We have used 1000 ksec of on-source, and 500 ksec of 10 degrees off-source INTEGRAL data of SN 1006 to characterize the synchrotron emission, and attempt to detect non-thermal bremsstrahlung, using the combination of IBIS and JEM-X spatial and spectral coverage. With the earlier 750 ksec on source data, we have detected the limbs of SN 1006 with JEM-X between 2.4 and 8.4 keV bands. The source was not detected with either ISGRI orSPI above 20 keV. The ISGRI upper limit is about a factor of four above current model predictions, but conrms the presence of steepening in the power-law extrapolated from lower energies (< 4 keV)

Boggs, Steven E.

Chenevez, Jerome

Kalemci, Emrah

Lund, Niels

Renaud, Matthieu

Reynolds, Stephen

English

Sabanci University Research Database

X-RAY OBSERVATIONS OF SN 1006 WITH INTEGRALE. Kalemci1, S. P. Reynolds2, S. E. Boggs3, N. Lund4, J. Chenevez4, and M. Renaud51Sabanc Universitesi, Orhanl-Tuzla, Istanbul, 34956, Turkey.2Department of Physics, NC State University, 2700 Stinson Drive, Box 8202, Raleigh, NC 27695, USA.3Space Sciences Laboratory, 7 Gauss Way, University of California, Berkeley, CA, 94720-7450, USA.4Danish National Space Center, Juliane Maries Vej 30, DK-2100 Copenhagen ∅, Denmark.5Service d’Astrophysique, CEA-Saclay, 91191, Gif-Sur-Yvette, FranceABSTRACTWe have used ∼1000 ksec of on-source, and 500 ksecof 10 degrees off-source INTEGRAL data of SN 1006 tocharacterize the synchrotron emission, and attempt to de-tect non-thermal bremsstrahlung, using the combinationof IBIS and JEM-X spatial and spectral coverage. Withthe earlier 750 ksec on source data, we have detected thelimbs of SN 1006 with JEM-X between 2.4 and 8.4 keVbands. The source was not detected with either ISGRI orSPI above 20 keV. The ISGRI upper limit is about a fac-tor of four above current model predictions, but confirmsthe presence of steepening in the power-law extrapolatedfrom lower energies (< 4 keV).Key words: ISM:individual (SN1006), supernova rem-nants, X-rays:observations, radiation mechanisms:non-thermal.1. INTRODUCTIONSN 1006 has been the prototype source for the study ofelectron acceleration to high energies in shocks. X-raysfrom this object were first reported by Winkler & Laird,1976 [1]. ASCA observations showed that the limbs havefeatureless power-law spectra, whereas the interior has athermal, line-dominated spectrum [2].High energy electrons in supernova remnants (SNR) pro-duce X-rays via non-thermal bremsstrahlung and syn-chrotron processes. In hard X-rays, synchrotron ra-diation from the tail of the electron distribution maycompete with non-thermal bremsstrahlung from the verylowest-energy accelerated electrons. Electrons producingkeV synchrotron emission could also produce very high-energy photons (in the TeV range) by IC upscatteringof CMB photons [3]. A detection of the northeast (NE)limb of SN 1006 was reported in ground-based TeV ob-servations by CANGAROO-I [4]. However, this result islater called into question by the observations of H.E.S.S.,and recent measurements by CANGAROO-III has alsoclaimed null result on SN 1006 [5]. Lower gamma-ray fluxes result in lower predicted bremsstrahlung fluxby an order of magnitude. The nature of X-ray emis-sion from SN 1006 above 10 keV is still uncertain.Below 10 keV, synchrotron emission should dominate.In principle, the INTEGRAL observatory can examinethe effects of both the lowest and highest energy non-thermal electrons by distinguishing the synchrotron andbremsstrahlung emission with its imaging and spectralcapabilities. The synchrotron emission should be concen-trated in two bright opposing limbs like the radio emis-sion, and should dominate the emission below 30 keV.The non-thermal bremsstrahlung will dominate at someenergy between 30 and 300 keV [6].Our group has observed SN 1006 for ∼1000 ks withthe INTEGRAL Observatory [7] in AO-1 with the mainaim of detecting and characterizing synchrotron emis-sion, and distinguishing synchrotron and non-thermalbremsstrahlung emission by comparing the IBIS/ISGRIand JEM-X images to the model images. Later, in AO-3,an additional 2.5 MSec to SN 1006 was awarded, thistime mainly to search for 511 keV emission from thesource. The AO-3 