- Email: [email protected]
Detection of a flare at Fermi LAT energies during a multiwavelength campaign on Markarian 180
Nuclear Instruments and Methods in Physics Research A 692 (2012) 272–274
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Detection of a flare at Fermi LAT energies during a multiwavelength campaign on Markarian 180 C. Sbarra n, D. Bastieri a
Department of Physics and INFN, Via Marzolo 8, 35131 Padova, Italy
Of the Fermi LAT collaboration a r t i c l e i n f o
a b s t r a c t
Available online 31 December 2011
We present the results of the analysis of Markarian 180 (1ES 1133 þ 704), a BL Lac object embedded in a giant elliptical galaxy, obtained for a period of 45 day, during which multi-wavelength observations were ongoing. The multi-wavelength campaign on Mrk180 was in 2008 (from 2008-10-24 to 2008-1208) and was coordinated by Stefan Rugamer (MAGIC Collaboration). The Mrk 180 is associated with a quasar-like object whose distance can be determined unambiguously, thanks to the measurement of absorption line that gives the redshift (z ¼ 0.046; Ulrich 1978). The source was observed by the LAT of the Fermi satellite, and it was possible to discover a change of flux, at the energies 100 MeV–300 GeV, during the multi-wavelength campaign period. Results of the analysis are shown. & 2011 Elsevier B.V. All rights reserved.
Keywords: Gamma ray astronomy Multiwavelength observations Space experiments
1. Introduction The all-sky observations of celestial objects by the Large Area Telescope (LAT) aboard the Fermi Gamma-ray Space Telescope reveal that there are at least two general classes of bright extragalactic sources of g-rays. One class comprises ‘radio-loud’ Active Galactic Nuclei (AGN) with powerful relativistic jets, including blazars and radio galaxies, which produce beamed high-energy emission via inverse-Compton scattering of soft photon fields on ultrarelativistic jet electrons. The other class consisting of galaxies with prominent starburst systems, which produce diffuse, unbeamed g-ray emission mainly resulting from the interactions of cosmic-ray particles with the interstellar medium (ISM). The Blazars are a particular type of AGN whose jets point to the observer. They are HE and VHE emitting sources with strong variability, which emit radiation from radio up to TeV energies. Multi-wavelength (MW) campaigns are strongly encouraged in order to better understand the various emission models.
2. The Fermi LAT detector The Large Area Telescope was designed to observe photons from 30 MeV to 300 GeV with a large field of view, over 2 sr (one-fifth of the entire sky), over short time intervals. The LAT detector, sketched n
Corresponding author. Tel.: þ39 051 2095035. E-mail address: [email protected] (C. Sbarra).
0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.12.073
in Fig. 1, consists of a Silicon Tracker, a calorimeter and a DAQ, all grouped in 16 modules surrounded by an anti-coincidence system.
3. AGN Markarian 180 The Markarian 180 source was discovered by the MAGIC telescope in 2006 following an optical trigger [1]. It was observed for 13.4 h and it was found to produce an integral flux above 200 GeV of F ¼(2.370.7) 10 11 cm 2 s 1, which corresponds to 1.27 10 11 ergs cm 2 s 1, or 11% of the Crab Nebula flux measured by MAGIC. The Markarian 180 is a nearby source (z¼0.046) at galactic coordinates L¼31.876, B¼45.645 (it is indicated by a green spot in Fig. 2).
4. Mrk180 multi-wavelength campaign and the LAT results Mrk180 is included in the Fermi-LAT 1-year Catalog [2]. A multi-wavelength campaign on the source was organized by the MAGIC collaboration in 2008 [3]. Due to its rather faint emission particularly at TeV energies, the campaign represented a quite challenging observation for the source and nevertheless it represented the first multi-wavelength campaign including VHE observations for this object. Two suitable observation windows were found, one in spring, the other one at the end of the year, and in the latter the Fermi observations were made.
C. Sbarra, D. Bastieri / Nuclear Instruments and Methods in Physics Research A 692 (2012) 272–274
Fig. 1. The Fermi LAT detector.
