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Simultaneous H.E.S.S. and Chandra observations of Sgr A* during an X-ray flare

The rapidly varying non-thermal X-ray emission observed from Sgr A* points to particle acceleration taking place close to the supermassive black hole. The TeV gamma-ray source HESS J1745-290 is coincident with Sgr A* and may be closely related to the X-ray emission. Simultaneous X-ray and TeV observations are required to elucidate the relationship between these two objects. Here we report on joint H.E.S.S./Chandra observations in July 2005, during which an X-ray flare was detected. Despite a factor>10 increase in the X-ray flux of Sgr A*, no evidence is found for an increase in the TeV gamma-ray flux. We find that an increase of the gamma-ray flux of a factor 2 or greater can be excluded at a confidence level of 99%. This finding disfavours scenarios in which the bulk of the gamma-ray emission observed is produced close to Sgr A*.


Introduction
The existence of a supermassive (3.6 ± 0.3 × 10 6 solar mass) black hole at the centre of our galaxy has been inferred using measurements of stellar orbits in the central parsec (see e.g. [1]). The supermassive black hole (SMBH) is coincident with the faint radio source: Sgr A ⋆ . The compact nature of Sgr A ⋆ has been demonstrated both by direct VLBI measurements [2] and by the observation of X-ray and near IR flares with timescales as short as a few minutes (see for example [3,4]). Variability on such short timescales limits the emission region (via causality arguments) to within < 10 Schwarzchild radii of the black hole. X-ray flares from Sgr A ⋆ have reached fluxes of 4 × 10 35 erg s −1 , two orders of magnitude brighter than the quiescent flux [4,5], and exhibit a range of spectral shapes [4]. Several models exist for the origin of this variable emission, all of which invoke nonthermal processes close to the event horizon of the central black hole to produce a population of relativistic particles.
Model independent evidence for the existence of ultra-relativistic particles close to Sgr A ⋆ can be provided by the observation of TeV γ-rays from this source. Indeed, TeV γ-ray emission has been detected from the Sgr A region by several groundbased instruments [6,7,8,9]. The most precise measurement of this source, HESS J1745−290, are those made using the H.E.S.S. telescope array. The centroid of the source is located 7 ′′ ± 14 ′′ stat ± 28 ′′ sys from Sgr A ⋆ , and has an rms extension of < 1.2 ′ [10]. TeV emission from Sgr A ⋆ is expected in several models of particle acceleration in the environment of the black hole. In some of these scenarios [11,12] TeV emission is produced in the immediate vicinity of the SMBH and variability is expected. In alternative scenarios particles are accelerated at Sgr A ⋆ but radiate in within the central ∼10 parsec region [13], or are accelerated at the termination shock of a wind driven by the SMBH [14]. However, several additional candidate objects exist for the origin of the observed γ-ray emission. The radio centroid of the supernova remnant (SNR) Sgr A East lies ∼ 1 ′ from Sgr A ⋆ , only marginally inconsistent with the position of the TeV source given in [10]. Shell-type SNR are now well established TeV γ-ray sources [15,16] and several authors have suggested Sgr A East as the origin of the TeV emission (see for example [17]). However, recent improvements in the statistical and systematic uncertainties of the centroid of HESS J1745−290 effectively exclude Sgr A East as the dominant γ-ray source in the region [18]. The recently discovered pulsar wind nebula candidate G 359.95-0.04 [19] lies only 9 arcseconds from Sgr A ⋆ and can plausibly explain the TeV emission [20]. Particle acceleration at stellar wind collision shocks within the central young stellar cluster has also been hypothesised to explain the γray source [21]. Finally, an origin of this source in the annihilation of WIMPs in a central dark matter cusp has been extensively discussed [22,23,10]. Given the limited angular resolution of current VHE γ-ray telescopes, the most promising tool for identification of the TeV source is the detection of correlated variability between the γ-ray and Xray and/or NIR regimes. A significant increase of the flux of HESS J1745−290 simultaneous with a flare in wavebands with sufficient angular resolution to isolate Sgr A ⋆ , would provide an unambiguous identification of the γ-ray source. Therefore, whilst not all models for TeV emission from Sgr A ⋆ predict variability of the VHE source, coordinated IR/keV/TeV observations can be seen as a key aspect of the ongoing program to understand the nature of this enigmatic source.

Observations & Results
A coordinated multi-wavelength campaign on Sgr A ⋆ took place during July/August 2005. As part of this campaign observations with H.E.S.S. occurred for 4-5 hours each night from the 27th of July to the 1st of August (MJD 53578-53584). Four Chandra observations with IDs 5950-5954 took place between the 24th of July and the 2nd of August. A search for flaring events in the Xray data yielded two significant events during the Chandra campaign, both during observation ID 5953 on the 30th of July. The second of these flares occurred during a period of H.E.S.S. coverage, at MJD 53581.94. The γ-ray data consist of 72 twenty-eight minute runs, 66 of which pass all the quality selection cuts described by [24]. All runs on the night of the X-ray flare pass these cuts and in addition we find no evidence for cloud cover in the simultaneous sky temperature (radiometer) measurements (see [24,25]). These data were analysed using the H.E.S.S. standard Hillas parameter based method described in [24]. An independent analysis based on the Model Analysis method described in [26] produced consistent results. Figure 1 shows a night-by-night TeV flux light-curve for this period. There is no evidence for variations of the flux on day timescales and the mean > 1 TeV γ-ray flux for this week of observations was 2.03 ± 0.09 stat × 10 −12 cm −2 s −1 , consistent with the average value for H.E.S.S. observations in 2004, 1.87 ± 0.1 stat ± 0.3 sys × 10 −12 cm −2 s −1 [10]. Figure 2 shows the X-ray and γ-ray light curves for the night MJD 53581-2. There is a clear increase in the X-ray flux of Sgr A ⋆ with an excess of 103 ± 10 events with respect to the quiescent level. The time-profile of this excess is consistent with a Gaussian of rms 13.1 ± 2.5 minutes. The time window for the γ-ray analysis is defined as the region within ±1.3σ of the X-ray flare (containing 80% of the signal). The lower panel of Figure 2 shows the mean TeV flux within this time window (grey shaded region) of 2.05±0.76 × 10 −12 cm −2 s −1 as a short dashed line. This flux level is almost identical to the mean flux level for this week of observations. There is therefore no evidence for an increase in γ-ray flux of HESS J1745-290 during the flare and a limit on the relative flux increase of < a factor 2 is derived at the 99% confidence level.
In principle a (positive or negative) time lag might be expected between the X-ray and any associated γ-ray flare. The existence of a counterpart γ-flare with a flux increase ≫ 2 requires a lag of at least 100 minutes.

Summary
For the first time simultaneous TeV γ-ray observations have been presented for a period of X-ray activity of Sgr A ⋆ . The non-detection of an increase in the TeV flux provides an important constraint on scenarios in which the source HESS J1745-290 is associated with the supermassive black hole.