10^{19}$ eV; and 2) synchrotron emission from p-$\gamma$-induced cascades as well as SSC emission from primary leptons, with $B \sim 0.1-1\ \textrm{G}$ and $E_{p;max} \lesssim 10^{17}$ eV. This can be realized with plausible, sub-Eddington values for the total (kinetic plus magnetic) power of the emitting plasma, in contrast to hadronic interpretations for other blazar classes that often warrant highly super-Eddington values.

SOURCE IDENTIFICATION IN THE IGR J17448-3232 FIELD: DISCOVERY OF THE SCORPIUS GALAXY CLUSTER

We use a 43-ks XMM-Newton observation to investigate the nature of sources first distinguished by a follow-up Chandra observation of the field surrounding INTEGRAL source IGR J17448-3232, which includes extended emission and a bright point source previously classified as a blazar. We establish that the extended emission is a heretofore unknown massive galaxy cluster hidden behind the Galactic bulge. The emission-weighted temperature of the cluster within the field of view is 8.8 keV, with parts of the cluster reaching temperatures of up to 12 keV; no cool core is evident. At a redshift of 0.055, the cluster is somewhat under-luminous relative to the X-ray luminosity-temperature relation, which may be attributable to its dynamical state. We present a preliminary analysis of its properties in this paper. We also confirm that the bright point source is a blazar, and we propose that it is either a flat spectrum radio quasar or a low-frequency peaked BL Lac object. We find four other fainter sources in the field, which we study and tentatively identify. Only one, which we propose is a foreground Galactic X-ray binary, is hard enough to contribute to IGR J17448-3232, but it is too faint to be significant. We thus determine that IGR J17448-3232 is in fact the galaxy cluster up to $\approx$45 keV and the blazar beyond.