273
Fig. 3. TS vs time during the period of the multi-wavelength campaign on Mrk 180. The trend of the graphic indicates that a flare of the source has been occurred between the 14th and 31th day from the 0th day (2008-10-24).
Fig. 2. The Fermi sky map with a green dot on the position of Mrk 180 (L¼ 31.876, B¼ 45.645).
In X-rays,1 an unprecedented flare with a flux increase of factor of 10 was detected for Mrk 180, and LAT data analysis were requested to search for possible correlations. 4.1. The likelihood analysis for LAT data The official analysis method of the Fermi collaboration to date is the maximum likelihood method. This method is used to estimate to what extent the observed data are consistent with a statistical hypothesis. The method allows the comparison between different models for which the likelihood ratio test is found to be the most powerful criterion. The test statistics TS¼ 2(ln L ln L0), were L and L0 are the likelihoods of the different models (null source hypothesis and alternative hypothesis), is distributed asintotically as a w2 with k¼ m h degrees of freedom (m and h are the parameters of the two models). The significance of a source detection at a position in the sky goes like s E(TS)1/2 [4]. All this statistical analysis is included in the Fermi Science Tools Package,2 which is available from the Fermi Science Support center. 4.2. The LAT results: flare detection Events satisfying the standard low-background event selection and coming from zenith angle o1051 (to greatly reduce the contribution of gamma rays from Earth’s limb) were used. Furthermore, we selected only events where the satellite rocking angle was less than 521. We further restricted the analysis to photon energies above 100 MeV (below, the effective area is relatively low and strongly dependent on energy). A Maximum
Fig. 4. Differential flux of the source Mrk180 in the LAT energy range during the period of the multi-wavelength campaign.
likelihood analysis was performed in unbinned mode using the tool gtlike. We analyzed the period, in MJD3: 54763 (t_start) 54808 (t_stop) that corresponds to 45 day: 2008-10-24 05:07:00 2008-12-08 06:48:00. We used P6_V34 standard analysis in the energy band 100 MeV–300 GeV. In normal situations, one expects that the TS should scale linearly with time, for this reason we computed the TS vs time, more precisely we plotted ‘‘after’’ , that is TS(t)¼TS in [t,t_stop] (it was expected a linear decrease with t) and ‘‘before’’, that is TS(t)¼TS in [t_start,t] (again, it was expected a linear increase with t). The results are shown in Fig. 3. The sudden increase in ‘‘TS(after)‘‘ and drop in ‘‘TS(before)‘‘ beginning 20day from the start of the campaign indicates a flare beginning at this time. By using an unbinned statistics, we found a TS value of 43.9 (well above 5 sigma of significance), and a flux of: N ¼ ð2:38 7 1:09Þ 108 ph cm2 s1 with a slope of:
g ¼ 1:68 7 0:21 3
1 2
X-ray coverage was provided by Too proposals to Swift [3] http://fermi.gsfc.nasa.gov/ssc/data/analysis/software/
Modified julian date Post-launch instrument response functions that take into account pile-up and accidental coincidence effects in the detector subsystem. 4
274
C. Sbarra, D. Bastieri / Nuclear Instruments and Methods in Physics Research A 692 (2012) 272–274
consistent with a source flare which lasted for 15day and with a quite hard resulting spectrum.
4.3. Resulting differential flux We computed the differential flux of Mrk180 in the observed period by simply making, in each energy chosen bin, all the analysis described in 4.1 and by computing the flux of the source in each bin. The selection of the data and the cuts made were the same as before. The preliminary resulting differential flux distribution for the whole period of the multi-wavelength campaign is shown in Fig. 4 (in each bin it is assumed g ¼2 to make the differential flux).