observations were amalgamated withCen X-4. The results of the AO-1 observations have al-ready been published [8]. For AO-3, only the 10◦ offsetdata were available at the time of the workshop. Here,we summarize the AO-1 results and add the ISGRI re-sults from AO-3.2. OBSERVATIONS AND ANALYSISThe INTEGRAL observations took place in three sets.The ∼250 ks first set (“Set I”) was conducted early inthe mission, between Jan, 11 and 20, 2003, correspond-ing to INTEGRAL revolutions 30 and 32. The ∼750 kssecond set (“Set II”) was conducted between Jan, 20 and30, 2004, during revolutions 155-158. The third set (“SetIII”) is part of an amalgamation of SN 1006 and Cen X-4and centered on on Cen X-4. It was conducted betweenJan 29 and Feb 4 2006, corresponding to revolutions 402-404.2We did not use SPI (Spectrometer on INTEGRAL [9]),as ISGRI (see § 2.2 for more information) places muchstricter limits in the hard X-ray band. Before the generalanalysis for all instruments, we filtered out the pointingswith high Anti-Coincidence Shield rates, mostly occur-ring during the entry and exit of the radiation belts.2.1. The JEM-X analysisSet I, which was before a new background rejection cri-teria was implemented, was not included in this analysis.Set III was not used either as the central pointing of thedither pattern was 10 degrees away.Although the total exposure for Set II is ∼750 ks, theeffective exposure time of the central object is approxi-mately 250 ks due to the vignetting of the JEM-X instru-ment during the 25 point dither. We have obtained JEM-X images in 2.4–4.2 keV, 4.2–8.4 keV, 8.4–14 keV, and14–35 keV bands using the JEM-X Midisky ofine soft-ware package [10]. The images from each pointing arethen mosaicked using the mosaic-weight program [11].These software were later implemented into OSA 5. Thefluxes and the significance values shown in Table 1 arederived using the inner ASCA contours enclosing the NEand SW limbs to define two shape templates. The countexcesses in the JEM-X mosaic images are then deter-mined inside these two regions. The relevant noise fig-ures are derived by defining a number of non-overlappingregions with the reference templates within a 60 × 60pixel field (90 × 90 arc minutes) centered on SN1006.The statistical properties of the excess counts in thesetwo sets of regions are used to derive RMS-noise of thebackground for regions of the two shapes. The fluxesare derived by comparing the excesses in the SN 1006mosaics with corresponding excesses in mosaic imagesof the Crab Nebula obtained with the same INTEGRALdither pattern, and correcting for the difference in the ef-fective observation time.2.2. The ISGRI analysisThe older background maps for Set I observations re-sulted in much noisier images compared to Set II images,and therefore we limit the analysis to the data from Set IIand Set III. We used OSA 4.2 [12] standard programs forSet II and OSA 5.1 for Set III.Since the ISGRI system point spread function is ∼13′,SN 1006 (∼30′ diameter) appears as an extended objectto the imager. One needs to use simulations to obtain theeffect of the extended nature of the source on the imageand the measured flux. Such simulations have been con-ducted for ISGRI for different extended source geome-tries, including SN 1006 [13]. A reduction factor of 0.7is obtained: that is, ISGRI would detect 70% of the trueflux at each limb.Figure 1. JEM-X reconstructed images of SN 1006 inthree energy bands. Top: 2.44.2 keV, middle: 4.28.4keV band, bottom: 8.4-14 keV. The ASCA contours areoverlaid. From Kalemci et al. [8]3. RESULTS3.1. JEM-X resultsThe source is detected at the limbs in 2.4–4.2 keV and4.2–8.4 keV bands (see Fig. 1 and Table 1). It appearsthat the South-West (SW) limb (the limb on the right inthe images) is stronger in the 2.4–4.2 keV band at aboutthe 1σ level, but the trend is reversed at higher energies.At 4.2–8.4 keV, the NE limb is stronger, a result withhigher significance. An excess at the position of the NElimb is present in the 8.4–14 keV band (1.7 σ), but no ex-cess is seen in the SW limb. We note that XMM-Newtondata also points to an asymmetry in flux coming fromthe NE and SW limbs such that the NE limb gets rela-tively stronger