5. Conclusion We studied the emission from Mrk 180 during a period when a multi-frequency campaign was ongoing. The period was divided in several sub-periods in which a standard Science Tools unbinned analysis was done. The results of the TS versus the increasing time of integration are consistent with a source flare which lasted for 15 days and with a resulting quite hard spectrum. References [1] [2] [3] [4]
J. Albert, et al., The Astrophysical Journal 648 (2006) L105. A. Abdo, Fermi–LAT Collab., et al., ApJS 188 (2010) 405. S..Rugamer et al., Proc. Of the 32th ICRC, 2011 Beijng. J.R. Mattox, et al., The Astrophysical Journal 461 (1996) 396.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Detection of a flare at Fermi LAT energies during a multiwavelength campaign on Markarian 180 C. Sbarra n, D. Bastieri a
Department of Physics and INFN, Via Marzolo 8, 35131 Padova, Italy
Of the Fermi LAT collaboration a r t i c l e i n f o
a b s t r a c t
Available online 31 December 2011
We present the results of the analysis of Markarian 180 (1ES 1133 þ 704), a BL Lac object embedded in a giant elliptical galaxy, obtained for a period of 45 day, during which multi-wavelength observations were ongoing. The multi-wavelength campaign on Mrk180 was in 2008 (from 2008-10-24 to 2008-1208) and was coordinated by Stefan Rugamer (MAGIC Collaboration). The Mrk 180 is associated with a quasar-like object whose distance can be determined unambiguously, thanks to the measurement of absorption line that gives the redshift (z ¼ 0.046; Ulrich 1978). The source was observed by the LAT of the Fermi satellite, and it was possible to discover a change of flux, at the energies 100 MeV–300 GeV, during the multi-wavelength campaign period. Results of the analysis are shown. & 2011 Elsevier B.V. All rights reserved.
Keywords: Gamma ray astronomy Multiwavelength observations Space experiments
1. Introduction The all-sky observations of celestial objects by the Large Area Telescope (LAT) aboard the Fermi Gamma-ray Space Telescope reveal that there are at least two general classes of bright extragalactic sources of g-rays. One class comprises ‘radio-loud’ Active Galactic Nuclei (AGN) with powerful relativistic jets, including blazars and radio galaxies, which produce beamed high-energy emission via inverse-Compton scattering of soft photon fields on ultrarelativistic jet electrons. The other class consisting of galaxies with prominent starburst systems, which produce diffuse, unbeamed g-ray emission mainly resulting from the interactions of cosmic-ray particles with the interstellar medium (ISM). The Blazars are a particular type of AGN whose jets point to the observer. They are HE and VHE emitting sources with strong variability, which emit radiation from radio up to TeV energies. Multi-wavelength (MW) campaigns are strongly encouraged in order to better understand the various emission models.
2. The Fermi LAT detector The Large Area Telescope was designed to observe photons from 30 MeV to 300 GeV with a large field of view, over 2 sr (one-fifth of the entire sky), over short time intervals. The LAT detector, sketched n
Corresponding author. Tel.: þ39 051 2095035. E-mail address: [email protected] (C. Sbarra).
0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2011.12.073
in Fig. 1, consists of a Silicon Tracker, a calorimeter and a DAQ, all grouped in 16 modules surrounded by an anti-coincidence system.
3. AGN Markarian 180 The Markarian 180 source was discovered by the MAGIC telescope in 2006 following an optical trigger [1]. It was observed for 13.4 h and it was found to produce an integral flux above 200 GeV of F ¼(2.370.7) 10 11 cm 2 s 1, which corresponds to 1.27 10 11 ergs cm 2 s 1, or 11% of the Crab Nebula flux measured by MAGIC. The Markarian 180 is a nearby source (z¼0.046) at galactic coordinates L¼31.876, B¼45.645 (it is indicated by a green spot in Fig. 2).
4. Mrk180 multi-wavelength campaign and the LAT results Mrk180 is included in the Fermi-LAT 1-year Catalog [2]. A multi-wavelength campaign on the source was organized by the MAGIC collaboration in 2008 [3]. Due to its rather faint emission particularly at TeV energies, the campaign represented a quite challenging observation for the source and nevertheless it represented the first multi-wavelength campaign including VHE observations for this object. Two suitable observation windows were found, one in spring, the other one at the end of the year, and in the latter the Fermi observations were made.
C. Sbarra, D. Bastieri / Nuclear Instruments and Methods in Physics Research A 692 (2012) 272–274
Fig. 1. The Fermi LAT detector.
273
Fig. 3. TS vs time during the period of the multi-wavelength campaign on Mrk 180. The trend of the graphic indicates that a flare of the source has been occurred between the 14th and 31th day from the 0th day (2008-10-24).
Fig. 2. The Fermi sky map with a green dot on the position of Mrk 180 (L¼ 31.876, B¼ 45.645).
In X-rays,1 an unprecedented flare with a flux increase of factor of 10 was detected for Mrk 180, and LAT data analysis were requested to search for possible correlations. 4.1. The likelihood analysis for LAT data The official analysis method of the Fermi collaboration to date is the maximum likelihood method. This method is used to estimate to what extent the observed data are consistent with a statistical hypothesis. The method allows the comparison between different models for which the likelihood ratio test is found to be the most powerful criterion. The test statistics TS¼ 2(ln L ln L0), were L and L0 are the likelihoods of the different models (null source hypothesis and alternative hypothesis), is distributed asintotically as a w2 with k¼ m h degrees of freedom (m and h are the parameters of the two models). The significance of a source detection at a position in the sky goes like s E(TS)1/2 [4]. All this statistical analysis is included in the Fermi Science Tools Package,2 which is available from the Fermi Science Support center. 4.2. The LAT results: flare detection Events satisfying the standard low-background event selection and coming from zenith angle o1051 (to greatly reduce the contribution of gamma rays from Earth’s limb) were used. Furthermore, we selected only events where the satellite rocking angle was less than 521. We further restricted the analysis to photon energies above 100 MeV (below, the effective area is relatively low and strongly dependent on energy). A Maximum
Fig. 4. Differential flux of the source Mrk180 in the LAT energy range during the period of the multi-wavelength campaign.
likelihood analysis was performed in unbinned mode using the tool gtlike. We analyzed the period, in MJD3: 54763 (t_start) 54808 (t_stop) that corresponds to 45 day: 2008-10-24 05:07:00 2008-12-08 06:48:00. We used P6_V34 standard analysis in the energy band 100 MeV–300 GeV. In normal situations, one expects that the TS should scale linearly with time, for this reason we computed the TS vs time, more precisely we plotted ‘‘after’’ , that is TS(t)¼TS in [t,t_stop] (it was expected a linear decrease with t) and ‘‘before’’, that is TS(t)¼TS in [t_start,t] (again, it was expected a linear increase with t). The results are shown in Fig. 3. The sudden increase in ‘‘TS(after)‘‘ and drop in ‘‘TS(before)‘‘ beginning 20day from the start of the campaign indicates a flare beginning at this time. By using an unbinned statistics, we found a TS value of 43.9 (well above 5 sigma of significance), and a flux of: N ¼ ð2:38 7 1:09Þ 108 ph cm2 s1 with a slope of:
g ¼ 1:68 7 0:21 3
1 2
X-ray coverage was provided by Too proposals to Swift [3] http://fermi.gsfc.nasa.gov/ssc/data/analysis/software/
Modified julian date Post-launch instrument response functions that take into account pile-up and accidental coincidence effects in the detector subsystem. 4
274
C. Sbarra, D. Bastieri / Nuclear Instruments and Methods in Physics Research A 692 (2012) 272–274
consistent with a source flare which lasted for 15day and with a quite hard resulting spectrum.
4.3. Resulting differential flux We computed the differential flux of Mrk180 in the observed period by simply making, in each energy chosen bin, all the analysis described in 4.1 and by computing the flux of the source in each bin. The selection of the data and the cuts made were the same as before. The preliminary resulting differential flux distribution for the whole period of the multi-wavelength campaign is shown in Fig. 4 (in each bin it is assumed g ¼2 to make the differential flux).
5. Conclusion We studied the emission from Mrk 180 during a period when a multi-frequency campaign was ongoing. The period was divided in several sub-periods in which a standard Science Tools unbinned analysis was done. The results of the TS versus the increasing time of integration are consistent with a source flare which lasted for 15 days and with a resulting quite hard spectrum. References [1] [2] [3] [4]
J. Albert, et al., The Astrophysical Journal 648 (2006) L105. A. Abdo, Fermi–LAT Collab., et al., ApJS 188 (2010) 405. S..Rugamer et al., Proc. Of the 32th ICRC, 2011 Beijng. J.R. Mattox, et al., The Astrophysical Journal 461 (1996) 396.