as energy increases [14]. A similar trendis seen in ASCA data [15], but the differences are small3Table 1. JEM-X SummaryNorth-East LimbEnergy band Flux Statistical Model uxa(keV) (10−4 phot/cm2s) signicance (10−4 phot/cm2s)2.44.2b 10 ± 4 2.6 144.28.4 11 ± 2 5.0 5.98.414 3.3± 1.9 1.7 1.4South-West Limb2.44.2 15 ± 4 4.0 134.28.4 3.6± 2.3 1.6 4.88.414 <5.7 3c -aFlux from extrapolated model fit to the ASCA spectrum (0.8 - 9keV), Dyer et al. 15bThe actual low energy threshold is variable over different parts ofthe JEM-X detector.c3σ upper limitFigure 2. Integrated hard X-ray spectrum of SN 1006.The upper limit bar is from ISGRI. Open circles are pre-ASCA (see text) observations, and the two solid lines arethe±1σ model ts to ASCA and RXTE data. The dotted-and-dashed line is the extrapolation from the Chandraspectral t [19]. The dashed line is the bremsstrahlungprediction of the model described in text. The Set III re-sults are not incorporated yet. From Kalemci et al. [8]in the ASCA band (below 8 keV). The small excess of theSW over NE limbs in the 2.4–4.2 keV band we see is notsupported by ASCA or XMM-Newton observations in thatenergy range.Figure 2 shows our combined JEM-X fluxes for bothlimbs, compared with previous spatially integrated pre-ASCA flux measurements (see [16, 17] for references).The integrated fluxes have been converted to flux den-sities assuming a photon index Γ of 3.0 [18]. Alsoshown are a pair of model curves for the escape modelthat provided a good fit to ASCA and RXTE data [15].The parameters of the best fit are a roll-off frequency ofνroll = 3.0×1017 Hz and an electron energy index of 2.2(implying a radio spectral index α = 0.6 or radio photonindex Γ ≡ α + 1 = 1.6). These parameters imply ane-folding energy of the exponential cutoff (due to escape)of 32(B2/10µGauss)−1/2 TeV (where B2 is the post-shock field). The curves in Figure 2 correspond to ±1σerrors on νroll: (2.8 − 3.1) × 1017 Hz. The dashed lineindicates the bremsstrahlung prediction for an upstreamdensity of 0.2 cm−3 and B2 = 10µGauss.3.2. ISGRI resultsWe obtained ISGRI images in different energy bands us-ing OSA 4.2 for Sets I and II, and OSA 5.1 for Set III. TheSets have not been merged yet. SN 1006 was not detectedin any band. The 20-40 keV sigma images for set II andIII are shown in Fig. 3. The 3 σ upper limit (sensitivitylimit) for a point source for ∼750 ks observing time is∼9 × 10−5 photon cm−2 s−1. If synchrotron dominatesin 20–40 keV band, the emission would be concentratedin two limbs. The extended nature of the source causesa reduction factor of 0.7 in flux (see § 2.2). Thereforethe 3 σ upper limit for a synchrotron dominated sourceis 1.3 × 10−4 photon cm−2 s−1 at each limb. For syn-chrotron dominated emission, the expected total flux inthe 20–40 keV band (based on the models shown in Fig-ure 2) is (2.9 − 3.6) × 10−5 photon cm−2 s−1 for theentire remnant, or roughly half this at each limb. Thebremsstrahlung emission from the model shown in Fig-ure 2, using the lower-limit 25 µG downstream magneticfield from H.E.S.S. TeV observations, is predicted to beabout 2×10−8 photon cm−2 s−1, far below ISGRI’s sen-sitivity. In fact, in this case, since the emission would becoming from a larger area, the upper limit set by our ob-servations is even higher. The predicted crossover energywhere bremsstrahlung and synchrotron emission becomecomparable is in the vicinity of 200 keV.Long et al. (2003) reported that a power-law photon indexof 2.30 fit the Chandra data well between 0.5 and 5 keVfor the emission at the limbs. An extrapolation of this fitfrom 5 keV to the ISGRI range is also shown in Fig. 2with a dotted-and-dashed line. Our ISGRI upper limit isabout a factor of 2 below the extrapolation of this power-law to 28 keV.4. DISCUSSIONCoded-mask imaging is a complex process, and optimaltechniques are still being developed to clean noisy im-ages, both for JEM-X and ISGRI. Our results confirmand extend results obtained with ASCA, Chandra X-rayObservatory, and XMM-Newton, and show that relativelysmall increases in sensitivity may allow ISGRI to detectpredicted synchrotron radiation for some models.The prospect of detecting bremsstrahlung X-rays orgamma-rays from SN 1006 has become considerablymore remote with the combination of lower estimates forthe ambient density and the much higher magnetic fieldthat would be required to explain the lack of IC fromCMB photons in the H.E.S.S. observations. However, itis quite possible that INTEGRAL observations conductedduring Cycle 3 will have sufficient sensitivity to detectsynchrotron emission in the 18-40 keV band. A modelthat can describe the observations of Figure 2 but with astrong enough magnetic field to satisfy the H.E.S.S. con-straints (downstream B > 25µG) is somewhat harderthan the spectra shown in Figure 2, and predicts a flux4-4 -3 -2 -1 0 1 2 3018224.000224.500225.000225.500226.000226.500227.000-42.000Figure 3. Top: The ISGRI sigma image around SN 1006in 2040 keV band for Set II. SN 1006 is not detected.A nearby source [VV 780, 11.6 σ detection 20] is alsoshown for comparison. The ASCA contours are overlaidto show the expected position of SN 1006. The feature atthe center of SN 1006 is not signicant, and is possiblydue to inaccurate background map. Bottom left: ISGRI20-40 keV image using Set III, showing offset. Bottomright: Set III, close to SN 1006. VV 780 with 4.2 σ is thebrightest source.in the 18-40 keV band of 0.3 × 10−4 photon cm−2 s−1.Some models predict considerably lower fluxes in the 18–40 keV band, so ISGRI detection would not only extendand confirm the presence of hard synchrotron X-rays, butcould provide useful model discrimination.We have obtained the first observations with JEM-X of anextended source, between 2.4 and 14 keV. The fluxes wederive are consistent with those of earlier imaging obser-vations where they overlap, and support the identificationof the continuum emission of SN 1006 as synchrotron ra-diation from a slowly dropping off electron distribution.Our ISGRI 3σ upper limit is about a factor of 4 higherthan the prediction of the model that best fits the soft X-ray continuum. With ISGRI, we confirm at higher en-ergies than has been previously reported that the X-rayspectrum of SN 1006 must be steepening. The resultsfrom the new datasets for both ISGRI and JEM-X will beincluded in the analysis in the near future.5. ACKNOWLEDGMENTSE.K. is supported by the European Comission through aFP6 Marie-Curie International Reintegration Grant (IN-DAM, MIRG-CT-2005-017203)). E.K. acknowledgespartial support of TU¨BI˙TAK. E.K. and S.E.B. acknowl-edge NASA grants NAG5-13142 and NAG5-13093.S.P.R. acknowledges support from NASA grant NAG5-13092.REFERENCES[1] Winkler, P. F. & Laird, F. N. 1976, ApJ, 204, L111[2] Koyama, K., Petre, R., Gotthelf, E. V., et al. 1995,Nature, 378, 255[3] Pohl, M. 1996, A&A, 307, L57[4] Tanimori, T. & et al. 1998, ApJ, 497, 25[5] Tanimori, T. & et al. 2005, in International CosmicRay Conference, 215–+[6] Reynolds, S. P. 1999, Astrophys. Lett. Commun., 38,425[7] Winkler, C., Courvoisier, T. J.-L., Di Cocco, G., et al.2003, A&A, 411, L1[8] Kalemci, E., Reynolds, S. P., Boggs, S. E., et al.2006, ApJ, 644[9] Vedrenne, G., Roques, J.-P., Scho¨nfelder, V., et al.2003, A&A, 411, L63[10] Lund, N., Westergaard, N. J., Budtz-Joergensen, C.,et al. 2004, in Proc. of The 5th INTEGRAL Workshop,The INTEGRAL Universe, Munich, 16-20 Feb. 2004[11] Chenevez, J., Lund, N., Westergaard, N. J., et al.2004, to appear in: Proc. of The 5th INTEGRALWorkshop, The INTEGRAL Universe, Munich, 16-20Feb. 2004, astro-ph/0406682[12] Goldwurm, A., David, P., Foschini, L., et al. 2003,A&A, 411, L223[13] Renaud, M., Lebrun, F., Terrier, R., Reynolds, S. P.,& Kalemci, E. 2006, A&A, 456,389[14] Rothenflug, R., Ballet, J., Dubner, G., et al. 2004,A&A, 425, 121[15] Dyer, K. K., Reynolds, S. P., & Borkowski, K. J.2004, ApJ, 600, 752[16] Reynolds, S. P. 1996, ApJ, 459, L13[17] Hamilton, A. J. S., Sarazin, C. L., & Szymkowiak,A. E. 1986, ApJ, 300, 698[18] Allen, G. E., Petre, R., & Gotthelf, E. V. 2001, ApJ,558, 739[19] Long, K. S., Reynolds, S. P., Raymond, J. C., et al.2003, ApJ, 586, 1162[20] Kalemci, E., Boggs, S. E., & Lund, N. 2005, TheAstronomer’s Telegram, 410

X-RAY OBSERVATIONS OF SN 1006 WITH INTEGRAL

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X-RAY OBSERVATIONS OF SN 1006 WITH INTEGRAL

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